The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 29

Pot 1 : 14 : 20.

Pot 2 : 15 : 14 4/8.

Pot 3 : 21 : 14.

Pot 3 : 18 4/8 : 13.

Total : 68.5 : 61.5.

The four crossed plants averaged 17.12, and the four self-fertilised 15.37 inches in height; or as 100 to 90. One of the self-fertilised plants in Pot 3 was killed by an accident, and its fellow pulled up; so that when they were again measured to the summits of their flowers, there were only three on each side; the crossed now averaged in height 32.83, and the self-fertilised 30.16 inches; or as 100 to 92.

20. COMPOSITAE.--Lactuca sativa. (5/19. The Compositae are well-adapted for cross-fertilisation, but a nurseryman on whom I can rely, told me that he had been in the habit of sowing several kinds of lettuce near together for the sake of seed, and had never observed that they became crossed. It is very improbable that all the varieties which were thus cultivated near together flowered at different times; but two which I selected by hazard and sowed near each other did not flower at the same time; and my trial failed.)

Three plants of Lettuce (Great London Cos var.) grew close together in my garden; one was covered by a net, and produced self-fertilised seeds, the other two were allowed to be naturally crossed by insects; but the season (1867) was unfavourable, and I did not obtain many seeds. Only one crossed and one self-fertilised plant were raised in Pot 1, and their measurements are given in Table 5/66. The flowers on this one self-fertilised plant were again self-fertilised under a net, not with pollen from the same floret, but from other florets on the same head.

The flowers on the two crossed plants were left to be crossed by insects, but the process was aided by some pollen being occasionally transported by me from plant to plant. These two lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of Pots 2 and 3, which were at first kept in the greenhouse and then turned out of doors. The plants were measured when in full flower. Table 5/66, therefore, includes plants belonging to two generations. When the seedlings of the two lots were only 5 or 6 inches in height they were equal. In Pot 3 one of the self-fertilised plants died before flowering, as has occurred in so many other cases.

TABLE 5/66. Lactuca sativa.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 27 : 21 4/8.

Pot 1 : 25 : 20.

First generation, planted in open ground.

Pot 2 : 29 4/8 : 24.

Pot 2 : 17 4/8 : 10.

Pot 2 : 12 4/8 : 11.

Second generation, planted in open ground.

Pot 3 : 14 : 9 4/8.

Pot 3 : 10 4/8 : 0.

Second generation, kept in the pot.

Total : 136 : 96.

The average height of the seven crossed plants is 19.43, and that of the six self-fertilised plants 16 inches; or as 100 to 82.

21. CAMPANULACEAE.--Specularia speculum.

In the closely allied genus, Campanula, in which Specularia was formerly included, the anthers shed at an early period their pollen, and this adheres to the collecting hairs which surround the pistil beneath the stigma; so that without some mechanical aid the flowers cannot be fertilised. For instance, I covered up a plant of Campanula carpathica, and it did not produce a single capsule, whilst the surrounding uncovered plants seeded profusely. On the other hand, the present species of Specularia appears to set almost as many capsules when covered up, as when left to the visits of the Diptera, which, as far as I have seen, are the only insects that frequent the flowers. (5/20. It has long been known that another species of the genus, Specularia perfoliata, produces cleistogene as well as perfect flowers, and the former are of course self-fertile.) I did not ascertain whether the naturally crossed and spontaneously self-fertilised capsules contained an equal number of seeds, but a comparison of artificially crossed and self-fertilised flowers, showed that the former were probably the most productive. It appears that this plant is capable of producing a large number of self-fertilised capsules owing to the petals closing at night, as well as during cold weather. In the act of closing, the margins of the petals become reflexed, and their inwardly projecting midribs then pa.s.s between the clefts of the stigma, and in doing so push the pollen from the outside of the pistil on to the stigmatic surfaces. (5/21. Mr.

Meehan has lately shown 'Proceedings of the Academy of Natural Science Philadelphia' May 16, 1876 page 84, that the closing of the flowers of Claytonia virginica and Ranunculus bulbosus during the night causes their self-fertilisation.)

Twenty flowers were fertilised by me with their own pollen, but owing to the bad season, only six capsules were produced; they contained on an average 21.7 seeds, with a maximum of forty-eight in one. Fourteen flowers were crossed with pollen from another plant, and these produced twelve capsules, containing on an average 30 seeds, with a maximum in one of fifty-seven seeds; so that the crossed seeds were to the self-fertilised from an equal number of capsules as 100 to 72. The former were also heavier than an equal number of self-fertilised seeds, in the ratio of 100 to 86. Thus, whether we judge by the number of capsules produced from an equal number of flowers, or by the average number of the contained seeds, or the maximum number in any one capsule, or by their weight, crossing does great good in comparison with self-fertilisation. The two lots of seeds were sown on the opposite sides of four pots; but the seedlings were not sufficiently thinned.

Only the tallest plant on each side was measured, when fully grown. The measurements are given in Table 5/67. In all four pots the crossed plants flowered first. When the seedlings were only about an inch and a half in height both lots were equal.

TABLE 5/67. Specularia speculum.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Tallest Crossed Plant in each Pot.

Column 3: Tallest Self-fertilised Plant in each Pot.

Pot 1 : 18 : 15 6/8.

Pot 2 : 17 : 19.

Pot 3 : 22 1/8 : 18.

Pot 4 : 20 : 23.

Total : 77.13 : 75.75.

The four tallest crossed plants averaged 19.28, and the four tallest self-fertilised 18.93 inches in height; or as 100 to 98. So that there was no difference worth speaking of between the two lots in height; though other great advantages are derived, as we have seen, from cross-fertilisation. From being grown in pots and kept in the greenhouse, none of the plants produced any capsules.

Lobelia ramosa. (5/22. I have adopted the name given to this plant in the 'Gardeners' Chronicle' 1866. Professor T. Dyer, however, informs me that it probably is a white variety of L. tenuior of R. Brown, from W.

Australia.)

VAR. SNOW-FLAKE.

The well-adapted means by which cross-fertilisation is ensured in this genus have been described by several authors. (5/23. See the works of Hildebrand and Delpino. Mr. Farrer also has given a remarkably clear description of the mechanism by which cross-fertilisation is effected in this genus, in the 'Annals and Magazine of Natural History' volume 2 4th series 1868 page 260. In the allied genus Isotoma, the curious spike which projects rectangularly from the anthers, and which when shaken causes the pollen to fall on the back of an entering insect, seems to have been developed from a bristle, like one of those which spring from the anthers in some of or all the species of Lobelia, as described by Mr. Farrer.) The pistil as it slowly increases in length pushes the pollen out of the conjoined anthers, by the aid of a ring of bristles; the two lobes of the stigma being at this time closed and incapable of fertilisation. The extrusion of the pollen is also aided by insects, which rub against the little bristles that project from the anthers. The pollen thus pushed out is carried by insects to the older flowers, in which the stigma of the now freely projecting pistil is open and ready to be fertilised. I proved the importance of the gaily-coloured corolla, by cutting off the large flowers of Lobelia erinus; and these flowers were neglected by the hive-bees which were incessantly visiting the other flowers.

A capsule was obtained by crossing a flower of L. ramosa with pollen from another plant, and two other capsules from artificially self-fertilised flowers. The contained seeds were sown on the opposite sides of four pots. Some of the crossed seedlings which came up before the others had to be pulled up and thrown away. Whilst the plants were very small there was not much difference in height between the two lots; but in Pot 3 the self-fertilised were for a time the tallest. When in full flower the tallest plant on each side of each pot was measured, and the result is shown in Table 5/68. In all four pots a crossed plant flowered before any one of its opponents.

TABLE 5/68. Lobelia ramosa (First Generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Tallest Crossed Plant in each Pot.

Column 3: Tallest Self-fertilised Plant in each Pot.

Pot 1 : 22 4/8 : 17 4/8.

Pot 2 : 27 4/8 : 24.

Pot 3 : 16 4/8 : 15.

Pot 4 : 22 4/8 : 17.