Desmidiacece 137 



cytoplasm is variable in its general disposition, the variability 

 depending largely on the nature and arrangement of the chloro- 

 plasts. In those Desmids with large, central chloroplasts, vactioles 

 may be absent or much reduced, and they are often confined to 

 one at each extremity of the cell ; in others with parietal chloro- 

 plasts large vacuoles are often present in the central portions of the 

 cell. The general transparency of the cell- wall enables the circula- 

 tion of the protoplasm to be seen extremely well, especially in the 

 larger species. The nucleus is usually embedded in a small mass 

 of protoplasm in the median part of the cell in the region of the 

 isthmus, and in some genera is readily visible without the use of 

 reagents. In the genera Gonatozygon, Closterium and Pleuro- 

 tcenium, and in certain species of the genus Penium, there is a 

 well marked terminal vacuole at each extremity of the cell, con- 

 taining one or many moving granules. These granules, which 

 may be quite irregular in form or of some definite shape, exhibit 

 rapid vibratory movements and have in the genus Closterium been 

 shown to be minute crystals of gypsum. Their movements cease 

 immediately on the death of the protoplasm. 



If Desmids are kept living in small glass vessels for some 

 time, and therefore under abnormal conditions, curious changes 

 frequently occur in their protoplasm, resulting in the formation 

 of large vacuoles which previously did not exist. These vacuoles 

 generally contain numbers of minute moving corpuscles which 

 are somewhat different in appearance from those normally present 

 in the apical vacuoles of Closterium. As many as six large 

 vacuoles can be noticed in a single semicell of Pleurotcenium 

 coronatum (Breb.) Rabenh., each one being partially filled with 

 an incessantly moving mass of minute corpuscles, which move 

 freely in the vacuole and always collect towards its base. These 

 corpuscles or granules are of a pale yellow colour and appear 

 brown in a thin stratum ; but when present in immense numbers 

 they give the Desmid almost a black appearance. Under abnormal 

 conditions these moving granules are developed in numerous 

 genera, such as Penium, Cosmarium, Euastrum, Micrasterias, 

 Staurastrum, Arthrodesmus, etc., and at the same time the cell- 

 sap in the vacuoles often becomes coloured violet with phycopor- 

 phyrin, a pigment which occurs normally in the cell-sap of very 

 few Desmids. 



The form and disposition of the chloroplasts are conspicuous 



