THE NATURE OF SEXUALITY. 901 



embryo, but brings about processes of growth in the mother-plant, in consequence of 

 which the cystocarp is produced in Floridese and the spore-fruit in Ascomycetes. In 

 the Orchideae the action of the pollen-tube is visible on the mother-plant even before 

 fertilisation; Hildebrand has shown (Bot. Zeit. 1863, p. 341) that in all Orchids 

 which he examined the ovules were not in a condition to be fertilised at the time of 

 pollination ; and in some (as Dendrobium nobile) they have not even begun to be 

 formed ; it is only during the growth of the pollen-tubes through the tissue of the 

 stigma and style that the ovules become so far developed that fertilisation can at 

 length be effected. In the Orchideae the formation of the female cell is therefore a 

 result of pollination; it is determined by the action of the male pollen- tube on the 

 tissue of the mother-plant^ 



When the embryo is being developed within the mother-plant, as in the Muscineae 

 and Vascular Cryptogams, it obtains its food-material from the plant; and this is 

 connected in the Vascular Cryptogams with complete exhaustion and the dying off 

 of the prothallium. In Phanerogams not only does the embryo usually acquire a 

 considerable development, even within the fruit, but a great quantity of the pro- 

 ducts of assimilation is also withdrawn from the plant by the accumulation of 

 reserve-material in the seed and by the development of the fruit; in many cases 

 the plant itself is also completely exhausted, all its disposable formative sub- 

 stances are given up to the seed and the fruit, and it dies off (monocarpous 

 plants). It is clear that all these changes and the various movements of materials 

 in the mother-plant connected with them are results of fertilisation, results of 

 immense importance caused by the union of microscopic cells, imponderable by 

 the best balance. 



(a) A careful consideration of the phenomena occurring among the Thallophytes 

 would probably lead to the formation of a tolerably clear idea of the mode of the 

 Development of Sexuality in the Vegetable Kingdom. The space at our disposal will 

 only suffice for a few general remarks. 



The labours of Pringsheim^, which open up the way for a complete theory of 

 sexuality in the future, seem to indicate that the conjugation of the motile cells of 

 Pandorina, Ulothrix, &c. is one of the primitive phases of a sexual act. If this be so, 

 then it follows that a sexual coalescence of cells first made its appearance when the 

 Thallophytes had already attained a considerable degree of morphological and of phy- 

 siological development. The same result is reached by a consideration of the fact 

 that sexuality is apparently absent in the Hydrodictyese^ and in the immediate allies 

 of Ulothrix, and that comparatively highly-developed forms, such as the Rivularieae, 

 exist among the Protophyta in which no trace of sexuality can be discovered. 



If it be also remembered that the simplest forms of sexuality, conjugation and the 

 formation of zygospores, occur in very different groups of the Thallophytes, and that 

 the mode of conjugation varies with the form and habit of the plants and that it may 

 be very different in different groups, the thought is at once suggested that the sexual 

 coalescence of cells may have commenced at different times and quite independently in 



' [For a summary of the instances in which pollen appears to have influenced the fruit of the- 

 mother-plant, see C. J. Maximowicz, Journ. Roy. Hort. Soc, new series, vol. III. p. 161 ; ^nd Darwin,^ 

 Animals and Plants under Domestication, vol. I. p. 397.] 



^ Monatsber. d. k. Akad. der Wiss. in Berlin, 1869. 



5> [Conjugation has since been observed in Hydrodiciyon (see page 251).] 



