90p PHENOMENA OF SEXUAL REPRODUCTION. 



ii 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 

 ^sexual spores. If the process of development of Rhizocarpeae and Selaginelleae 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 

 ;"elation 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 

 jn the course of development of the individual. 



The process of development brought about by fertilisation 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 Characeae the enveloping tubes of the 

 jcarpogonium grow after fertilisation, their coils increase in number, and their mem- 

 -branes 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 Muscineae 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 

 ndeyeiopment 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 



