Fertility of plants of crossed and self-fertilised parentage, both lots being fertilised in the same manner.  Fertility of the parent-plants when first crossed and self-fertilised, and of their crossed and self-fertilised offspring when again crossed and self-fertilised.  Comparison of the fertility of flowers fertilised with their own pollen and with that from other flowers on the same plant.  Self-sterile plants.  Causes of self-sterility.  The appearance of highly self-fertile varieties.  Self-fertilisation apparently in some respects beneficial, independently of the assured 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 classes 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 constitutional 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