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 pass 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.