EUPHYCOPHYTA II5 



and early summer, and their resistance in the vegetative state to 

 adverse conditions would seem to be very great. The evidence 

 suggests that, as a group, they have been evolved from filamentous 

 ancestors, possibly by over-specialization of the process of frag- 

 mentation. Recent cytological work on Desmids has revealed 

 great variation within clones of the same species. Thus, Cosmarium 

 cucumis gave chromosome counts of 44 or 52, and Netrium digitus 

 counts of 122, 172-82, and 592. Further counts and analyses of 

 clones are obviously essential. 



REFERENCES 



General 



Fritsch, F. E. (1935). Structure and Reproduction of the AlgaejVol. I, 



pp. 310-68. Camb. Univ. Press. 

 Lefebre, M. and Manguin, E. (1938). Rev. Gen. Bot., 30, 501. 

 King, G. C. (i960). New Phyt., 59 (1)5 65. 



Charophyceae 



The division comprises but a single order, the Charales. The 

 plants represent a very highly speciaUzed group that must have 

 diverged very early in the course of evolution from the rest of the 

 green algae, the intermediate forms subsequently being lost. There 

 is a small amount of evidence which suggests that they may have 

 originated in or along with the Chaetophorales, e.g. presence of 

 nodes (Draparnaldia) and cortication in that order. There is no 

 asexual reproduction but the process of sexual reproduction is very 

 complex. The young plants develop from a protonemal stage, the 

 erect plants having a structure which is more elaborate than any 

 type so far described, whilst the thallus is sometimes lime en- 

 crusted (Char a spp.). The group is known to be very ancient 

 because fossil members are found from almost the earliest strata. 

 The living forms are widely distributed in quiet waters, fresh or 

 brackish, where they may descend to considerable depths so long 

 as the bottom is either sandy or muddy. 



Nitella: (a littie star). Figs. 66-9 



The plants have the appearance of miniature horsetails {Equi- 

 setum) because they bear whorls of lateral branches arising from the 



