900 PHENOMENA OF SEXUAL REPRODUCTION. 
a morphological point of view ; they follow altogether different laws of development ; 
one of their limits always occurs in the fertilised oosphere. The prothallium de- 
veloped from the asexual spore of Ferns and Equisetaceae is, for example, morpho- 
logically a thallus without leaves or roots, while its physiological significance is 
determined by the production of antheridia and archegonia. From the fertilised 
oosphere on the other hand is produced the Fern or Horsetail, characterised 
morphologically by the differentiation of stem, root, and leaf ; but sexually this 
differentiated plant is neuter, producing neither male nor female cells, but only 
asexual spores. If the process of development of Rhizocarpese and Selaginellese is 
compared with these phenomena, it will be seen that in these classes the two genera- 
tions, the prothallium and the spore-forming leafy plant, stand essentially in the same 
relation to one another as in Ferns and Equisetaceae, only that the sexual differentia- 
tion goes back to the spore itself ; the spores are of two kinds, large female spores 
which produce the small female prothallium, and small male spores which produce 
a still smaller prothallium and antherozoids. The preparation for this sexual difference 
is manifested even in the asexual generation, by the sporangia producing only female 
or only male spores according to their position. In Salvinia the preparation goes 
back still further, each entire capsule producing only female or only male sporangia. 
It has already been pointed out how in Phanerogams the embryo-sac corresponds to 
the large, the pollen-grain to the small spore of heterosporous Vascular Cryptogams, 
and the endosperm to the prothallium. The endosperm of Phanerogams no longer 
appears as an independent structure, but only as a constituent part of the preceding 
generation ; in Angiosperms it is often from the first rudimentary and sometimes 
entirely absent, and the female sexual cell, the oosphere, is then the immediate pro- 
duct of the embryo-sac which corresponds to the large spore. The true sexual 
generation therefore becomes less and less important ; as such it becomes devoid of 
significance, while the sexual differentiation is carried back to the spore-forming 
generation, in which it determines the formation of the two kinds of reproductive 
organs, i. e. the pollen-sacs and ovules ; the flower may be exclusively male or 
female (monoecious diclinous), and, where the plant is dioecious, the sexual differ: 
entiation affects the entire individual, which is either male or female. In all Crypto- 
gams, on the other hand, dioecism is only displayed in one (the sexual) generation 
in the course of development of the individual. 
The process of development brought about by fertihsation or the union of the 
reproductive cells is usually not confined to the resulting embryo, but shows itself 
also in a variety of changes in the mother-plant itself. In ColeochcEte the oospore 
becomes invested with a cortical layer; in Characese the enveloping tubes of the 
carpogonium grow after fertilisation, their coils increase in number, and their mem- 
branes become lignified on the inside ; in the Hepaticse a variety of envelopes arise 
from the mother-plant ; in the Mosses the vaginule and in all Muscinese the calyptra 
becomes developed ; the tissue of the prothaUium 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 oosphere. 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 
