CHARALES 



13 



filaments, the outer cell-walls of which are thickened, while their tips unite at 

 the apex to form the crown. After fertilisation has been brought about by 

 the entrance of a spermatozoid through the cells of the crown, the nucule 

 remains in a dormant condition for some months. After this resting period 

 it germinates, putting out a slender filament known as the pro-embryo y 

 which suggests a comparison with the protonema of the mosses, and from 

 which, as in the mosses, the new plant arises as a lateral bud. The pro- 

 embryo is shown in Fig. 8. 



Fig. 7. Chara polystachya Gant. a, globule (</"), 

 60/ 1; c, the same in section, 70/1 ; /?, nucule 

 ($), 40/1 ; d, the same in section, 30/1. 

 w. t. s. 



Fig. 8. Chara fragilh Desv. Proto- 

 nema. sp, germinating oospore ; 

 b, d, p, ap, form together the 

 protonema; from d originate the 

 rhizoids rh ; r\ the so-called 

 principal root; /, the first leaves of 

 the foliage plant; ap, apex of the 

 protonema. (After Pringsheim) 



VIII. DIATOMACEAE. 



The diatoms are all microscopic, but they frequently occur in vast 

 numbers, and their cell-walls are so impregnated with silica as to be almost 

 indestructible. Fossil diatoms, for this reason, are often exceedingly well 

 preserved, some strata formed almost exclusively of their remains being 



