THALLOPHYTA: AWAE 



25 



Pennales auxospore formation is due to a fusion of cells. In some forms 

 two vegetative protoplasts escape and conjugate to produce a single 

 auxospore (Fig. 15). In other forms two diatoms unite to produce two 

 auxospores. Here the two fusing cells lie within a gelatinous matrix and 

 each produces two gametes. Then each of the gametes derived from 

 one cell conjugates with one of those derived from the other cell. It is 

 apparent that an "auxospore" formed by sexual fusion is really a zygote. 



Fig. 15. Conjugation in Cocconeis placentula, X 1,500. A and B, meiosis in conjugating 

 cells; C, fusion of protoplasts; D, zygote with sexual nuclei not yet fused. (After Geitler.) 



Just previous to conjugation the nucleus of each of the pairing proto- 

 plasts undergoes a reduction of chromosomes, giving rise to four haploid 

 nuclei. Some of these degenerate. 



In some of the Centrales many small biciliate protoplasts arise within 

 a vegetative cell and later escape into the water. These have been 

 called "microspores." Some observers think that they function as zoo- 

 spores, while others regard them as gametes, claiming that they fuse in 

 pairs. The occurrence of these ciliated cells in the Bacillariophyceae 

 suggests that the group may have been derived from flagellates with 

 brown plastids. The connection, however, is a remote one. 



7. CHLOROPHYCEAE 



The Chlorophyceae, or green algae, are predominantly fresh-water 

 forms whose plastids contain a preponderance of chlorophyll over its 

 associated carotinoids, the green and yellow pigments occurring in 

 approximately the same proportions as in the groups above the thal- 

 lophyte level. ^ Onlj^ a comparatively few members are marine, but 



' In a few members accessory pigments in the cell sap may mask the green color of 

 the chloroplasts. 



