364 



BOTANY OF THE LIVING PLANT 



numerous motile spermatozoids. The series will be found to run 

 parallel in these respects to those in other groups of Algae. 



In organisms such as Chlamydomonas and Sphaerella there may be a 

 brief, non-motile phase at the time of vegetative reproduction. In the 

 Chlorococcales this non-motile phase tends to be prolonged, the zoo 

 sporic phase being of comparatively brief duration. The common soil 

 alga, Chlorococcum humicolum, Fig. 269, consists of small spherical cells 



Fig. 269. 



Left : Chlorococcum humicolum. A-F, various stages in the life history. A, B, C, 

 non-motile cells; D, E, F, motile stages ( x 800). (From Chapman's, "An Intro- 

 duction to the Study of Algae." Cambr. Univ. Press.) 



Right : Pleurococcus Naegelii Chod. H, cell structure and division ; n, nucleus : 

 c, chloroplast. (After Fritsch and Salisbury.) G, packet of cells resulting from 

 division. (After Chodat.) 



each with a cell wall, a parietal chloroplast and internal structure not 

 unlike that of Chlamydomonas, except that the stigma and contractile 

 vacuole are absent. This type of cell is typical of many of the Chloro- 

 coccales. On attaining a certain size, the protoplast undergoes a suc- 

 cession of divisions into 2, 4, 8, etc. parts; each part becomes 

 ovoid in shape and develops two flagella. The retaining membrane 

 is then ruptured and these naked zoospores are set free. After 

 a period of movement, the flagella are withdrawn, a cell wall is 

 formed, and a new non-motile vegetative phase is begun. The close 

 assemblage of many zoospores may lead to the production of a green 

 stratum consisting of closely addressed cells of irregular shape and 

 unequal size. In some instances the swarmers behave as gametes, 

 sometimes of unequal size, and these fuse together in pairs to form 

 a spherical zygote which becomes a new individual. No cases of 

 oogamy are known in this Order. Like the Volvocales, which are pro- 

 bably closely related, the Chlorococcales include both unicellular and 



