DESMIDIACE^E. 



286 



hyaline sheath or covering, as in Didymoprium Grevillii 

 and Staurastrum tumidum ; in others it is more attenu- 

 ated, and the fact that it forms a covering is discerned 



Fig. 155. 



21, Penium. 24, Pediastrum biradiatum. 25, Closterium, showing conjugation 

 or self-division. 27, Penium Jenneri. 28, Aptogonum desmidium. 29, Pedias- 

 trum pecticum. 30, AnTcistrodesmus falcatus. 33, Conjugation of Peniurr 

 margaritaceum. 34. Spirotcemia. 35, Closterium. 



only by its preventing the contact of the coloured cells. 

 In general its quantity is merely sufficient to hold the 

 fronds together in a kind of filmy cloud, which is dispersed 

 by the slightest touch. When they are left exposed by 

 the evaporation of the water, this mucus becomes denser, 

 and is apparently secreted in larger quantities, to protect 

 them from the effects of drought. Meyen states, " that the 

 large and small granules contain starch, and were some- 

 times even entirely composed of it ; " and " in the month 

 of May he observed many specimens of Closterium in 

 which the whole interior was granulated; these grains gave 

 with iodine the beautiful blue colour, indicative of the 

 presence of starch." 1 



(1) The test for starch can be easily applied, and so remove any doubt that 

 nay exist. It is snly necessary to bear in mind that unless granular matter 



