CHARALES 113 



octants, only one of which develops to maturity. When mature, the 

 investing threads part somewhat to form a neck, and the apex of the 

 oogonium gelatinizes in order to permit the antherozoids to enter. 

 After fertilization the zygote nucleus travels to the apex of the 

 oospore and a coloured cellulose membrane is excreted around it, 

 whilst the oogonium wall, together with inner walls of the investing 

 threads, thicken and silicify. Four nuclei are formed by 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 

 nodes which become separated by elongation of the middle cell 

 (fig. 74 D-F). The lower node develops rhizoids whilst the upper 

 produces 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 protonemata 

 which develop from the primary rhizoid ring or else from dormant 

 apices. 



Chora (of a mountain stream). Figs. 74-77. 



This genus is very similar to Nitella in its method of reproduction, 

 but the plants are usually larger and coarser as a result of lime 

 encrustation, whilst the stem is corticated, the corticating cells 

 arising from the basal nodes of the short laterals, one thread 

 growing up and another down. 



*XANTHOPHYCEAE 



As a group the Xanthophyceae exhibit considerably less differ- 

 entiation than the Chlorophyceae. Two of the most characteristic 

 features are the replacement of starch as a food reserve by oil and a 

 greater quantity of xanthophyll in the plastids, although the actual 

 amount of the latter is partially dependent upon the external 

 conditions. The pigment turns blue-green when the cells are 

 heated in concentrated hydrochloric acid and this forms a con- 

 venient test for distinguishing them from the Chlorophyceae. 

 The walls are frequently in two equal or unequal portions which 



CSA 8 



