The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 25

A few flowers were crossed with pollen from a distinct plant, but owing to the unfavourable season only two crossed seeds were produced. Nine seeds were saved from flowers spontaneously self-fertilised under a net, on the same plant which yielded the two crossed seeds. One of these crossed seeds was sown in a pot with two self-fertilised seeds on the opposite side; the latter came up between two and three days before the crossed seed. The second crossed seed was sown in like manner with two self-fertilised seeds on the opposite side; these latter also came up about a day before the crossed one. In both pots, therefore, the crossed seedlings from germinating later, were at first completely beaten by the self-fertilised; nevertheless, this state of things was afterwards completely reversed. The seeds were sown late in the autumn, and the pots, which were much too small, were kept in the greenhouse. The plants in consequence grew badly, and the self-fertilised suffered most in both pots. The two crossed plants when in flower during the following spring were 9 inches in height; one of the self-fertilised plants was 8, and the three others only 3 inches in height, being thus mere dwarfs. The two crossed plants produced thirteen pods, whilst the four self-fertilised plants produced only a single one. Some other self-fertilised plants which had been raised separately in larger pots produced several spontaneously self-fertilised pods under a net, and seeds from these were used in the following experiment.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The spontaneously self-fertilised seeds just mentioned, and crossed seeds obtained by intercrossing the two crossed plants of the last generation, after germinating on sand, were planted in pairs on the opposite sides of three large pots. When the seedlings were only 4 inches in height, the crossed had a slight advantage over their opponents. When grown to their full height, every one of the crossed plants exceeded its opponent in height. Nevertheless the self-fertilised plants in all three pots flowered before the crossed! The measurements are given in Table 5/52.

TABLE 5/52. Lupinus luteus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 33 2/8 : 24 4/8.

Pot 1 : 30 4/8 : 18 4/8.

Pot 1 : 30 : 28.

Pot 2 : 29 4/8 : 26.

Pot 2 : 30 : 25.

Pot 3 : 30 4/8 : 28.

Pot 3 : 31 : 27 2/8.

Pot 3 : 31 4/8 : 24 4/8.

Total : 246.25 : 201.75.

The eight crossed plants here average 30.78, and the eight self-fertilised 25.21 inches in height; or as 100 to 82. These plants were left uncovered in the greenhouse to set their pods, but they produced very few good ones, perhaps in part owing to few bees visiting them. The crossed plants produced nine pods, containing on an average 3.4 seeds, and the self-fertilised plants seven pods, containing on an average 3 seeds, so that the seeds from an equal number of plants were as 100 to 88.

Two other crossed seedlings, each with two self-fertilised seedlings on the opposite sides of the same large pot, were turned out of their pots early in the season, without being disturbed, into open ground of good quality. They were thus subjected to but little compet.i.tion with one another, in comparison with the plants in the above three pots. In the autumn the two crossed plants were about 3 inches taller than the four self-fertilised plants; they looked also more vigorous and produced many more pods.

Two other crossed and self-fertilised seeds of the same lot, after germinating on sand, were planted on the opposite sides of a large pot, in which a Calceolaria had long been growing, and were therefore exposed to unfavourable conditions: the two crossed plants ultimately attained a height of 20 1/2 and 20 inches, whilst the two self-fertilised were only 18 and 9 1/2 inches high.

Lupinus pilosus.

From a series of accidents I was again unfortunate in obtaining a sufficient number of crossed seedlings; and the following results would not be worth giving, did they not strictly accord with those just given with respect to Lupinus luteus. I raised at first only a single crossed seedling, which was placed in compet.i.tion with two self-fertilised ones on the opposite side of the same pot. These plants, without being disturbed, were soon afterwards turned into the open ground. By the autumn the crossed plant had grown to so large a size that it almost smothered the two self-fertilised plants, which were mere dwarfs; and the latter died without maturing a single pod. Several self-fertilised seeds had been planted at the same time separately in the open ground; and the two tallest of these were 33 and 32 inches, whereas the one crossed plant was 38 inches in height. This latter plant also produced many more pods than did any one of the self-fertilised plants, although growing separately. A few flowers on the one crossed plant were crossed with pollen from one of the self-fertilised plants, for I had no other crossed plant from which to obtain pollen. One of the self-fertilised plants having been covered by a net produced plenty of spontaneously self-fertilised pods.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

From crossed and self-fertilised seeds obtained in the manner just described, I succeeded in raising to maturity only a pair of plants, which were kept in a pot in the greenhouse. The crossed plant grew to a height of 33 inches, and the self-fertilised to that of 26 1/2 inches.

The former produced, whilst still kept in the greenhouse, eight pods, containing on an average 2.77 seeds; and the latter only two pods, containing on an average 2.5 seeds. The average height of the two crossed plants of the two generations taken together was 35.5, and that of the three self-fertilised plants of the same two generations 30.5; or as 100 to 86. (5/3. We here see that both Lupinus luteus and pilosus seed freely when insects are excluded; but Mr. Swale, of Christchurch, in New Zealand, informs me 'Gardeners' Chronicle' 1858 page 828, that the garden varieties of the lupine are not there visited by any bees, and that they seed less freely than any other introduced leguminous plant, with the exception of red clover. He adds "I have, for amus.e.m.e.nt, during the summer, released the stamens with a pin, and a pod of seed has always rewarded me for my trouble, the adjoining flowers not so served having all proved blind." I do not know to what species this statement refers.)

Phaseolus multiflorus.

This plant, the scarlet-runner of English gardeners and the Phaseolus coccineus of Lamarck, originally came from Mexico, as I am informed by Mr. Bentham. The flowers are so constructed that hive and humble-bees, which visit them incessantly, almost always alight on the left wing-petal, as they can best suck the nectar from this side. Their weight and movements depress the petal, and this causes the stigma to protrude from the spirally-wound keel, and a brush of hairs round the stigma pushes out the pollen before it. The pollen adheres to the head or proboscis of the bee which is at work, and is thus placed either on the stigma of the same flower, or is carried to another flower. (5/4.

The flowers have been described by Delpino, and in an admirable manner by Mr. Farrer in the 'Annals and Magazine of Natural History' volume 2 4th series October 1868 page 256. My son Francis has explained 'Nature'

January 8, 1874 page 189, the use of one peculiarity in their structure, namely, a little vertical projection on the single free stamen near its base, which seems placed as if to guard the entrance into the two nectar-holes in the staminal sheath. He shows that this projection prevents the bees reaching the nectar, unless they go to the left side of the flower, and it is absolutely necessary for cross-fertilisation that they should alight on the left wing-petal.) Several years ago I covered some plants under a large net, and these produced on one occasion about one-third, and on another occasion about one-eighth, of the number of pods which the same number of uncovered plants growing close alongside produced. (5/5. 'Gardeners' Chronicle' 1857 page 725 and more especially ibid 1858 page 828. Also 'Annals and Magazine of Natural History' 3rd series volume 2 1858 page 462.) This lessened fertility was not caused by any injury from the net, as I moved the wing-petals of several protected flowers, in the same manner as bees do, and these produced remarkably fine pods. When the net was taken off, the flowers were immediately visited by bees, and it was interesting to observe how quickly the plants became covered with young pods. As the flowers are much frequented by Thrips, the self-fertilisation of most of the flowers under the net may have been due to the action of these minute insects.

Dr. Ogle likewise covered up a large portion of a plant, and "out of a vast number of blossoms thus protected not a single one produced a pod, while the unprotected blossoms were for the most part fruitful." Mr.

Belt gives a more curious case; this plant grows well and flowers in Nicaragua; but as none of the native bees visit the flowers, not a single pod is ever produced. (5/6. Dr. Ogle 'Popular Science Review'

1870 page 168. Mr. Belt 'The Naturalist in Nicaragua' 1874 page 70. The latter author gives a case 'Nature' 1875 page 26, of a late crop of Phaseolus multiflorus near London which "was rendered barren" by the humble-bees cutting, as they frequently do, holes at the bases of the flowers instead of entering them in the proper manner.)

From the facts now given we may feel nearly sure that individuals of the same variety or of different varieties, if growing near each other and in flower at the same time, would intercross; but I cannot myself advance any direct evidence of such an occurrence, as only a single variety is commonly cultivated in England. I have, however, received an account from the Reverend W.A. Leighton, that plants raised by him from ordinary seed produced seeds differing in an extraordinary manner in colour and shape, leading to the belief that their parents must have been crossed. In France M. Fermond more than once planted close together varieties which ordinarily come true and which bear differently coloured flowers and seeds; and the offspring thus raised varied so greatly that there could hardly be a doubt that they had intercrossed. (5/7.

'Fecondation chez les Vegetaux' 1859 pages 34-40. He adds that M.

Villiers has described a spontaneous hybrid, which he calls Phaseolus coccineus hybridus, in the 'Annales de la Soc. R. de Horticulture' June 1844.) On the other hand, Professor H. Hoffman does not believe in the natural crossing of the varieties; for although seedlings raised from two varieties growing close together produced plants which yielded seeds of a mixed character, he found that this likewise occurred with plants separated by a s.p.a.ce of from 40 to 150 paces from any other variety; he therefore attributes the mixed character of the seed to spontaneous variability. (5/8. 'Bestimmung des Werthes von Species und Varietat'

1869 pages 47-72.) But the above distance would be very far from sufficient to prevent intercrossing: cabbages have been known to cross at several times this distance; and the careful Gartner gives many instances of plants growing at from 600 to 800 yards apart fertilising one another. (5/9. 'Kenntnis der Befruchtung' 1844 pages 573, 577.) Professor Hoffman even maintains that the flowers of the kidney-bean are specially adapted for self-fertilisation. He enclosed several flowers in bags; and as the buds often dropped off, he attributes the partial sterility of these flowers to the injurious effects of the bags, and not to the exclusion of insects. But the only safe method of experimenting is to cover up a whole plant, which then never suffers.

Self-fertilised seeds were obtained by moving up and down in the same manner as bees do the wing-petals of flowers protected by a net; and crossed seeds were obtained by crossing two of the plants under the same net. The seeds after germinating on sand were planted on the opposite sides of two large pots, and equal-sized sticks were given them to twine up. When 8 inches in height, the plants on the two sides were equal. The crossed plants flowered before the self-fertilised in both pots. As soon as one of each pair had grown to the summit of its stick both were measured.

TABLE 5/53. Phaseolus multiflorus.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 87 : 84 6/8.

Pot 1 : 88 : 87.

Pot 1 : 82 4/8 : 76.

Pot 2 : 90 : 76 4/8.

Pot 2 : 82 4/8 : 87 4/8.

Total : 430.00 : 411.75.

The average height of the five crossed plants is 86 inches, and that of the five self-fertilised plants 82.35; or as 100 to 96. The pots were kept in the greenhouse, and there was little or no difference in the fertility of the two lots. Therefore as far as these few observations serve, the advantage gained by a cross is very small.

Phaseolus vulgaris.

With respect to this species, I merely ascertained that the flowers were highly fertile when insects were excluded, as indeed must be the case, for the plants are often forced during the winter when no insects are present. Some plants of two varieties (namely Canterbury and Fulmer's Forcing Bean) were covered with a net, and they seemed to produce as many pods, containing as many beans, as some uncovered plants growing alongside; but neither the pods nor the beans were actually counted.

This difference in self-fertility between Phaseolus vulgaris and multifloris is remarkable, as these two species are so closely related that Linnaeus thought that they formed one. When the varieties of Phaseolus vulgaris grow near one another in the open ground, they sometimes cross largely, notwithstanding their capacity for self-fertilisation. Mr. Coe has given me a remarkable instance of this fact with respect to the negro and a white-seeded and a brown-seeded variety, which were all grown together. The diversity of character in the seedlings of the second generation raised by me from his plants was wonderful. I could add other a.n.a.logous cases, and the fact is well-known to gardeners. (5/10. I have given Mr. Coe's case in the 'Gardeners'

Chronicle' 1858 page 829. See also for another case ibid page 845.)

Lathyrus odoratus.

Almost everyone who has studied the structure of papilionaceous flowers has been convinced that they are specially adapted for cross-fertilisation, although many of the species are likewise capable of self-fertilisation. The case therefore of Lathyrus odoratus or the sweet-pea is curious, for in this country it seems invariably to fertilise itself. I conclude that this is so, as five varieties, differing greatly in the colour of their flowers but in no other respect, are commonly sold and come true; yet on inquiry from two great raisers of seed for sale, I find that they take no precautions to insure purity--the five varieties being habitually grown close together. (5/11.

See Mr. W. Earley in 'Nature' 1872 page 242, to the same effect. He once, however, saw bees visiting the flowers, and supposed that on this occasion they would have been intercrossed.) I have myself purposely made similar trials with the same result. Although the varieties always come true, yet, as we shall presently see, one of the five well-known varieties occasionally gives birth to another, which exhibits all its usual characters. Owing to this curious fact, and to the darker-coloured varieties being the most productive, these increase, to the exclusion of the others, as I was informed by the late Mr. Masters, if there be no selection.

In order to ascertain what would be the effect of crossing two varieties, some flowers on the Purple sweet-pea, which has a dark reddish-purple standard-petal with violet-coloured wing-petals and keel, were castrated whilst very young, and were fertilised with pollen of the Painted Lady. This latter variety has a pale cherry-coloured standard, with almost white wings and keel. On two occasions I raised from a flower thus crossed plants perfectly resembling both parent-forms; but the greater number resembled the paternal variety. So perfect was the resemblance, that I should have suspected some mistake in the label, had not the plants, which were at first identical in appearance with the father or Painted Lady, later in the season produced flowers blotched and streaked with dark purple. This is an interesting example of partial reversion in the same individual plant as it grows older. The purple-flowered plants were thrown away, as they might possibly have been the product of the accidental self-fertilisation of the mother-plant, owing to the castration not having been effectual. But the plants which resembled in the colour of their flowers the paternal variety or Painted Lady were preserved, and their seeds saved. Next summer many plants were raised from these seeds, and they generally resembled their grandfather the Painted Lady, but most of them had their wing-petals streaked and stained with dark pink; and a few had pale purple wings with the standard of a darker crimson than is natural to the Painted Lady, so that they formed a new sub-variety. Amongst these plants a single one appeared having purple flowers like those of the grandmother, but with the petals slightly streaked with a paler tint: this was thrown away. Seeds were again saved from the foregoing plants, and the seedlings thus raised still resembled the Painted Lady, or great-grandfather; but they now varied much, the standard petal varying from pale to dark red, in a few instances with blotches of white; and the wing-petals varied from nearly white to purple, the keel being in all nearly white.

As no variability of this kind can be detected in plants raised from seeds, the parents of which have grown during many successive generations in close proximity, we may infer that they cannot have intercrossed. What does occasionally occur is that in a row of plants raised from seeds of one variety, another variety true of its kind appears; for instance, in a long row of Scarlets (the seeds of which had been carefully gathered from Scarlets for the sake of this experiment) two Purples and one Painted Lady appeared. Seeds from these three aberrant plants were saved and sown in separate beds. The seedlings from both the Purples were chiefly Purples, but with some Painted Ladies and some Scarlets. The seedlings from the aberrant Painted Lady were chiefly Painted Ladies with some Scarlets. Each variety, whatever its parentage may have been, retained all its characters perfect, and there was no streaking or blotching of the colours, as in the foregoing plants of crossed origin. Another variety, however, is often sold, which is striped and blotched with dark purple; and this is probably of crossed origin, for I found, as well as Mr. Masters, that it did not transmit its characters at all truly.

From the evidence now given, we may conclude that the varieties of the sweet-pea rarely or never intercross in this country; and this is a highly remarkable fact, considering, firstly, the general structure of the flowers; secondly, the large quant.i.ty of pollen produced, far more than is requisite for self-fertilisation; and thirdly, the occasional visit of insects. That insects should sometimes fail to cross-fertilise the flowers is intelligible, for I have thrice seen humble-bees of two kinds, as well as hive-bees, sucking the nectar, and they did not depress the keel-petals so as to expose the anthers and stigma; they were therefore quite inefficient for fertilising the flowers. One of these bees, namely, Bombus lapidarius, stood on one side at the base of the standard and inserted its proboscis beneath the single separate stamen, as I afterwards ascertained by opening the flower and finding this stamen prised up. Bees are forced to act in this manner from the slit in the staminal tube being closely covered by the broad membranous margin of the single stamen, and from the tube not being perforated by nectar-pa.s.sages. On the other hand, in the three British species of Lathyrus which I have examined, and in the allied genus Vicia, two nectar-pa.s.sages are present. Therefore British bees might well be puzzled how to act in the case of the sweet-pea. I may add that the staminal tube of another exotic species, Lathyrus grandiflorus, is not perforated by nectar-pa.s.sages, and this species has rarely set any pods in my garden, unless the wing-petals were moved up and down, in the same manner as bees ought to do; and then pods were generally formed, but from some cause often dropped off afterwards. One of my sons caught an elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but this insect would not depress the wing-petals and keel. On the other hand, I have seen on one occasion hive-bees, and two or three occasions the Megachile willughbiella in the act of depressing the keel; and these bees had the under sides of their bodies thickly covered with pollen, and could not thus fail to carry pollen from one flower to the stigma of another. Why then do not the varieties occasionally intercross, though this would not often happen, as insects so rarely act in an efficient manner? The fact cannot, as it appears, be explained by the flowers being self-fertilised at a very early age; for although nectar is sometimes secreted and pollen adheres to the viscid stigma before the flowers are fully expanded, yet in five young flowers which were examined by me the pollen-tubes were not exserted. Whatever the cause may be, we may conclude, that in England the varieties never or very rarely intercross. But it does not follow from this, that they would not be cross by the aid of other and larger insects in their native country, which in botanical works is said to be the south of Europe and the East Indies. Accordingly I wrote to Professor Delpino, in Florence, and he informs me "that it is the fixed opinion of gardeners there that the varieties do intercross, and that they cannot be preserved pure unless they are sown separately."

It follows also from the foregoing facts that the several varieties of the sweet-pea must have propagated themselves in England by self-fertilisation for very many generations, since the time when each new variety first appeared. From the a.n.a.logy of the plants of Mimulus and Ipomoea, which had been self-fertilised for several generations, and from trials previously made with the common pea, which is in nearly the same state as the sweet-pea, it appeared to me very improbable that a cross between the individuals of the same variety would benefit the offspring. A cross of this kind was therefore not tried, which I now regret. But some flowers of the Painted Lady, castrated at an early age, were fertilised with pollen from the Purple sweet-pea; and it should be remembered that these varieties differ in nothing except in the colour of their flowers. The cross was manifestly effectual (though only two seeds were obtained), as was shown by the two seedlings, when they flowered, closely resembling their father, the Purple pea, excepting that they were a little lighter coloured, with their keels slightly streaked with pale purple. Seeds from flowers spontaneously self-fertilised under a net were at the same time saved from the same mother-plant, the Painted Lady. These seeds unfortunately did not germinate on sand at the same time with the crossed seeds, so that they could not be planted simultaneously. One of the two crossed seeds in a state of germination was planted in a pot (Number 1) in which a self-fertilised seed in the same state had been planted four days before, so that this latter seedling had a great advantage over the crossed one. In Pot 2 the other crossed seed was planted two days before a self-fertilised one; so that here the crossed seedling had a considerable advantage over the self-fertilised one. But this crossed seedling had its summit gnawed off by a slug, and was in consequence for a time quite beaten by the self-fertilised plant. Nevertheless I allowed it to remain, and so great was its const.i.tutional vigour that it ultimately beat its uninjured self-fertilised rival. When all four plants were almost fully grown they were measured, as here shown:--

TABLE 5/54. Lathyrus odoratus.