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 Florideae 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 prothalHum. 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ducts of assimilation is also withdrawn from the plant by the accumulation of 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 2, 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 vv^hich 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 ; and Darwin, 
Animals and Plants under Domestication, vol. I. p. 397.] 
^ Monatsber. d. k. Akad. der Wiss. in Berlin, 1869. 
3 [Conjugation has since been observed in Hydrodictyon (see page 251).] 
