Desmidiaceas 363 



though still one half of it is older than the other half. In a few species of 

 Closterium and Penium the wall is composed of more than two pieces owing 

 to the development of one or more girdle-bands subsequent to cell-division 

 (consult p. 369). 



The protoplast of the Desmid varies much, but there is always a nucleus 

 of moderate size, usually embedded in a small band of cytoplasm traversing 

 the median part of the cell and, in the constricted types, lying in the isthmus. 

 The nucleus consists of a fine reticulum, with few chromatin granules, but 

 with a large compound nucleolus 1 . There is the usual lining layer of cyto- 

 plasm, but the general disposition of the rest of the cytoplasm depends entirely 

 on the nature and arrangement of the chlorpplasts. In those Desmids with 

 large axile chloroplasts the sap-vacuoles are much reduced and they are often 

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

 plasts one large vacuole usually occupies the central part of each semicell. 

 Owing to transparency of the cell-wall the circulation of the protoplasm is 

 often seen extremely well, especially in the larger species of Closterium, 

 Netrium, etc. In the genera Gonatozygon and Closterium, and in certain 

 species of Pleurot&nium and Penitim, there is a conspicuous terminal vacuole 

 at each extremity of the cell containing one or more moving granules. These 

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

 exhibit rapid vibratory movements and in the genus Closterium have been 

 shown to be minute crystals of gypsum. 



The chloroplasts of Desmids present a very interesting study 2 . They are 

 always of comparatively large size and usually of some definite form ; in most 

 cases they are situated in an axile position in the cell or semicell, although 

 less frequently they take the form of parietal cushions or bands. In the cells 

 of Spirottenia (fig. 230 A and B), Mesotzenium (fig. 230 E and F\ fig. 231 F I) 

 and Roya there is only one chloroplast and, except for a few species of 

 Spirotienia, the nucleus consequently occupies an asymmetrical position. The 

 same is true of some forms of Cosmocladium (fig. 226 A), Gonatozygon and 

 Cosmarium 3 . The majority of Desmids possess two axile chloroplasts, 

 symmetrically arranged one in each semicell (figs. 229 A and B, D F\ 

 fig. 230 H, J and K), although in many species of Cosmarium there are two 

 more or less distinct axile chloroplasts in each semicell. In Pleurotsenium, 

 and in a few species of Cosmarium, Staurastrum and Xanthidium there are 



1 The nuclear structure is well seen in various species of Gonatozygon, Closterium, Roya and 

 Micrasterias. For nuclear structure in Closterium consult Lutman ('10). 



2 But for the untimely death (in 1912) of Dr J. Ltitkemuller of Vienna we should have had by 

 now an exhaustive study of the chloroplasts of Desmids. Much of his work and most of his 

 drawings were communicated to the present author, who deplores the loss of a valued corre- 

 spondent and of the unwritten work founded upon many years of patient study. 



3 As for example Cosmarium subtile (W. & G. S. West) Liitkem. and C. subtilissimum 

 G. S. West. 



