THALLOPHYTES 



37 



Conclusions. — The body of Siphonales is coenocytic and usually 

 complexly branched and differentiated. Zoospores are formed, which 

 sometimes take on the character of the coenocytic body and may be 

 regarded . as compound zoospores, as in Vaucheria. Sexual repro- 

 duction ranges from isogamy to heterogamy, in the latter condition 

 extending to the formation of special branches bearing the sex 

 organs. Aplanospores or non-motile protected cells are produced 

 under conditions unfavorable for vegetative activity, and from all 

 spores or resting cells there is a direct development of new filaments. 

 A possible connection of Siphonales with Confervales is suggested 

 by such partially coenocytic forms as Cladophora and Sphaeroplca, 

 but in any event it is to be regarded as a highly speciahzed group. 



(e) Conjugales 



General character. — This is also a very 

 distinct group, which is separated by some 

 writers from Chlorophyceae on account of 

 its pecuhar features. It is a fresh-water 

 group, the bodies being single cells or simple 

 filaments, and characterized by remarkable 

 chloroplasts, by the absence of asexual spores, 

 and by a pecuhar method of sexual reproduc- 

 tion. The absence of all ciliated cells, includ- 

 ing both spores and gametes, in an aquatic 

 group is remarkable. The following families 

 may be used to illustrate the group. 



Desmidiaceae. — The desmids are one-celled 

 plants that exhibit the greatest possible variety 

 of form and are often extremely beautiful. 

 The cell is peculiar in being organized into 

 two symmetrical halves, often separated by 

 a deep constriction which makes the isthmus. 

 In each half there is a large, irregular, often 

 complex chloroplast and several pyrenoids, 

 while in the isthmus connecting the two halves 

 the single nucleus is placed. Many of the 

 desmids can move about, and they are very 

 sensitive to light, taking a position determined 

 by the incident rays. 



Figs, ioi, 102. — Clos- 

 terium: loi, cell dividing, 

 the two nuclei and the wall 

 having been formed ; 102, the 

 plastid in an old " half cell " 

 having divided, and the 

 nucleus becoming: placed be- 

 tween the two plastids. — 

 After A. Fischer. 



