The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 60

Therefore the earliest terrestrial plants known to us, namely, the Coniferae and Cycadiae, no doubt were anemophilous, like the existing species of these same groups. A vestige of this early state of things is likewise shown by some other groups of plants which are anemophilous, as these on the whole stand lower in the scale than entomophilous species.

There is no great difficulty in understanding how an anemophilous plant might have been rendered entomophilous. Pollen is a nutritious substance, and would soon have been discovered and devoured by insects; and if any adhered to their bodies it would have been carried from the anthers to the stigma of the same flower, or from one flower to another.

One of the chief characteristics of the pollen of anemophilous plants is its incoherence; but pollen in this state can adhere to the hairy bodies of insects, as we see with some Leguminosae, Ericaceae, and Melastomaceae. We have, however, better evidence of the possibility of a transition of the above kind in certain plants being now fertilised partly by the wind and partly by insects. The common rhubarb (Rheum rhapontic.u.m) is so far in an intermediate condition, that I have seen many Diptera sucking the flowers, with much pollen adhering to their bodies; and yet the pollen is so incoherent, that clouds of it are emitted if the plant be gently shaken on a sunny day, some of which could hardly fail to fall on the large stigmas of the neighbouring flowers. According to Delpino and Hermann Muller, some species of Plantago are in a similar intermediate condition. (10/44. 'Die Befruchtung' etc. page 342.)

Although it is probable that pollen was aboriginally the sole attraction to insects, and although many plants now exist whose flowers are frequented exclusively by pollen-devouring insects, yet the great majority secrete nectar as the chief attraction. Many years ago I suggested that primarily the saccharine matter in nectar was excreted as a waste product of chemical changes in the sap; and that when the excretion happened to occur within the envelopes of a flower, it was utilised for the important object of cross-fertilisation, being subsequently much increased in quant.i.ty and stored in various ways.

(10/45. Nectar was regarded by De Candolle and Dunal as an excretion, as stated by Martinet in 'Annal des Sc. Nat.' 1872 tome 14 page 211.) This view is rendered probable by the leaves of some trees excreting, under certain climatic conditions, without the aid of special glands, a saccharine fluid, often called honey-dew. This is the case with the leaves of the lime; for although some authors have disputed the fact, a most capable judge, Dr. Maxwell Masters, informs me that, after having heard the discussions on this subject before the Horticultural Society, he feels no doubt on this head. The leaves, as well as the cut stems, of the manna ash (Fraxinus ornus) secrete in a like manner saccharine matter. (10/46. 'Gardeners' Chronicle' 1876 page 242.) According to Trevira.n.u.s, so do the upper surfaces of the leaves of Carduus arctioides during hot weather. Many a.n.a.logous facts could be given. (10/47. Kurr 'Untersuchungen uber die Bedeutung der Nektarien' 1833 page 115.) There are, however, a considerable number of plants which bear small glands on their leaves, petioles, phyllodia, stipules, bracteae, or flower peduncles, or on the outside of their calyx, and these glands secrete minute drops of a sweet fluid, which is eagerly sought by sugar-loving insects, such as ants, hive-bees, and wasps. (10/48. A large number of cases are given by Delpino in the 'Bulletino Entomologico' Anno 6 1874.

To these may be added those given in my text, as well as the excretion of saccharine matter from the calyx of two species of Iris, and from the bracteae of certain Orchideae: see Kurr 'Bedeutung der Nektarien' 1833 pages 25, 28. Belt 'Nicaragua' page 224, also refers to a similar excretion by many epiphytal orchids and pa.s.sion-flowers. Mr. Rodgers has seen much nectar secreted from the bases of the flower-peduncles of Vanilla. Link says that the only example of a hypopetalous nectary known to him is externally at the base of the flowers of Chironia decussata: see 'Reports on Botany, Ray Society' 1846 page 355. An important memoir bearing on this subject has lately appeared by Reinke 'Gottingen Nachrichten' 1873 page 825, who shows that in many plants the tips of the serrations on the leaves in the bud bear glands which secrete only at a very early age, and which have the same morphological structure as true nectar-secreting glands. He further shows that the nectar-secreting glands on the petioles of Prunus avium are not developed at a very early age, yet wither away on the old leaves. They are h.o.m.ologous with those on the serrations of the blades of the same leaves, as shown by their structure and by transition-forms; for the lowest serrations on the blades of most of the leaves secrete nectar instead of resin (harz).) In the case of the glands on the stipules of Vicia sativa, the excretion manifestly depends on changes in the sap, consequent on the sun s.h.i.+ning brightly; for I repeatedly observed that as soon as the sun was hidden behind clouds the secretion ceased, and the hive-bees left the field; but as soon as the sun broke out again, they returned to their feast.

(10/49. I published a brief notice of this case in the 'Gardeners'

Chronicle' 1855 July 21 page 487, and afterwards made further observations. Besides the hive-bee, another species of bee, a moth, ants, and two kinds of flies sucked the drops of fluid on the stipules.

The larger drops tasted sweet. The hive-bees never even looked at the flowers which were open at the same time; whilst two species of humble-bees neglected the stipules and visited only the flowers.) I have observed an a.n.a.logous fact with the secretion of true nectar in the flowers of Lobelia erinus.

Delpino, however, maintains that the power of secreting a sweet fluid by any extra-floral organ has been in every case specially gained, for the sake of attracting ants and wasps as defenders of the plant against their enemies; but I have never seen any reason to believe that this is so with the three species observed by me, namely, Prunus laurocerasus, Vicia sativa, and V. faba. No plant is so little attacked by enemies of any kind as the common bracken-fern (Pteris aquilina); and yet, as my son Francis has discovered, the large glands at the bases of the fronds, but only whilst young, excrete much sweetish fluid, which is eagerly sought by innumerable ants, chiefly belonging to Myrmica; and these ants certainly do not serve as a protection against any enemy. Delpino argues that such glands ought not to be considered as excretory, because if they were so, they would be present in every species; but I cannot see much force in this argument, as the leaves of some plants excrete sugar only during certain states of the weather. That in some cases the secretion serves to attract insects as defenders of the plant, and may have been developed to a high degree for this special purpose, I have not the least doubt, from the observations of Delpino, and more especially from those of Mr. Belt on Acacia sphaerocephala, and on pa.s.sion-flowers. This acacia likewise produces, as an additional attraction to ants, small bodies containing much oil and protoplasm, and a.n.a.logous bodies are developed by a Cecropia for the same purpose, as described by Fritz Muller. (10/50. Mr. Belt 'The Naturalist in Nicaragua' 1874 page 218, has given a most interesting account of the paramount importance of ants as defenders of the above Acacia. With respect to the Cecropia see 'Nature' 1876 page 304. My son Francis has described the microscopical structure and development of these wonderful food-bodies in a paper read before the Linnean Society.)

The excretion of a sweet fluid by glands seated outside of a flower is rarely utilised as a means for cross-fertilisation by the aid of insects; but this occurs with the bracteae of the Marcgraviaceae, as the late Dr. Cruger informed me from actual observation in the West Indies, and as Delpino infers with much acuteness from the relative position of the several parts of their flowers. (10/51. 'Ult. Osservaz. Dicogamia'

1868-69 page 188.) Mr. Farrer has also shown that the flowers of Coronilla are curiously modified, so that bees may fertilise them whilst sucking the fluid secreted from the outside of the calyx. (10/52.

'Nature' 1874 page 169.) It further appears probable from the observations of the Reverend W.A. Leighton, that the fluid so abundantly secreted by glands on the phyllodia of the Australian Acacia magnifica, which stand near the flowers, is connected with their fertilisation.

(10/53. 'Annals and Magazine of Natural History' volume 16 1865 page 14.

In my work on the 'Fertilisation of Orchids' and in a paper subsequently published in the 'Annals and Magazine of Natural History' it has been shown that although certain kinds of orchids possess a nectary, no nectar is actually secreted by it; but that insects penetrate the inner walls and suck the fluid contained in the intercellular s.p.a.ces. I further suggested, in the case of some other orchids which do not secrete nectar, that insects gnawed the labellum; and this suggestion has since been proved true. Hermann Muller and Delpino have now shown that some other plants have thickened petals which are sucked or gnawed by insects, their fertilisation being thus aided. All the known facts on this head have been collected by Delpino in his 'Ult. Osserv.' part 2 fasc. 2 1875 pages 59-63.)

The amount of pollen produced by anemophilous plants, and the distance to which it is often transported by the wind, are both surprisingly great. Mr. Ha.s.sall found that the weight of pollen produced by a single plant of the Bulrush (Typha) was 144 grains. Bucketfuls of pollen, chiefly of Coniferae and Gramineae, have been swept off the decks of vessels near the North American sh.o.r.e; and Mr. Riley has seen the ground near St. Louis, in Missouri, covered with pollen, as if sprinkled with sulphur; and there was good reason to believe that this had been transported from the pine-forests at least 400 miles to the south.

Kerner has seen the snow-fields on the higher Alps similarly dusted; and Mr. Blackley found numerous pollen-grains, in one instance 1200, adhering to sticky slides, which were sent up to a height of from 500 to 1000 feet by means of a kite, and then uncovered by a special mechanism.

It is remarkable that in these experiments there were on an average nineteen times as many pollen-grains in the atmosphere at the higher than at the lower levels. (10/54. For Mr. Ha.s.sall's observations see 'Annals and Magazine of Natural History' volume 8 1842 page 108. In the 'North American Journal of Science' January 1842, there is an account of the pollen swept off the decks of a vessel. Riley 'Fifth Report on the Noxious Insects of Missouri' 1873 page 86. Kerner 'Die Schutzmittel des Pollens' 1873 page 6. This author has also seen a lake in the Tyrol so covered with pollen, that the water no longer appeared blue. Mr.

Blackley 'Experimental Researches on Hay-fever' 1873 pages 132, 141-152.) Considering these facts, it is not so surprising as it at first appears that all, or nearly all, the stigmas of anemophilous plants should receive pollen brought to them by mere chance by the wind.

During the early part of summer every object is thus dusted with pollen; for instance, I examined for another purpose the labella of a large number of flowers of the Fly Ophrys (which is rarely visited by insects), and found on all very many pollen-grains of other plants, which had been caught by their velvety surfaces.

The extraordinary quant.i.ty and lightness of the pollen of anemophilous plants are no doubt both necessary, as their pollen has generally to be carried to the stigmas of other and often distant flowers; for, as we shall soon see, most anemophilous plants have their s.e.xes separated. The fertilisation of these plants is generally aided by the stigmas being of large size or plumose; and in the case of the Coniferae, by the naked ovules secreting a drop of fluid, as shown by Delpino. Although the number of anemophilous species is small, as the author just quoted remarks, the number of individuals is large in comparison with that of entomophilous species. This holds good especially in cold and temperate regions, where insects are not so numerous as under a warmer climate, and where consequently entomophilous plants are less favourably situated. We see this in our forests of Coniferae and other trees, such as oaks, beeches, birches, ashes, etc.; and in the Gramineae, Cyperaceae, and Juncaceae, which clothe our meadows and swamps; all these trees and plants being fertilised by the wind. As a large quant.i.ty of pollen is wasted by anemophilous plants, it is surprising that so many vigorous species of this kind abounding with individuals should still exist in any part of the world; for if they had been rendered entomophilous, their pollen would have been transported by the aid of the senses and appet.i.tes of insects with incomparably greater safety than by the wind. That such a conversion is possible can hardly be doubted, from the remarks lately made on the existence of intermediate forms; and apparently it has been effected in the group of willows, as we may infer from the nature of their nearest allies. (10/55. Hermann Muller 'Die Befruchtung' etc. page 149.)

It seems at first sight a still more surprising fact that plants, after having been once rendered entomophilous, should ever again have become anemophilous; but this has occasionally though rarely occurred, for instance, with the common Poterium sanguisorba, as may be inferred from its belonging to the Rosaceae. Such cases are, however, intelligible, as almost all plants require to be occasionally intercrossed; and if any entomiphilous species ceased to be visited by insects, it would probably perish unless it were rendered anemophilous. A plant would be neglected by insects if nectar failed to be secreted, unless indeed a large supply of attractive pollen was present; and from what we have seen of the excretion of saccharine fluid from leaves and glands being largely governed in several cases by climatic influences, and from some few flowers which do not now secrete nectar still retaining coloured guiding-marks, the failure of the secretion cannot be considered as a very improbable event. The same result would follow to a certainty, if winged insects ceased to exist in any district, or became very rare. Now there is only a single plant in the great order of the Cruciferae, namely, Pringlea, which is anemophilous, and this plant is an inhabitant of Kerguelen Land, where there are hardly any winged insects, owing probably, as was suggested by me in the case of Madeira, to the risk which they run of being blown out to sea and destroyed. (10/56. The Reverend A.E. Eaton in 'Proceedings of the Royal Society' volume 23 1875 page 351.)

A remarkable fact with respect to anemophilous plants is that they are often diclinous, that is, they are either monoecious with their s.e.xes separated on the same plant, or dioecious with their s.e.xes on distinct plants. In the cla.s.s Monoecia of Linnaeus, Delpino shows that the species of twenty-eight genera are anemophilous, and of seventeen genera entomophilous. (10/57. 'Studi sopra un Lignaggio anemofilo delle Compositae' 1871.) The larger proportion of entomophilous genera in this latter cla.s.s is probably the indirect result of insects having the power of carrying pollen to another and sometimes distant plant much more securely than the wind. In the above two cla.s.ses taken together there are thirty-eight anemophilous and thirty-six entomophilous genera; whereas in the great ma.s.s of hermaphrodite plants the proportion of anemophilous to entomophilous genera is extremely small. The cause of this remarkable difference may be attributed to anemophilous plants having retained in a greater degree than the entomophilous a primordial condition, in which the s.e.xes were separated and their mutual fertilisation effected by means of the wind. That the earliest and lowest members of the vegetable kingdom had their s.e.xes separated, as is still the case to a large extent, is the opinion of a high authority, Nageli. (10/58. 'Entstehung und Begriff der Naturhist. Art' 1865 page 22.) It is indeed difficult to avoid this conclusion, if we admit the view, which seems highly probable, that the conjugation of the Algae and of some of the simplest animals is the first step towards s.e.xual reproduction; and if we further bear in mind that a greater and greater degree of differentiation between the cells which conjugate can be traced, thus leading apparently to the development of the two s.e.xual forms. (10/59. See the interesting discussion on this whole subject by O. Butschli in his 'Studien uber die ersten Entwickelungsvorgange der Eizelle; etc. 1876 pages 207-219. Also Engelmann "Ueber Entwickelung von Infusorien" 'Morphol. Jahrbuch' B. 1 page 573. Also Dr. A. Dodel "Die Kraushaar-Algae" 'Pringsheims Jahrbuch f. Wiss. Bot.' B. 10.) We have also seen that as plants became more highly developed and affixed to the ground, they would be compelled to be anemophilous in order to intercross. Therefore all plants which have not since been greatly modified, would tend still to be both diclinous and anemophilous; and we can thus understand the connection between these two states, although they appear at first sight quite disconnected. If this view is correct, plants must have been rendered hermaphrodites at a later though still very early period, and entomophilous at a yet later period, namely, after the development of winged insects. So that the relations.h.i.+p between hermaphroditism and fertilisation by means of insects is likewise to a certain extent intelligible.

Why the descendants of plants which were originally dioecious, and which therefore profited by always intercrossing with another individual, should have been converted into hermaphrodites, may perhaps be explained by the risk which they ran, especially as long as they were anemophilous, of not being always fertilised, and consequently of not leaving offspring. This latter evil, the greatest of all to any organism, would have been much lessened by their becoming hermaphrodites, though with the contingent disadvantage of frequent self-fertilisation. By what graduated steps an hermaphrodite condition was acquired we do not know. But we can see that if a lowly organised form, in which the two s.e.xes were represented by somewhat different individuals, were to increase by budding either before or after conjugation, the two incipient s.e.xes would be capable of appearing by buds on the same stock, as occasionally occurs with various characters at the present day. The organism would then be in a monoecious condition, and this is probably the first step towards hermaphroditism; for if very simple male and female flowers on the same stock, each consisting of a single stamen or pistil, were brought close together and surrounded by a common envelope, in nearly the same manner as with the florets of the Compositae, we should have an hermaphrodite flower.

There seems to be no limit to the changes which organisms undergo under changing conditions of life; and some hermaphrodite plants, descended as we must believe from aboriginally diclinous plants, have had their s.e.xes again separated. That this has occurred, we may infer from the presence of rudimentary stamens in the flowers of some individuals, and of rudimentary pistils in the flowers of other individuals, for example in Lychnis dioica. But a conversion of this kind will not have occurred unless cross-fertilisation was already a.s.sured, generally by the agency of insects; but why the production of male and female flowers on distinct plants should have been advantageous to the species, cross-fertilisation having been previously a.s.sured, is far from obvious.

A plant might indeed produce twice as many seeds as were necessary to keep up its numbers under new or changed conditions of life; and if it did not vary by bearing fewer flowers, and did vary in the state of its reproductive organs (as often occurs under cultivation), a wasteful expenditure of seeds and pollen would be saved by the flowers becoming diclinous.

A related point is worth notice. I remarked in my Origin of Species that in Britain a much larger proportion of trees and bushes than of herbaceous plants have their s.e.xes separated; and so it is, according to Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in the 'London Catalogue of British Plants' that there are thirty-two indigenous trees and bushes in Great Britain, cla.s.sed under nine families; but to err on the safe side, I have counted only six species of willows. Of the thirty-two trees and bushes, nineteen, or more than half, have their s.e.xes separated; and this is an enormous proportion compared with other British plants. New Zealand abounds with diclinous plants and trees; and Dr. Hooker calculates that out of about 756 phanerogamic plants inhabiting the islands, no less than 108 are trees, belonging to thirty-five families. Of these 108 trees, fifty-two, or very nearly half, have their s.e.xes more or less separated. Of bushes there are 149, of which sixty-one have their s.e.xes in the same state; whilst of the remaining 500 herbaceous plants only 121, or less than a fourth, have their s.e.xes separated. Lastly, Professor Asa Gray informs me that in the United States there are 132 native trees (belonging to twenty-five families) of which ninety-five (belonging to seventeen families) "have their s.e.xes more or less separated, for the greater part decidedly separated.") It is, however, doubtful how far this rule holds good generally, and it certainly does not do so in Australia. But I have been a.s.sured that the flowers of the prevailing Australian trees, namely, the Myrtaceae, swarm with insects, and if they are dichogamous they would be practically diclinous. (10/61. With respect to the Proteaceae of Australia, Mr. Bentham 'Journal of the Linnean Society Botany' volume 13 1871 pages 58, 64, remarks on the various contrivances by which the stigma in the several genera is screened from the action of the pollen from the same flower. For instance, in Synaphea "the stigma is held by the eunuch (i.e., one of the stamens which is barren) safe from all pollution from her brother anthers, and is preserved intact for any pollen that may be inserted by insects and other agencies.") As far as anemophilous plants are concerned, we know that they are apt to have their s.e.xes separated, and we can see that it would be an unfavourable circ.u.mstance for them to bear their flowers very close to the ground, as their pollen is liable to be blown high up in the air (10/62. Kerner 'Schutzmittel des Pollens' 1873 page 4.); but as the culms of gra.s.ses give sufficient elevation, we cannot thus account for so many trees and bushes being diclinous. We may infer from our previous discussion that a tree bearing numerous hermaphrodite flowers would rarely intercross with another tree, except by means of the pollen of a distinct individual being prepotent over the plants' own pollen. Now the separation of the s.e.xes, whether the plant were anemophilous are entomophilous, would most effectually bar self-fertilisation, and this may be the cause of so many trees and bushes being diclinous. Or to put the case in another way, a plant would be better fitted for development into a tree, if the s.e.xes were separated, than if it were hermaphrodite; for in the former case its numerous flowers would be less liable to continued self-fertilisation. But it should also be observed that the long life of a tree or bush permits of the separation of the s.e.xes, with much less risk of evil from impregnation occasionally failing and seeds not being produced, than in the case of short-lived plants. Hence it probably is, as Lecoq has remarked, that annual plants are rarely dioecious.

Finally, we have seen reason to believe that the higher plants are descended from extremely low forms which conjugated, and that the conjugating individuals differed somewhat from one another,--the one representing the male and the other the female--so that plants were aboriginally dioecious. At a very early period such lowly organised dioecious plants probably gave rise by budding to monoecious plants with the two s.e.xes borne by the same individual; and by a still closer union of the s.e.xes to hermaphrodite plants, which are now much the commonest form. (10/63. There is a considerable amount of evidence that all the higher animals are the descendants of hermaphrodites; and it is a curious problem whether such hermaphroditism may not have been the result of the conjugation of two slightly different individuals, which represented the two incipient s.e.xes. On this view, the higher animals may now owe their bilateral structure, with all their organs double at an early embryonic period, to the fusion or conjugation of two primordial individuals.) As soon as plants became affixed to the ground, their pollen must have been carried by some means from flower to flower, at first almost certainly by the wind, then by pollen-devouring, and afterwards by nectar-seeking insects. During subsequent ages some few entomophilous plants have been again rendered anemophilous, and some hermaphrodite plants have had their s.e.xes again separated; and we can vaguely see the advantages of such recurrent changes under certain conditions.

Dioecious plants, however fertilised, have a great advantage over other plants in their cross-fertilisation being a.s.sured. But this advantage is gained in the case of anemophilous species at the expense of the production of an enormous superfluity of pollen, with some risk to them and to entomophilous species of their fertilisation occasionally failing. Half the individuals, moreover, namely, the males, produce no seed, and this might possibly be a disadvantage. Delpino remarks that dioecious plants cannot spread so easily as monoecious and hermaphrodite species, for a single individual which happened to reach some new site could not propagate its kind; but it may be doubted whether this is a serious evil. Monoecious plants can hardly fail to be to a large extent dioecious in function, owing to the lightness of their pollen and to the wind blowing laterally, with the great additional advantage of occasionally or often producing some self-fertilised seeds. When they are also dichogamous, they are necessarily dioecious in function.

Lastly, hermaphrodite plants can generally produce at least some self-fertilised seeds, and they are at the same time capable, through the various means specified in this chapter, of cross-fertilisation.

When their structure absolutely prevents self-fertilisation, they are in the same relative position to one another as monoecious and dioecious plants, with what may be an advantage, namely, that every flower is capable of yielding seeds.

CHAPTER XI.

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.

Insects visit the flowers of the same species as long as they can.

Cause of this habit.

Means by which bees recognise the flowers of the same species.

Sudden secretion of nectar.

Nectar of certain flowers unattractive to certain insects.

Industry of bees, and the number of flowers visited within a short time.

Perforation of the corolla by bees.

Skill shown in the operation.

Hive-bees profit by the holes made by humble-bees.

Effects of habit.

The motive for perforating flowers to save time.

Flowers growing in crowded ma.s.ses chiefly perforated.

Bees and various other insects must be directed by instinct to search flowers for nectar and pollen, as they act in this manner without instruction as soon as they emerge from the pupa state. Their instincts, however, are not of a specialised nature, for they visit many exotic flowers as readily as the endemic kinds, and they often search for nectar in flowers which do not secrete any; and they may be seen attempting to suck it out of nectaries of such length that it cannot be reached by them. (11/1. See, on this subject Hermann Muller 'Befruchtung' etc. page 427; and Sir J. Lubbock's 'British Wild Flowers'

etc. page 20. Muller 'Bienen Zeitung' June 1876 page 119, a.s.signs good reasons for his belief that bees and many other Hymenoptera have inherited from some early nectar-sucking progenitor greater skill in robbing flowers than that which is displayed by insects belonging to the other Orders.) All kinds of bees and certain other insects usually visit the flowers of the same species as long as they can, before going to another species. This fact was observed by Aristotle with respect to the hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper published in 1736 in the Philosophical Transactions. It may be observed by any one, both with hive and humble-bees, in every flower-garden; not that the habit is invariably followed. Mr. Bennett watched for several hours many plants of Lamium alb.u.m, L. purpureum, and another l.a.b.i.ate plant, Nepeta glechoma, all growing mingled together on a bank near some hives; and he found that each bee confined its visits to the same species. (11/2. 'Nature' 1874 June 4 page 92.) The pollen of these three plants differs in colour, so that he was able to test his observations by examining that which adhered to the bodies of the captured bees, and he found one kind on each bee.

Humble and hive-bees are good botanists, for they know that varieties may differ widely in the colour of their flowers and yet belong to the same species. I have repeatedly seen humble-bees flying straight from a plant of the ordinary red Dictamnus fraxinella to a white variety; from one to another very differently coloured variety of Delphinium consolida and of Primula veris; from a dark purple to a bright yellow variety of Viola tricolor; and with two species of Papaver, from one variety to another which differed much in colour; but in this latter case some of the bees flew indifferently to either species, although pa.s.sing by other genera, and thus acted as if the two species were merely varieties.

Hermann Muller also has seen hive-bees flying from flower to flower of Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and repens; and even from blue hyacinths to blue violets. (11/3. 'Bienen Zeitung' July 1876 page 183.)

Some species of Diptera or flies keep to the flowers of the same species with almost as much regularity as do bees; and when captured they are found covered with pollen. I have seen Rhingia rostrata acting in this manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici sepium. Volucella plumosa and Empis cheiroptera flew straight from flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved in the same manner with Potentilla tormentilla; and other Diptera with Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica hederaefolia and chamoedrys; but some flies visited indifferently the flowers of these two latter species. I have seen more than once a minute Thrips, with pollen adhering to its body, fly from one flower to another of the same kind; and one was observed by me crawling about within a convolvulus with four grains of pollen adhering to its head, which were deposited on the stigma.

Fabricius and Sprengel state that when flies have once entered the flowers of Aristolochia they never escape,--a statement which I could not believe, as in this case the insects would not aid in the cross-fertilisation of the plant; and this statement has now been shown by Hildebrand to be erroneous. As the spathes of Arum maculatum are furnished with filaments apparently adapted to prevent the exit of insects, they resemble in this respect the flowers of Aristolochia; and on examining several spathes, from thirty to sixty minute Diptera belonging to three species were found in some of them; and many of these insects were lying dead at the bottom, as if they had been permanently entrapped. In order to discover whether the living ones could escape and carry pollen to another plant, I tied in the spring of 1842 a fine muslin bag tightly round a spathe; and on returning in an hour's time several little flies were crawling about on the inner surface of the bag. I then gathered a spathe and breathed hard into it; several flies soon crawled out, and all without exception were dusted with arum pollen. These flies quickly flew away, and I distinctly saw three of them fly to another plant about a yard off; they alighted on the inner or concave surface of the spathe, and suddenly flew down into the flower. I then opened this flower, and although not a single anther had burst, several grains of pollen were lying at the bottom, which must have been brought from another plant by one of these flies or by some other insect. In another flower little flies were crawling about, and I saw them leave pollen on the stigmas.

I do not know whether Lepidoptera generally keep to the flowers of the same species; but I once observed many minute moths (I believe Lamp.r.o.nia (Tinea) calth.e.l.la) apparently eating the pollen of Mercurialis annua, and they had the whole front of their bodies covered with pollen. I then went to a female plant some yards off, and saw in the course of fifteen minutes three of these moths alight on the stigmas. Lepidoptera are probably often induced to frequent the flowers of the same species, whenever these are provided with a long and narrow nectary, as in this case other insects cannot suck the nectar, which will thus be preserved for those having an elongated proboscis. No doubt the Yucca moth visits only the flowers whence its name is derived, for a most wonderful instinct guides this moth to place pollen on the stigma, so that the ovules may be developed on which the larvae feed. (11/4. Described by Mr. Riley in the 'American Naturalist' volume 7 October 1873.)With respect to Coleoptera, I have seen Meligethes covered with pollen flying from flower to flower of the same species; and this must often occur, as, according to M. Brisout, 'many of the species affect only one kind of plant." (11/5. As quoted in 'American Nat.' May 1873 page 270.)

It must not be supposed from these several statements that insects strictly confine their visits to the same species. They often visit other species when only a few plants of the same kind grow near together. In a flower-garden containing some plants of Oenothera, the pollen of which can easily be recognised, I found not only single grains but ma.s.ses of it within many flowers of Mimulus, Digitalis, Antirrhinum, and Linaria. Other kinds of pollen were likewise detected in these same flowers. A large number of the stigmas of a plant of Thyme, in which the anthers were completely aborted, were examined; and these stigmas, though scarcely larger than a split needle, were covered not only with pollen of Thyme brought from other plants by the bees, but with several other kinds of pollen.

That insects should visit the flowers of the same species as long as they can, is of great importance to the plant, as it favours the cross-fertilisation of distinct individuals of the same species; but no one will suppose that insects act in this manner for the good of the plant. The cause probably lies in insects being thus enabled to work quicker; they have just learnt how to stand in the best position on the flower, and how far and in what direction to insert their proboscides.

(11/6. Since these remarks were written, I find that Hermann Muller has come to almost exactly the same conclusion with respect to the cause of insects frequenting as long as they can the flowers of the same species: 'Bienen Zeitung' July 1876 page 182.) They act on the same principle as does an artificer who has to make half-a-dozen engines, and who saves time by making consecutively each wheel and part for all of them.

Insects, or at least bees, seem much influenced by habit in all their manifold operations; and we shall presently see that this holds good in their felonious practice of biting holes through the corolla.

It is a curious question how bees recognise the flowers of the same species. That the coloured corolla is the chief guide cannot be doubted.

On a fine day, when hive-bees were incessantly visiting the little blue flowers of Lobelia erinus, I cut off all the petals of some, and only the lower striped petals of others, and these flowers were not once again sucked by the bees, although some actually crawled over them. The removal of the two little upper petals alone made no difference in their visits. Mr. J. Anderson likewise states that when he removed the corollas of the Calceolaria, bees never visited the flowers. (11/7.

'Gardeners' Chronicle' 1853 page 534. Kurr cut off the nectaries from a large number of flowers of several species, and found that the greater number yielded seeds; but insects probably would not perceive the loss of the nectary until they had inserted their proboscides into the holes thus formed, and in doing so would fertilise the flowers. He also removed the whole corolla from a considerable number of flowers, and these likewise yielded seeds. Flowers which are self-fertile would naturally produce seeds under these circ.u.mstances; but I am greatly surprised that Delphinium consolida, as well as another species of Delphinium, and Viola tricolor, should have produced a fair supply of seeds when thus treated; but it does not appear that he compared the number of the seeds thus produced with those yielded by unmutilated flowers left to the free access of insects: 'Bedeutung der Nektarien'

1833 pages 123-135.) On the other hand, in some large ma.s.ses of Geranium phaeum which had escaped out of a garden, I observed the unusual fact of the flowers continuing to secrete an abundance of nectar after all the petals had fallen off; and the flowers in this state were still visited by humble-bees. But the bees might have learnt that these flowers with all their petals lost were still worth visiting, by finding nectar in those with only one or two lost. The colour alone of the corolla serves as an approximate guide: thus I watched for some time humble-bees which were visiting exclusively plants of the white-flowered Spiranthes autumnalis, growing on short turf at a considerable distance apart; and these bees often flew within a few inches of several other plants with white flowers, and then without further examination pa.s.sed onwards in search of the Spiranthes. Again, many hive-bees which confined their visits to the common ling (Calluna vulgaris), repeatedly flew towards Erica tetralix, evidently attracted by the nearly similar tint of their flowers, and then instantly pa.s.sed on in search of the Calluna.

That the colour of the flower is not the sole guide, is clearly shown by the six cases above given of bees which repeatedly pa.s.sed in a direct line from one variety to another of the same species, although they bore very differently coloured flowers. I observed also bees flying in a straight line from one clump of a yellow-flowered Oenothera to every other clump of the same plant in the garden, without turning an inch from their course to plants of Eschscholtzia and others with yellow flowers which lay only a foot or two on either side. In these cases the bees knew the position of each plant in the garden perfectly well, as we may infer by the directness of their flight; so that they were guided by experience and memory. But how did they discover at first that the above varieties with differently coloured flowers belonged to the same species? Improbable as it may appear, they seem, at least sometimes, to recognise plants even from a distance by their general aspect, in the same manner as we should do. On three occasions I observed humble-bees flying in a perfectly straight line from a tall larkspur (Delphinium) which was in full flower to another plant of the same species at the distance of fifteen yards which had not as yet a single flower open, and on which the buds showed only a faint tinge of blue. Here neither odour nor the memory of former visits could have come into play, and the tinge of blue was so faint that it could hardly have served as a guide. (11/8.

A fact mentioned by Hermann Muller 'Die Befruchtung' etc. page 347, shows that bees possess acute powers of vision and discrimination; for those engaged in collecting pollen from Primula elatior invariably pa.s.sed by the flowers of the long-styled form, in which the anthers are seated low down in the tubular corolla. Yet the difference in aspect between the long-styled and short-styled forms is extremely slight.)

The conspicuousness of the corolla does not suffice to induce repeated visits from insects, unless nectar is at the same time secreted, together perhaps with some odour emitted. I watched for a fortnight many times daily a wall covered with Linaria cymbalaria in full flower, and never saw a bee even looking at one. There was then a very hot day, and suddenly many bees were industriously at work on the flowers. It appears that a certain degree of heat is necessary for the secretion of nectar; for I observed with Lobelia erinus that if the sun ceased to s.h.i.+ne for only half an hour, the visits of the bees slackened and soon ceased. An a.n.a.logous fact with respect to the sweet excretion from the stipules of Vicia sativa has been already given. As in the case of the Linaria, so with Pedicularis sylvatica, Polygala vulgaris, Viola tricolor, and some species of Trifolium, I have watched the flowers day after day without seeing a bee at work, and then suddenly all the flowers were visited by many bees. Now how did so many bees discover at once that the flowers were secreting nectar? I presume that it must have been by their odour; and that as soon as a few bees began to suck the flowers, others of the same and of different kinds observed the fact and profited by it. We shall presently see, when we treat of the perforation of the corolla, that bees are fully capable of profiting by the labour of other species.

Memory also comes into play, for, as already remarked, bees know the position of each clump of flowers in a garden. I have repeatedly seen them pa.s.sing round a corner, but otherwise in as straight a line as possible, from one plant of Fraxinella and of Linaria to another and distant one of the same species; although, owing to the intervention of other plants, the two were not in sight of each other.

It would appear that either the taste or the odour of the nectar of certain flowers is unattractive to hive or to humble-bees, or to both; for there seems no other reason why certain open flowers which secrete nectar are not visited by them. The small quant.i.ty of nectar secreted by some of these flowers can hardly be the cause of their neglect, as hive-bees search eagerly for the minute drops on the glands on the leaves of the Prunus laurocerasus. Even the bees from different hives sometimes visit different kinds of flowers, as is said to be the case by Mr. Grant with respect to the Polyanthus and Viola tricolor. (11/9.

'Gardeners' Chronicle' 1844 page 374.) I have known humble-bees to visit the flowers of Lobelia fulgens in one garden and not in another at the distance of only a few miles. The cupful of nectar in the labellum of Epipactis latifolia is never touched by hive- or humble-bees, although I have seen them flying close by; and yet the nectar has a pleasant taste to us, and is habitually consumed by the common wasp. As far as I have seen, wasps seek for nectar in this country only from the flowers of this Epipactis, Scrophularia aquatica, Symphoricarpus racemosa (11/10.

The same fact apparently holds good in Italy, for Delpino says that the flowers of these three plants are alone visited by wasps: 'Nettarii Estranuziali, Bulletino Entomologico' anno 6.), and Tritoma; the two former plants being endemic, and the two latter exotic. As wasps are so fond of sugar and of any sweet fluid, and as they do not disdain the minute drops on the glands of Prunus laurocerasus, it is a strange fact that they do not suck the nectar of many open flowers, which they could do without the aid of a proboscis. Hive-bees visit the flowers of the Symphoricarpus and Tritoma, and this makes it all the stranger that they do not visit the flowers of the Epipactis, or, as far as I have seen, those of the Scrophularia aquatica; although they do visit the flowers of Scrophularia nodosa, at least in North America. (11/11. 'Silliman's American Journal of Science' August 1871.)

The extraordinary industry of bees and the number of flowers which they visit within a short time, so that each flower is visited repeatedly, must greatly increase the chance of each receiving pollen from a distinct plant. When the nectar is in any way hidden, bees cannot tell without inserting their proboscides whether it has lately been exhausted by other bees, and this, as remarked in a former chapter, forces them to visit many more flowers than they otherwise would. But they endeavour to lose as little time as they can; thus in flowers having several nectaries, if they find one dry they do not try the others, but as I have often observed, pa.s.s on to another flower. They work so industriously and effectually, that even in the case of social plants, of which hundreds of thousands grow together, as with the several kinds of heath, every single flower is visited, of which evidence will presently be given. They lose no time and fly very quickly from plant to plant, but I do not know the rate at which hive-bees fly. Humble-bees fly at the rate of ten miles an hour, as I was able to ascertain in the case of the males from their curious habit of calling at certain fixed points, which made it easy to measure the time taken in pa.s.sing from one place to another.

With respect to the number of flowers which bees visit in a given time, I observed that in exactly one minute a humble-bee visited twenty-four of the closed flowers of the Linaria cymbalaria; another bee visited in the same time twenty-two flowers of the Symphoricarpus racemosa; and another seventeen flowers on two plants of a Delphinium. In the course of fifteen minutes a single flower on the summit of a plant of Oenothera was visited eight times by several humble-bees, and I followed the last of these bees, whilst it visited in the course of a few additional minutes every plant of the same species in a large flower-garden. In nineteen minutes every flower on a small plant of Nemophila insignis was visited twice. In one minute six flowers of a Campanula were entered by a pollen-collecting hive-bee; and bees when thus employed work slower than when sucking nectar. Lastly, seven flower-stalks on a plant of Dictamnus fraxinella were observed on the 15th of June 1841 during ten minutes; they were visited by thirteen humble-bees each of which entered many flowers. On the 22nd the same flower-stalks were visited within the same time by eleven humble-bees. This plant bore altogether 280 flowers, and from the above data, taking into consideration how late in the evening humble-bees work, each flower must have been visited at least thirty times daily, and the same flower keeps open during several days.