CONJUGALES 105 



Desmidiaceae : Mesotaenium (meso, middle; taenium, band). Fig. 



71- 

 This is an example of one of the saccoderm desmids, which as a 

 group are characterized by a smooth wall in one complete piece 

 and without any pores. The rod-shaped cells of Mesotaenium are 

 single, have no median constriction, and are circular in transverse 

 section. The chloroplast is a flat axile plate containing several 

 pyrenoids, whilst in some species the presence of phycoporphyrin 

 imparts a violet colour. The inner cell wall is composed of cellulose 

 and the outer of pectose. Multiplication takes place by cell division, 

 the daughter cells being liberated by dissolution of the middle 

 lamella after a constriction has been formed, though in some cases 

 this may not occur until a number of cells have been enclosed in a 

 common mucilaginous envelope. Sexual reproduction is by means 

 of conjugation, two processes being put out just as in the fila- 

 mentous forms: these unite and then 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 thai either two or four new individuals 

 arise. The species are to be found in upland pools, peat bogs or on 

 the soil. 



*Desmidiaceae : Closterium (enclosed space). Figs. 72, 73. 



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, whilst in transverse section 

 it either has the appearance of a hub with radiating spokes or else 

 looks Hke a coarsely cogged wheel. Cell division is pecuHar and 

 takes place by one of two methods producing either (a) connecting 

 bands which appear as striae in the older semi-cells or (b) girdle 



