Sexual reproduction 



397 



unicellular dwarf male and the upper cell becomes the antheridium (antheri- 

 dium interior ; fig. 251J. and B) ; in other species the antheridium may consist 

 of one or more cells which arise by the division of the first-formed cell in a 

 manner characteristic of cell-division in (Edogonium (antheridium exterior ; 

 fig. 250 C a). 



When the oosphere is ready for fertilization there is formed in it a clear 

 pellucid space, the receptive spot, opposite the opening in the wall of the 

 oogonium. The latter opens in two distinct ways, either by a pore or by a 

 circular split all round the wall. In both cases the opening may be superior, 

 median or inferior. The antherozoid enters by the opening and sexual fusion 

 results in the oospore (fig. 247 6r). The wall of the ripe oospore consists of 

 from one to three layers and not infrequently it exhibits various sculptures. 

 In (Edogonium it is sometimes clothed with short spines. The oospore is a 

 resting spore and a short time after the formation of its wall the chlorophyll 

 disappears, the protoplast becoming largely filled with a fatty oil in which 

 there is commonly dissolved a brown or red pigment. It is liberated by the 

 decay of the wall of the oogonium. On germination the thick walls are burst 

 open and the protoplast, now green again, divides into four cells each of 

 which becomes a swarm-cell of the typical (Edogonium-type (fig. 252). After 

 swarming for a time the motile cells come to rest and begin to grow into new 

 filaments. In some cases these filaments are asexual and they give rise to 

 several other asexual generations before forming a sexual plant. It has been 

 suggested that the four swarm-cells formed on the germination of the fertilized 

 egg represent a rudimentary sporophyte generation, but this is doubtful and 

 there is no cytological evidence in support of it. 



(Edogonium fonticola A. Br., which has male plants of small size, is apparently a 

 species intermediate between the truly dioecious macrandrous forms arid those with large 

 nannandria. Tt thus sheds much light on the possible evolution of the nannandrous types 

 (G. S. W., '12). It seems not im- 

 probable that the macrandrous species 

 arose from the monoecious species by 

 the physiological differentiation of 

 the zoogonidia, some of which gave 

 rise only to female filaments and 

 others only to male filaments. Thus 

 there would have arisen androzoo- 

 gonidia and gynozoogonidia. In the 

 nannandrous species there is a 

 further differentiation between these 

 two types of zoogonidia in point of 

 view of size, the androzoogonidia being 

 as a rule not more than half the size of 

 the gynozoogonidia, and intermediate 

 between the gynozoogonidium and the antherozoid. 



Fig. 252. A and B, germination of the oospore of 

 (Edogonium pluviale Nordst. C, the four swarin- 

 spores produced on the germination of the oospore 

 of BulbochsBte elachistandra Wittr., x 350. (A and 

 B, after Juranyi ; C, after Pringsheim.) 



All these motile bodies exhibit 



