244 



Siphonales 



Considering the diameter of the filaments the cell-wall is as a rule 

 relatively thin. This fact, combined with an absence of elasticity of the wall, 

 causes the filaments to collapse very readily unless carefully handled. 



The cytoplasm forms a thick lining layer in which numerous minute 

 nuclei are embedded. The chloroplasts are very small and discoidal, with an 

 oval or subcircular outline, and occur in very large numbers in the lining 

 layer. They are without pyrenoids. The stored reserve material consists of 

 globules of oil in Vauch eria and grains of starch in Dichotomosiphon. As in 

 other cases in the Chlorophycese, and as pointed out by Fleissig ('00), these 

 reserves are physiologically analogous. 



Septa normally appear in the tubular thallus only in connection with the 

 reproductive organs. Injury, however, results in the appearance of septa 

 cutting off the injured parts, the uninjured portions developing into new 

 plants (consult fig. 157 A and B). 



Fig. 157. A and 11, portions of thallus of Vauchcria showing formation of septa on injury ; 

 A, Vauchcria g<-iniiuit<i. (Vauch.) DC.; B, V. ornithocephala Ag. C, apex of filament of 

 V. sessilis (Vauch.) DC. showing the zoogonidanginm. D, germination of the zoogonidium of 

 V. ornithocephala. E, V. sessilis showing developing oogoniuin (on right) and antheridium 

 (on left). F H, V. geminata (Vauch.) DC. showing development of sexual organs. 

 All x 75. a, antheridium ; oo, oogoniurn ; zg, zoogouidangium. 



Asexual reproduction usually takes place by large solitary zoogonidia of a 

 unique character, which have received the name of ' synzoospores ' owing to 

 their obviously compound nature. During the development of the zoogoni- 

 dium the growing apex of a filament assumes a club-shaped form and 

 becomes of an intense green colour, after which a transverse septum appears 

 and the swollen end is cut off as a zoogonidangium. The contents of the 

 gonidangium then become rounded off, forming an oval zoogonidium of large 

 size, which ultimately escapes by an apical opening of much smaller diameter 

 than itself through which it pushes its way (fig. 158 B). The whole 

 surface of the zoogonidium is clothed with numerous short cilia, arranged in 



