t>22 PHYSIOLOGY. 



head of cell-formation, p. 585, and which is adverted to in the 

 description of the Thallophytes. The spore resulting from such 

 union is termed a zyyospore (Mucorini, Algae, &c.). The conju- 

 gating cells are either motionless, or, as in Botrydium and Aceta- 

 bularia, the motile particles of: protoplasm (zoospores provided with 

 cilia) combine one with another to reproduce the plant. In other 

 cases the germ particle is stationary while the sperm particle 

 exhibits active movements. Generally both sperm particles and 

 germ particles are uncovered masses of protoplasm, the cell-wall 

 not being formed around the germ mass until after fertilization. 



Further details relating to the various modifications observable in the 

 reproductive cells are given in the sections relating to Cell-formation and 

 to the separate natural orders of Cryptogams. 



Sect. 3. KEPRODTJCTIO^ OE PHA:N T EKOGAMS. 



The remarkable distinguishing character of this group of plants 

 is the possession of stamens producing pollen and of carpels pro- 

 ducing ovules, the latter containing a large cell, the embryo-sac, 

 within which is the germ or germinal vesicle. The sperm-cell 

 or pollen-grain falls on the stigma, elongates into a long tube, 

 which traverses the style and comes into contact with the embryo- 

 sac containing the germinal vesicle. As a consequence of this con- 

 tact, the germ- vesicle becomes a cell, and this ultimately forms an 

 embryo, as described in other sections. In G-ymnosperras the 

 pollen or sperm-cells are applied directly, without the intervention 

 of style or stigma, to the nucleus of the ovule. The germ-cell 

 here differs from that of Angiosperms, and is called the corpuscle 

 (see ante, p. 358). It is supposed to be homologous with the germ- 

 cell or central cell of Lycopods (p. 424). The differences in the 

 development of the embryo of G-ynmosperms and Angiosperms are 

 treated elsewhere. 



The formation of the reproductive organs closes the life-cycle of 

 the plant either permanently, or, in the case of perennial plants, 

 the periodic cycle of growth and the progeny thrown off, after 

 passing through a quiescent stage of rest, germinate afresh into a 

 new perfect morphological representative of the species. 



Alternation of Generations. Vegetative reproduction (agamo- 

 (jenesis) in its varied forms and sexual reproduction (gamogenesis) 

 may be manifested in the plant at the same time, or they may 

 alternate one with the other ; thus the production of spores and 

 the formation of a prothallus in Ferns is an asexual process alter- 

 nating with the development of a perfect plant from the action of 

 a spermatozoid in the central cell of the archegonium. Among 



