THALLOPHYTES 



33 



sporophyte. In fact, in this case, the doubling of chromosomes in the 

 formation of the oospore is followed immediately by their reduction 

 during the divisions of the oospore. 



Conclusions. — Upon considering the assemblage of green algae here 

 brought together under Confervales, the following general statements 

 may be made : The body is a simple filament, a branching filament, or 

 a fiat thallus, and is either completely septate or partially coenocytic. 

 Zoospores are produced abundantly, and are generally biciliate, Oedo- 

 gonium being a notable exception. The sexual reproduction ranges 

 from isogamy to a stage of heterogamy in which distinct oogonia and 

 antheridia are developed. The zygospores may give rise directly to 

 new plants or may produce zoospores ; but the oospores always develop 

 zoospores, a process which culminates in Coleochaete in a specially or- 

 ganized zoospore-producing body. 



(d) Siphonales 



General character. — This is a very well-defined group, comprising 

 mostly marine forms. The distinguishing character, suggested by the 

 name, is that the plant body has no cross walls, being one continuous 

 multinucleate protoplast enclosed by the peripheral wall, a kind of body 

 called coenocytic. In this case the body is completely coenocytic, as 

 distinguished from such partially coenocytic bodies as those of Clado- 

 phora and Sphaeroplea. The bodies are more or less diffusely branch- 

 ing, and in some of the marine forms the differentiation of the body is 

 remarkable, as in Bryopsis, Caulerpa, Acetabularia, etc. The two 

 prominent fresh-water forms are as follows. 



Botrydium. — This interesting plant develops in damp places, such 

 as the mud of drying-up ponds, wet plowed ground, boggy fields, and 

 especially on flood plains. It appears as groups of little green, balloon- 

 shaped bladders, about the size of a pin's head, which grip the substratum 

 by means of colorless, rhizoidal branches (fig. 90). Numerous small 

 nuclei are embedded in the thin wall layer of cytoplasm, and in the 

 aerial part there are numerous chloroplasts. 



The asexual methods of reproduction are various, dependent upon 

 var)dng conditions. A new bladder may bud out from the aerial part, 

 send rhizoidal branches into the substratum, and then become separated 

 from the parent by a wall. If covered with water, the whole inflated 

 aerial portion may be converted into a sporangium producing a multi- 

 tude of uniciliate zoospores (figs. 91, 92). These zoospores germinate 



