8o6 PHENOMENA OF SEXUAL REPRODUCTION. 



These general observations show that the sexual differentiation stands, in the various 

 classes of plants, in a very different relationship to the morphological differentiation 

 manifested in the alternation of generations. Hence it arises that the product of the 

 fertilised oosphere has from a morphological point of view a very different value in 

 different groups of plants. In the Conjugatae it is a zygospore from which generations 

 of cells are subsequently developed similar to the mother-cells of the zygospore ; in 

 Vaucheria, GEdogonium, and Coleochsete it is an oospore from which an asexual gener- 

 ation developes, proceeding from it in different ways ; in the Muscineae the asexual 

 generation constitutes the so-called fruit of Mosses ; in Vascular Cryptogams and Pha- 

 nerogams it is the plant furnished with leaves and rooting in the soil. 



The process of development brought about by fertilisation or the union of the repro- 

 ductive cells is usually not confined to the resulting embryo, but shows itself also in a 

 variety of changes in the mother-plant itself. In Coleochaete the oospore becomes in- 

 vested with a cortical layer ; in Characeae the enveloping tubes of the nucule grow after 

 fertilisation, their coils increase in number, and their membranes become lignified on 

 the inside ; in the Hepaticae a variety of envelopes arise from the mother-plant ; in the 

 Mosses the vaginule and in all IMuscineae the calyptra becomes developed ; the tissue of 

 the prothallium which surrounds the growing embryo of Ferns grows at first rapidly 

 along with it ; in Phanerogams the entire development of the seed and fruit depends on 

 the changes caused in the mother-plant by the fertilisation of the embryonic vesicles. 

 The two most remarkable cases occur in Florideae and Ascomycetes on the one hand, 

 and in Orchideae on the other hand. In the former fertilisation does not in general 

 directly cause the formation of an embryo, but brings about processes of growth in the 

 mother-plant, in consequence of which the cystocarp is produced in Florideae and the 

 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 nohili) 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 developed within the mother-plant, as in the Muscineae and 

 Vascular Cryptogams, it withdraws its food-material from the plant; this being con- 

 nected 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 products 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 substances 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. 



Achlya polymidra ; the only difference between the parthenogenetic germ-cells and those produced by 

 the ordinary process of fertilisation being that the former remain dormant for a longer period. (See 

 Braun, Abhand. der Berlin. Akad. 1856; and Pringsheim, Jahrb. fur wiss. Bot. vol. IX, p. 191). 

 — Ed.] 



^ [For a summary of the instances in which pollen appears to have influenced tlie fruit of the 

 mother-pknt, 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.— Ed.] 



