112 



THE ALGAE 



the middle septum breaks down so that the two protoplasts can 

 meet in the centre, after which the conjugation tube may widen. 

 The thick-walled zygote divides twice, the first division being 

 heterotype, whilst in one species the divisions result in two macro- 

 and two micronuclei. It is from these divisions that either two or 

 four new individuals arise. The species are to be found in upland 

 pools, peat bogs or on the soil. 



Fig. 63 Mesotaenium. 1-5, 6-12, conjugation of Cylindrocystis 

 hrehissonii and germination of zygospore. (After Fritsch.) 



* Desmidiaceae : Closterium (enclosed space). Figs. 64, 65 



This genus is an example of one of the placoderm desmids, a 

 group that is commonly characterized by the highly perforated cell 

 wall composed of two parts. 



The curved cells have attenuated apices with a vacuole in each 

 apex which contains crystals of gypsum that appear to have no 

 physiological function and are probably purely excretory. The 

 pores are arranged in rows in narrow grooves, cell movement being 

 secured by the exudation of mucilage through large pores near the 

 apices. Each semi-cell has one axile chloroplast which is in the form 

 of a curved cone with ridges on it with one to several pyrenoids, 



