424 



A TEXTBOOK OF BOTANY 



[On. X 



Ecologically the Confervales are typical sedentary fila- 

 mentous hydrophytes. Phylogenetically the lower forms 

 seem clearly related to Protococcales, and were no doubt 

 evolved therefrom. The' higher kinds presumably gave 



A 



a 



FIG. 293. Coleochcete scutata. 



Left, a plant, disk-shaped, seen from above, X 50 ; right, an oospore in 

 germination, showing formation of the enveloping case, X 500. (From 

 Oltmanns.) 



origin to the Bryophytes as shown in our phylogenetic tree 

 (Fig. 275). 



ORDER 3. CONJUGALES (ZYGOPHYCEM] : THE CONJUGATING 

 A.LGM. These are unicellular or filamentous floating forms, 

 mostly microscopic but often conspicuous in mass. They 

 are notable for the remarkably diverse and symmetrical 

 elaboration of the chloroplasts and walls, on which account 

 they are beautiful and ever-favorite objects of study with 

 the microscope. Though diverse in certain of their charac- 

 teristics, to such a degree that not all authorities include 

 them in one class, they are brought into one group by their 

 principal method of sexual reproduction, viz., the conjuga- 

 tion of the contents of two cells to form a zygospore, from 

 which later arises a single new plant. The process obviously 

 represents a very simple, perhaps a simplified, form of 

 sexual reproduction. They comprise three groups, (1) the 

 filamentous Zygnemacece or Pond Scums, (2) the elaborate 

 unicellular Desmids, and (3) the equally elaborate Diatoms. 



