248 Siphonales 



Oltmanns ('95) stated that in V. sessilis just before the appearance of the basal wall 

 all the nuclei except one were withdrawn from the young oogonium into the filament ; 

 but Davis ('04), from studies on V. geminata var. racemosa, declared that there were from 

 20 to 50 nuclei in the young oogonium, and that the oogonium was multinucleate at the 

 time of the formation of the separating wall, after which all the nuclei degenerated except 

 one, which grew to three or four times its original size. Heidinger ('08) studied the 

 development of the oogonia in a number of species, and his observations support Oltmanns' 

 statement that all the nuclei except one are withdrawn from the oogonium just before it 

 is cut off by the transverse wall. 



The antheridia develop simultaneously with the oogonia and generally in 

 close proximity to them (figs. 157 E\ 160, 1). Each antheridium arises as a 

 short cylindrical branch which usually becomes much curved on approaching 

 maturity (figs. 159 A, B, and D; 160, 5). The terminal portion of this 

 curved branch is cut off by a septum and becomes the actual antheridium. 

 In some species, such as in the submarine Vaucheria synandra, a number of 

 antheridia occur on a structure known as an androphore. The protoplasm of 

 each antheridium at the time that it is cut off from the thallus by a 

 separating wall contains numerous chloroplasts and nuclei. The latter collect 

 in the central portion of the antheridium and it is from the protoplasm of 

 this central region that the antherozoids are ultimately formed. The 

 antherozoids are very minute, each one consisting of a small amount of 

 cytoplasm surrounding a nucleus and possessing two cilia. The cilia are 

 attached far apart and point in opposite directions. The antherozoids swarm 

 for a short time within the antheridium, which soon opens at the apex and 

 sets them free. A certain proportion of unused protoplasm is expelled with 

 the antherozoids and some is also left behind in the antheridium. 



Oltmanns found that in V. sessilis the oogonia and antheridia opened 

 within a few minutes of each other and between 2 and 4 a.m. In most 

 species self-fertilization is apparently the rule. The antherozoids swarm in 

 the vicinity of the oogonium, the beak of which has opened by the dissolution 

 of the wall, a small quantity of mucilaginous protoplasm having been exuded 

 from the open apex (figs. 159 A ; 160, 6). One antherozoid fuses with the 

 oosphere and its nucleus passes towards the egg-nucleus, alongside which it 

 remains for some time : according to Davis ('04) the male nucleus increases 

 in size while in this position. At this stage the egg-cell becomes invested 

 with a thick cell-wall. The fusion of the nuclei takes place slowly and the 

 fusion-nucleus remains in a central position until the germination of the 

 oospore, which takes place only after a rest of from one to three months, or 

 more. Germination is direct. 



In general it would appear that good nourishment and light of consider- 

 able intensity are necessary factors in the formation of the sexual organs. 

 In Dichotomosiphon the sexual organs mostly terminate the branches. 



