The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 28

Six genera in this family were experimented on, and the results are in some respects remarkable. The crossed plants of the two species of Lupinus were conspicuously superior to the self-fertilised plants in height and fertility; and when grown under very unfavourable conditions, in vigour. The scarlet-runner (Phaseolus multiflorus) is partially sterile if the visits of bees are prevented, and there is reason to believe that varieties growing near one another intercross. The five crossed plants, however, exceeded in height the five self-fertilised only by a little. Phaseolus vulgaris is perfectly self-sterile; nevertheless, varieties growing in the same garden sometimes intercross largely. The varieties of Lathyrus odoratus, on the other hand, appear never to intercross in this country; and though the flowers are not often visited by efficient insects, I cannot account for this fact, more especially as the varieties are believed to intercross in North Italy.

Plants raised from a cross between two varieties, differing only in the colour of their flowers, grew much taller and were under unfavourable conditions more vigorous than the self-fertilised plants; they also transmitted, when self-fertilised, their superiority to their offspring.

The many varieties of the common Pea (Pisum sativum), though growing in close proximity, very seldom intercross; and this seems due to the rarity in this country of the visits of bees sufficiently powerful to effect cross-fertilisation. A cross between the self-fertilised individuals of the same variety does no good whatever to the offspring; whilst a cross between distinct varieties, though closely allied, does great good, of which we have excellent evidence. The flowers of the Broom (Sarothamnus) are almost sterile if they are not disturbed and if insects are excluded. The pollen from a distinct plant is more effective than that from the same flower in producing seeds. The crossed seedlings have an enormous advantage over the self-fertilised when grown together in close compet.i.tion. Lastly, only four plants of the Ononis minutissima were raised; but as these were observed during their whole growth, the advantage of the crossed over the self-fertilised plants may, I think, be fully trusted.

[15. ONAGRACEAE.--Clarkia elegans.

Owing to the season being very unfavourable (1867), few of the flowers which I fertilised formed capsules; twelve crossed flowers produced only four, and eighteen self-fertilised flowers yielded only one capsule. The seeds after germinating on sand were planted in three pots, but all the self-fertilised plants died in one of them. When the two lots were between 4 and 5 inches in height, the crossed began to show a slight superiority over the self-fertilised. When in full flower they were measured, with the following result:--

TABLE 5/62. Clarkia elegans.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 40 4/8 : 33.

Pot 1 : 35 : 24.

Pot 1 : 25 : 23.

Pot 2 : 33 4/8 : 30 4/8.

Total : 134.0 : 110.5.

The average height of the four crossed plants is 33.5, and that of the four self-fertilised plants 27.62 inches, or as 100 to 82. The crossed plants altogether produced 105 and the self-fertilised plants 63 capsules; or as 100 to 60. In both pots a self-fertilised plant flowered before any one of the crossed plants.

16. LOASACEAE.--Bartonia aurea.

Some flowers were crossed and self-fertilised in the usual manner during two seasons; but as I reared on the first occasion only two pairs, the results are given together. On both occasions the crossed capsules contained slightly more seeds than the self-fertilised. During the first year, when the plants were about 7 inches in height, the self-fertilised were the tallest, and in the second year the crossed were the tallest.

When the two lots were in full flower they were measured, as in Table 5/63.

TABLE 5/63. Bartonia aurea.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 31 : 37.

Pot 2 : 18 4/8 : 20 4/8.

Pot 3 : 19 4/8 : 40 4/8.

Pot 4 : 25 : 35.

Pot 4 : 36 : 15 4/8.

Pot 5 : 31 : 18.

Pot 5 : 16 : 11 4/8.

Pot 6 : 20 : 32 4/8.

Total : 197.0 : 210.5.

The average height of the eight crossed plants is 24.62, and that of the eight self-fertilised 26.31 inches; or as 100 to 107. So that the self-fertilised had a decided advantage over the crossed. But the plants from some cause never grew well, and finally became so unhealthy that only three crossed and three self-fertilised plants survived to set any capsules, and these were few in number. The two lots seemed to be about equally unproductive.

17. Pa.s.sIFLORACEAE.--Pa.s.siflora gracilis.

This annual species produces spontaneously numerous fruits when insects are excluded, and behaves in this respect very differently from most of the other species in the genus, which are extremely sterile unless fertilised with pollen from a distinct plant. (5/17. 'Variation of Animals and Plants under Domestication' chapter 17 2nd edition volume 2 page 118.) Fourteen fruits from crossed flowers contained on an average 24.14 seeds. Fourteen fruits (two poor ones being rejected), spontaneously self-fertilised under a net, contained on an average 20.58 seeds per fruit; or as 100 to 85. These seeds were sown on the opposite sides of three pots, but only two pairs came up at the same time; and therefore a fair judgment cannot be formed.

TABLE 5/64. Pa.s.siflora gracilis.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 56 : 38.

Pot 2 : 42 : 64.

Total : 98 : 102.

The mean of the two crossed is 49 inches, and that of the two self-fertilised 51 inches; or as 100 to 104.

18. UMBELLIFERAE.--Apium petroselinum.

The Umbelliferae are proterandrous, and can hardly fail to be cross-fertilised by the many flies and small Hymenoptera which visit the flowers. (5/18. Hermann Muller 'Befruchtung' etc. page 96. According to M. Mustel as stated by G.o.dron 'De l'espece' tome 2 page 58 1859, varieties of the carrot growing near each other readily intercross.) A plant of the common parsley was covered by a net, and it apparently produced as many and as fine spontaneously self-fertilised fruits or seeds as the adjoining uncovered plants. The flowers on the latter were visited by so many insects that they must have received pollen from one another. Some of these two lots of seeds were left on sand, but nearly all the self-fertilised seeds germinated before the others, so that I was forced to throw all away. The remaining seeds were then sown on the opposite sides of four pots. At first the self-fertilised seedlings were a little taller in most of the pots than the naturally crossed seedlings, and this no doubt was due to the self-fertilised seeds having germinated first. But in the autumn all the plants were so equal that it did not seem worth while to measure them. In two of the pots they were absolutely equal; in a third, if there was any difference, it was in favour of the crossed plants, and in a somewhat plainer manner in the fourth pot. But neither side had any substantial advantage over the other; so that in height they may be said to be as 100 to 100.

19. DIPSACEAE.--Scabiosa atro-purpurea.

The flowers, which are proterandrous, were fertilised during the unfavourable season of 1867, so that I got few seeds, especially from the self-fertilised heads, which were extremely sterile. The crossed and self-fertilised plants raised from these seeds were measured before they were in full flower, as in Table 5/65.

TABLE 5/65. Scabiosa atro-purpurea.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.