118 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[26. 



of the spore. In some cases, however (e.g. Hydropteridese), a coat, termed 

 perinium, is formed round the spore, externally to its proper coats, not from its 

 own cytoplasm, but from cytoplasm in the sporangium derived from the cells 

 of the disintegrated tapetal layer. 



(b) Cell- formation by Conjugation takes place solely in connection 

 with the sexual process. The fusion of vegetative cells is not un- 

 common, but it does not lead to the formation of a single cell, 

 but of a syncyte (p. 91). In the sexual process, however, two 

 gametes, whether externally alike, as in isogamous plants, or 

 externally dissimilar, as in heterogamous plants, fuse together, 

 cytoplasm with cytoplasm, centrosphere with centrosphere, nucleus 

 with nucleus, completely losing their individuality, and constitute 

 a single new cell, a zygospore or an oospore, which is the starting 

 point for the development of a new organism. 



Fro. 76. Conjugation in Spirogyra (x400). At A two cells of adjacent filaments are 

 about to conjugate, and are putting out protuberances (a) towards each other : cl chromato- 

 phore ; Tc nucleus. At B the gamete p of the one cell is passing over and fusing with that 

 of the other (p). At C the process of conjugation is completed, the zygospore Z being the 

 product. 



It will have been observed, from the preceding instances, that cell-formation 

 is frequently preceded by nuclear fusion. This is so in cell-formation by conjuga- 

 tion ; and in many cases of cell-formation from a coenocyte (e.g. development of 

 endosperm in the embryo-sac of some Angiosperms; development of the oosphere 

 in incompletely septate plants, such as Vaucheria, Peronospora, Pythium). 



2. Cell- Formation with Cell-Division. Since in all cases, with 

 possibly occasional exceptions in coenocytes, this mode of cell- 

 formation is preceded by indirect or mitotic nuclear division, the 

 consideration of it may be preceded by an account of this form of 

 nuclear division. 



Indirect or Mitotic Division of the Nucleus. The indirect division of the 

 nucleus presents a series of remarkable phenomena which are collectively de- 

 signated by the term karyokiiiesis. Beginning with the nucleus in the resting- 



