6 ISK1TISH DESMIDIAOE.E. 



which become filled with a dense swarming mass of 

 granular material. This change has been observed to 

 take place in all the genera. 



The liquid contained in the vacuoles is known as 

 rt'U-xnp and is usually colourless. In certain species, 

 such as Mesotssnmm r'wlnx<-<'itx De Bary and M. pnr- 

 pureum West & Gr. S. West, the cell-sap is coloured 

 violet or purple by a pigment wliich has been termed 

 by Lagerheim phycoporphyrin. 



The chloroplasts occur embedded in the protoplasm, 

 either one or more in each cell. Sometimes they are 

 parietal cushions or bands on the walls of the cells, but 

 more frequently they are central. In Spirotaenia, 

 Mesotavnium, Roy a, and in some species of Gonatozygon 

 and Pemum, there is only one chloroplast. In those 

 species with parietal chloroplasts there may be four, 

 six, or eight, but the vast majority of Desxnids possess 

 two central chloroplasts, one in each semicell. The 

 chloroplasts of the Desmidiacea? are chiefly remarkable 

 for their large size, their variability in different 

 genera, and their peculiar complex character. They 

 may be straight and rod-like, ridged and spirally 

 twisted, or they may exhibit a radiating structure. In 

 some of the larger Desmids, such as in certain species 

 of Euastrum and Micrasterias, they are plate-like and 

 their margins are incised or lobed corresponding to the 

 incisions or lobes of the cell-Avail. Embedded in each 

 chloroplast are one or more large conspicuous pyre- 

 noids (Cosmarium, Staurastrum, etc.), or in some cases 

 numerous small pyrenoids (Gonatozygon, Genicularia, 

 etc.). The pyrenoids are crystalloidal nitrogenous 

 bodies, wliich usually become covered with minute 

 starch-grains after exposure to light. They consist of 

 reserve materials, and, with the exception of Anthoceros 

 among the Hepaticas, are quite peculiar to Alga3. The 

 pyrenoids with their envelope of starch-grains are 

 termed amylospheres. 



There is one nucleus in each cell, generally situated 

 in the central portion of the cell, and in those Desmids 



