DESMIDIACE^. 



283 



Iiyaline sheath or covering, as in Didi/moprium Grevillii 

 and Staurastrum tumidum ; in others it is more attenu- 

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



Fig. 155. 



21, Pe7ilv.m. 2-1, Pcdlastrum biradiatum. 25, Closfermm, sliowiug conjugation 



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



truin pecticum. 30, Ankistrodcsm^is falcatus. 33, Conjugation of Penlwm, 



margaritaceuvi. 34. Spirotcumia. 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. Me^^en states, " that the 

 large and small granules contain starchy 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) The test for starch can be easily applied, and so remove any doubt that 

 may exist. It is only necessary to bear in mind that unle'ss granular matter 



