120 



THE ALGAE 



two successive divisions of the zygote nucleus, meiosis taking place 

 during this process. One of these nuclei becomes cut off by a cell 

 wall whilst the other three degenerate. The small cell so formed 

 then divides and two threads grow out in opposite directions, one a 

 rhizoid, the other a protonema. The cell next to the basal cell of the 

 protonema divides into three cells, the upper and lower forming 







Fig. 69 Charales. A-F, Char a vulgaris, stages in formation of 

 oogonium. A, first division, B, C, division of periphery to form 

 envelope cells. D, coronal cells cut off. F, mature oogonium. G-I, 

 Nitella flexilis stages in formation of oogonium. J, fertile branch of 

 C.fragilis. a = oogonium, he = bract cell, hn = branch nodal cell, d = 

 coronal cells, e= envelope cells, /= flask cell, z=intemode, « = 

 nodal cell, t = turning cell, os = oogonium stalk cell. (After Grove.) 



nodes which become separated by elongation of the middle cell 

 (Fig. 66). The lower node develops rhizoids whilst the upper pro- 

 duces a whorl of laterals from all the peripheral cells except the 

 oldest, which instead forms the apex of the new plant. The mature 

 plant is therefore morphologically a branch of the protonema. 

 Vegetative reproduction can take place from secondary protone- 

 mata which develop from the primary rhizoid ring or else from 

 dormant apices. 



