The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 53

This table includes thirty-three cases relating to twenty-three species, and shows the degree of innate fertility of plants of crossed parentage in comparison with those of self-fertilised parentage; both lots being fertilised in the same manner. With several of the species, as with Eschscholtzia, Reseda, Viola, Dianthus, Petunia, and Primula, both lots were certainly cross-fertilised by insects, and so it probably was with several of the others; but in some of the species, as with Nemophila, and in some of the trials with Ipomoea and Dianthus, the plants were covered up, and both lots were spontaneously self-fertilised. This also was necessarily the case with the capsules produced by the cleistogene flowers of Vandellia.

The fertility of the crossed plants is represented in Table 9/D by 100, and that of the self-fertilised by the other figures. There are five cases in which the fertility of the self-fertilised plants is approximately equal to that of the crossed; nevertheless, in four of these cases the crossed plants were plainly taller, and in the fifth somewhat taller than the self-fertilised. But I should state that in some of these five cases the fertility of the two lots was not strictly ascertained, as the capsules were not actually counted, from appearing equal in number and from all apparently containing a full complement of seeds. In only two instances in the table, namely, with Vandellia and in the third generation of Dianthus, the capsules on the self-fertilised plants contained more seed than those on the crossed plants. With Dianthus the ratio between the number of seeds contained in the self-fertilised and crossed capsules was as 125 to 100; both sets of plants were left to fertilise themselves under a net; and it is almost certain that the greater fertility of the self-fertilised plants was here due merely to their having varied and become less strictly dichogamous, so as to mature their anthers and stigmas more nearly at the same time than is proper to the species. Excluding the seven cases now referred to, there remain twenty-six in which the crossed plants were manifestly much more fertile, sometimes to an extraordinary degree, than the self-fertilised with which they grew in compet.i.tion. The most striking instances are those in which plants derived from a cross with a fresh stock are compared with plants of one of the later self-fertilised generations; yet there are some striking cases, as that of Viola, between the intercrossed plants of the same stock and the self-fertilised, even in the first generation. The results most to be trusted are those in which the productiveness of the plants was ascertained by the number of capsules produced by an equal number of plants, together with the actual or average number of seeds in each capsule. Of such cases there are twelve in the table, and the mean of their mean fertility is as 100 for the crossed plants, to 59 for the self-fertilised plants. The Primulaceae seem eminently liable to suffer in fertility from self-fertilisation.

The following short table, Table 9/E, includes four cases which have already been partly given in the last table.

TABLE 9/E.--INNATE FERTILITY OF PLANTS FROM A CROSS WITH A FRESH STOCK, COMPARED WITH THAT OF INTERCROSSED PLANTS OF THE SAME STOCK, AND WITH THAT OF SELF-FERTILISED PLANTS, ALL OF THE CORRESPONDING GENERATION.

FERTILITY JUDGED OF BY THE NUMBER OR WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS.

Column 1: Name of plant and feature observed.

Column 2: Plants from a cross with a fresh stock.

Column 3: Intercrossed plants of the same stock.

Column 4: Self-fertilised plants.

Mimulus luteus--the intercrossed plants are derived from a cross between two plants of the 8th self-fertilised generation. The self-fertilised plants belong to the 9th generation: 100 : 4 : 3.

Eschscholtzia californica--the intercrossed and self-fertilised plants belong to the 2nd generation: 100 : 45 : 40.

Dianthus caryophyllus--the intercrossed plants are derived from self-fertilised of the 3rd generation, crossed by intercrossed plants of the 3rd generation. The self-fertilised plants belong to the 4th generation: 100 : 45 : 33.

Petunia violacea--the intercrossed and self-fertilised plants belong to the 5th generation: 100 : 54 : 46.

NB.--In the above cases, excepting in that of Eschscholtzia, the plants derived from a cross with a fresh stock belong on the mother-side to the same stock with the intercrossed and self-fertilised plants, and to the corresponding generation.

These cases show us how greatly superior in innate fertility the seedlings from plants self-fertilised or intercrossed for several generations and then crossed by a fresh stock are, in comparison with the seedlings from plants of the old stock, either intercrossed or self-fertilised for the same number of generations. The three lots of plants in each case were left freely exposed to the visits of insects, and their flowers without doubt were cross-fertilised by them.

Table 9/E further shows us that in all four cases the intercrossed plants of the same stock still have a decided though small advantage in fertility over the self-fertilised plants.

With respect to the state of the reproductive organs in the self-fertilised plants of Tables 9/D and 9/E, only a few observations were made. In the seventh and eighth generation of Ipomoea, the anthers in the flowers of the self-fertilised plants were plainly smaller than those in the flowers of the intercrossed plants. The tendency to sterility in these same plants was also shown by the first-formed flowers, after they had been carefully fertilised, often dropping off, in the same manner as frequently occurs with hybrids. The flowers likewise tended to be monstrous. In the fourth generation of Petunia, the pollen produced by the self-fertilised and intercrossed plants was compared, and they were far more empty and shrivelled grains in the former.

RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE FIRST OR A SUCCEEDING GENERATION.

I will first treat of the parent-plants, which were raised from seeds purchased from nursery-gardens, or taken from plants growing in my garden, or growing wild, and surrounded in every case by many individuals of the same species. Plants thus circ.u.mstanced will commonly have been intercrossed by insects; so that the seedlings which were first experimented on will generally have been the product of a cross.

Consequently any difference in the fertility of their flowers, when crossed and self-fertilised, will have been caused by the nature of the pollen employed; that is, whether it was taken from a distinct plant or from the same flower. The degrees of fertility shown in Table 9/F, were determined in each case by the average number of seeds per capsule, ascertained either by counting or weighing.

Another element ought properly to have been taken into account, namely, the proportion of flowers which yielded capsules when they were crossed and self-fertilised; and as crossed flowers generally produce a larger proportion of capsules, their superiority in fertility, if this element had been taken into account, would have been much more strongly marked than appears in Table 9/F. But had I thus acted, there would have been greater liability to error, as pollen applied to the stigma at the wrong time fails to produce any effect, independently of its greater or less potency. A good ill.u.s.tration of the great difference in the results which sometimes follows, if the number of capsules produced relatively to the number of flowers fertilised be included in the calculation, was afforded by Nolana prostrata. Thirty flowers on some plants of this species were crossed and produced twenty-seven capsules, each containing five seeds; thirty-two flowers on the same plants were self-fertilised and produced only six capsules, each containing five seeds. As the number of seeds per capsule is here the same, the fertility of the crossed and self-fertilised flowers is given in Table 9/F as equal, or as 100 to 100. But if the flowers which failed to produce capsules be included, the crossed flowers yielded on an average 4.50 seeds, whilst the self-fertilised flowers yielded only 0.94 seeds, so that their relative fertility would have been as 100 to 21. I should here state that it has been found convenient to reserve for separate discussion the cases of flowers which are usually quite sterile with their own pollen.

TABLE 9/f.--relative fertility of the flowers on the parent-plants used in my experiments, when fertilised with pollen from a distinct plant and with their own pollen. Fertility judged of by the average number of seeds per capsule. Fertility of crossed flowers taken as 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression 100 to x.

Ipomoea purpurea--crossed and self-fertilised flowers yielded seeds as (about): 100.

Mimulus luteus--crossed and self-fertilised flowers yielded seeds as (by weight): 79.

Linaria vulgaris--crossed and self-fertilised flowers yielded seeds as: 14.

Vandellia nummularifolia--crossed and self-fertilised flowers yielded seeds as: 67?

Gesneria pendulina--crossed and self-fertilised flowers yielded seeds as (by weight): 100.

Salvia coccinea--crossed and self-fertilised flowers yielded seeds as (about): 100.

Bra.s.sica oleracea--crossed and self-fertilised flowers yielded seeds as: 25.

Eschscholtzia californica--(English stock) crossed and self-fertilised flowers yielded seeds as (by weight): 71.

Eschscholtzia californica--(Brazilian stock grown in England) crossed and self-fertilised flowers yielded seeds (by weight) as (about): 15.

Delphinium consolida--crossed and self-fertilised flowers (self-fertilised capsules spontaneously produced, but result supported by other evidence) yielded seeds as: 59.

Viscaria oculata--crossed and self-fertilised flowers yielded seeds as (by weight): 38.

Viscaria oculata--crossed and self-fertilised flowers (crossed capsules compared on following year with spontaneously self-fertilised capsules) yielded seeds as : 58.

Dianthus caryophyllus--crossed and self-fertilised flowers yielded seeds as: 92.

Tropaeolum minus--crossed and self-fertilised flowers yielded seeds as: 92.

Tropaeolum tricolorum--crossed and self-fertilised flowers yielded seeds as: 115. (9/1. Tropaeolum tricolorum and Cuphea purpurea have been introduced into this table, although seedlings were not raised from them; but of the Cuphea only six crossed and six self-fertilised capsules, and of the Tropaeolum only six crossed and eleven self-fertilised capsules, were compared. A larger proportion of the self-fertilised than of the crossed flowers of the Tropaeolum produced fruit.)

Limnanthes douglasii--crossed and self-fertilised flowers yielded seeds as (about): 100.

Sarothamnus scoparius--crossed and self-fertilised flowers yielded seeds as: 41.

Ononis minutissima--crossed and self-fertilised flowers yielded seeds as: 65.

Cuphea purpurea--crossed and self-fertilised flowers yielded seeds as: 113.

Pa.s.siflora gracilis--crossed and self-fertilised flowers yielded seeds as: 85.

Specularia speculum--crossed and self-fertilised flowers yielded seeds as: 72.

Lobelia fulgens--crossed and self-fertilised flowers yielded seeds as (about): 100.

Nemophila insignis--crossed and self-fertilised flowers yielded seeds as (by weight): 69.

Borago officinalis--crossed and self-fertilised flowers yielded seeds as: 60.

Nolana prostrata--crossed and self-fertilised flowers yielded seeds as: 100.

Petunia violacea--crossed and self-fertilised flowers yielded seeds as (by weight): 67.

Nicotiana tabac.u.m--crossed and self-fertilised flowers yielded seeds as (by weight): 150.