Self-sterile plants.

Causes of self-sterility.

The appearance of highly self-fertile varieties.

Self-fertilisation apparently in some respects beneficial, independently of the a.s.sured production of seeds.

Relative weights and rates of germination of seeds from crossed and self-fertilised flowers.

The present chapter is devoted to the Fertility of plants, as influenced by cross-fertilisation and self-fertilisation. The subject consists of two distinct branches; firstly, the relative productiveness or fertility of flowers crossed with pollen from a distinct plant and with their own pollen, as shown by the proportional number of capsules which they produce, together with the number of the contained seeds. Secondly, the degree of innate fertility or sterility of the seedlings raised from crossed and self-fertilised seeds; such seedlings being of the same age, grown under the same conditions, and fertilised in the same manner.

These two branches of the subject correspond with the two which have to be considered by any one treating of hybrid plants; namely, in the first place the comparative productiveness of a species when fertilised with pollen from a distinct species and with its own pollen; and in the second place, the fertility of its hybrid offspring. These two cla.s.ses of cases do not always run parallel; thus some plants, as Gartner has shown, can be crossed with great ease, but yield excessively sterile hybrids; while others are crossed with extreme difficulty, but yield fairly fertile hybrids.

The natural order to follow in this chapter would have been first to consider the effects on the fertility of the parent-plants of crossing them, and of fertilising them with their own pollen; but as we have discussed in the two last chapters the relative height, weight, and const.i.tutional vigour of crossed and self-fertilised plants--that is, of plants raised from crossed and self-fertilised seeds--it will be convenient here first to consider their relative fertility. The cases observed by me are given in Table 9/D, in which plants of crossed and self-fertilised parentage were left to fertilise themselves, being either crossed by insects or spontaneously self-fertilised. It should be observed that the results cannot be considered as fully trustworthy, for the fertility of a plant is a most variable element, depending on its age, health, nature of the soil, amount of water given, and temperature 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 circ.u.mstance 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 capsules on crossed plants to those on self-fertilised plants, spontaneously self-fertilised under a net: 26.

Mimulus luteus--an equal number of capsules on plants descended from self-fertilised plants of the 8th generation crossed by a fresh stock, and on plants of the 9th self-fertilised generation, both sets having been left uncovered and spontaneously fertilised, contained seeds, by weight: 30.

Mimulus luteus--productiveness of the same plants, as judged by the number of capsules produced, and the average weight of seeds per capsule: 3.

Vandellia nummularifolia--seeds per capsule from cleistogene flowers on the crossed and self-fertilised plants, in number: 106.

Salvia coccinea--crossed plants, compared with self-fertilised plants, produced flowers, in number: 57.

Iberis umbellata--plants left uncovered in greenhouse; intercrossed plants of the 3rd generation, compared with self-fertilised plants of the 3rd generation, yielded seeds, in number: 75.

Iberis umbellata--plants from a cross between two varieties, compared with self-fertilised plants of the 3rd generation, yielded seeds, by weight : 75.

Papaver vagum--crossed and self-fertilised plants, left uncovered, produced capsules, in number: 99.

Eschscholtzia californica--Brazilian stock; plants left uncovered and cross-fertilised by bees; capsules on intercrossed plants of the 2nd generation, compared with capsules on self-fertilised plants of 2nd generation, contained seeds, in number: 78.

Eschscholtzia californica--productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 89.

Eschscholtzia californica--plants left uncovered and cross-fertilised by bees; capsules on plants derived from intercrossed plants of the 2nd generation of the Brazilian stock crossed by English stock, compared with capsules on self-fertilised plants of 2nd generation, contained seeds, in number: 63.

Eschscholtzia californica--productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 40.

Reseda odorata--crossed and self-fertilised plants, left uncovered and cross-fertilised by bees; produced capsules in number (about): 100.

Viola tricolor--crossed and self-fertilised plants, left uncovered and cross-fertilised by bees, produced capsules in number: 10.

Delphinium consolida--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 56.

Viscaria oculata--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 77.

Dianthus caryophyllus--plants spontaneously self-fertilised under a net; capsules on intercrossed and self-fertilised plants of the 3rd generation contained seeds in number: 125.

Dianthus caryophyllus--plants left uncovered and cross-fertilised by insects: offspring from plants self-fertilised for three generations and then crossed by an intercrossed plant of the same stock, compared with plants of the 4th self-fertilised generation, produced seeds by weight: 73.

Dianthus caryophyllus--plants left uncovered and cross-fertilised by insects: offspring from plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the 4th self-fertilised generation, produced seeds by weight: 33.

Tropaeolum minus--crossed and self-fertilised plants, left uncovered in the greenhouse, produced seeds in number: 64.

Limnanthes douglasii--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number (about): 100.

Lupinus luteus--crossed and self-fertilised plants of the 2nd generation, left uncovered in the greenhouse, produced seeds in number (judged from only a few pods): 88.

Phaseolus multiflorus--crossed and self-fertilised plants, left uncovered in the greenhouse, produced seeds in number (about): 100.

Lathyrus odoratus--crossed and self-fertilised plants of the 2nd generation, left uncovered in the greenhouse, but certainly self-fertilised, produced pods in number: 91.

Clarkia elegans--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 60.

Nemophila insignis--crossed and self-fertilised plants, covered by a net and spontaneously self-fertilised in the greenhouse, produced capsules in number: 29.

Petunia violacea--left uncovered and cross-fertilised by insects: plants of the 5th intercrossed and self-fertilised generations produced seeds, as judged by the weight of an equal number of capsules: 86.

Petunia violacea--left uncovered as above: offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the 5th self-fertilised generation, produced seeds, as judged by the weight of an equal number of capsules: 46.

Cyclamen persic.u.m--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 12.

Anagallis collina--crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 8.

Primula veris--left uncovered in open ground and cross-fertilised by insects: offspring from plants of the 3rd illegitimate generation crossed by a fresh stock, compared with plants of the 4th illegitimate and self-fertilised generation, produced capsules in number: 5.

Same plants in the following year: 3.5.

Primula veris--(equal-styled variety): left uncovered in open ground and cross-fertilised by insects: offspring from plants self-fertilised for two generations and then crossed by another variety, compared with plants of the 3rd self-fertilised generation, produced capsules in number: 15.

Primula veris--(equal-styled variety) same plants; average number of seeds per capsule: 71.

Primula veris--(equal-styled variety) productiveness of the same plants, as judged by the number of capsules produced and the average number of seeds per capsule: 11.