154 



STRUCTURAL BOTANY 



oogonium now opens, either by the formation of a 

 round hole in the membrane, or by the transverse 

 splitting of the cell-wall near the top, in which case the 

 upper part of the membrane acts as a lid (Figs. 66 and 

 69, A). The gap is at first closed by a new membrane 

 formed from the neighbouring protoplasm of the 

 oogonium, but this soon disappears 

 again, leaving a free passage to the 

 ovum. 



Before describing the mode of 

 fertilisation, we will consider the 

 peculiar distribution of the sexes 

 already mentioned, as differing 

 from the ordinary monoecious and 

 dioecious conditions. It is this 

 form which our figures illustrate. 

 The peculiarity consists in the 

 production of dwarf male plants 

 quite different from the ordinary 

 form of the species. 



By repeated transverse divisions 

 in parts of the filament a chain 

 of small cells is produced much 

 shorter than the ordinary vegetative 

 cells of the plant. Each of these 

 short cells becomes the mother- 

 cell of a single zoospore of the usual structure, but of a 

 size intermediate between a normal vegetative zoospore and 

 a spermatozoid (Fig. 68, an). These small spores (called 

 androspores) are most commonly produced from the same 

 filaments which bear the oogonia ; more rarely they occur 

 on distinct filaments. Each androspore swims about for 

 a time, and then comes to rest, attaching itself to the 



FIG. 68. Androspore (art) 

 of (Edogonium ciliatum 

 escaping. At $ is an 

 obgouium. Magnified 

 350. (After Pring- 

 sheim.) 



