CONJUGALES 



107 



from the Antarctic and is scarce in waters containing much lime, the 

 individual species thriving best in soft or peaty waters. The most 

 favourable seasons for their development are the late spring and 



g cl d d I b 



Fig. 73. Closterium. Diagrams to explain cell division in species of Closterium 

 with (B) and without (A) girdle bands. The different segments of the wall are 

 indicated by shading, i, 2, 3, and A, b, c = the successive generations. The 

 individuals in i and A have each arisen from a zygote and have not undergone 

 division, a, b, c/= semi-cells of various ages; c = the connecting band demarcated 

 by the two sutures s, of the previous generation, and t, of the present ;5', /-girdle 

 bands developed before (g) and after division ; s = suture bet^veen young and older 

 semi-cells; r = the line of the next division, (After Fritsch.) 



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 fragmentation. 



REFERENCES 



Zygnemaceae. Czurda, V. (193 1). Beth. hot. Zbl. 48/2, 238. 

 Spirogyra, Zygnema. Czurda, V. (i933)- Beih. hot. Zbl. 50/1, 196. 

 Zygogonium. Hodgetts, W. J. (19 18). New Phytol. 17, 238. 

 General. Lefevre, M. and Manguin, E. (1938). Rev. gen. Bot. 50, 501. 

 Spirogyra. Lloyd, F. E. (1926). Trajis. Roy. Can. Inst. 15, 151. 



