Chapter 9 - Page 2

to which it is exposed. The number of the capsules produced and the number of the contained seeds, ought to have been ascertained on a large number of crossed and self-fertilised plants of the same age and treated in every respect alike. In these two latter respects my observations may be trusted, but a sufficient number of capsules were counted only in a few instances. The fertility, or as it may perhaps better be called the productiveness, of a plant depends on the number of capsules produced, and on the number of seeds which these contain. But from various causes, chiefly from the want of time, I was often compelled to rely on the number of the capsules alone. Nevertheless, in the more interesting cases, the seeds were also counted or weighed. The average number of seeds per capsule is a more valuable criterion of fertility than the number of capsules produced. This latter circumstance depends partly on the size of the plant; and we know that crossed plants are generally taller and heavier than the self-fertilised; but the difference in this respect is rarely sufficient to account for the difference in the number of the capsules produced. It need hardly be added that in Table 9/D the same number of crossed and self-fertilised plants are always compared. Subject to the foregoing sources of doubt I will now give the table, in which the parentage of the plants experimented on, and the manner of determining their fertility are explained. Fuller details may be found in the previous part of this work, under the head of each species.

TABLE 9/D.--RELATIVE FERTILITY OF PLANTS OF CROSSED AND SELF-FERTILISED PARENTAGE, BOTH SETS BEING FERTILISED IN THE SAME MANNER. FERTILITY JUDGED OF BY VARIOUS STANDARDS. THAT OF THE CROSSED PLANTS TAKEN AS 100.

Column 1: Name of plant and feature observed.

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

Ipomoea purpurea--first generation: seeds per capsule on crossed and self-fertilised plants, not growing much crowded, spontaneously self-fertilised under a net, in number: 99.

Ipomoea purpurea--seeds per capsule on crossed and self-fertilised plants from the same parents as in the last case, but growing much crowded, spontaneously self-fertilised under a net, in number: 93.

Ipomoea purpurea--productiveness of the same plants, as judged by the number of capsules produced, and average number of seeds per capsule: 45.

Ipomoea purpurea--third generation: seeds per capsule on crossed and self-fertilised plants, spontaneously self-fertilised under a net, in number: 94.

Ipomoea purpurea--productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 35.

Ipomoea purpurea--fifth generation: seeds per capsule on crossed and self-fertilised plants, left uncovered in the hothouse, and spontaneously fertilised: 89.

Ipomoea purpurea--ninth generation: number of