138 Chlorophycece 



features of many Desmids. They may be situated in a central 

 position in the cell or semicell, or they may take the form of 

 parietal cushions or bands. In the cells of Spirotcenia, Mesotce- 

 nium, Roya, and in some forms of Cosmocladium, Gonatozygon and 

 Penium, there is only one chloroplast, and the nucleus conse- 

 quently occupies an asymmetrical position. The majority of 

 Desmids possess two centrally disposed (axile) chloroplasts, sym- 

 metrically arranged, one in each semicell, but in Pleurotamium 

 and a few species of Cosmarium, Staurastrum and Xanthidium 

 there are a number of parietal, cushion-like chloroplasts in each 

 semicell. The chloroplasts are very variable in character and dis- 

 position, and many intermediate conditions are noticed between 

 truly axile and truly parietal masses of chlorophyll. There can 

 be little doubt that the axile chloroplast was the original type and 

 that the parietal condition has been secondarily acquired by 

 certain Desmids. In those forms in which the cell is deeply lobed 

 or incised the chloroplasts often closely follow the cell-outlines, 

 being themselves symmetrically lobed. Pyrenoids are present in 

 the chloroplasts of all Desmids. In most forms one or two pyre- 

 noids are present in each semicell, but in the large flattened forms 

 of Euastrum and Micrasterias, and in the elongated cells of 

 Closterium, Penium, Pleurotcenium, Tetmemorus, etc., the chloro- 

 plasts usually contain many pyrenoids. In certain species, such 

 as Spirotcenia acuta Hilse and Penium subtile W. & G. S. West, 

 only a solitary pyrenoid occurs in each cell. In certain genera the 

 pyrenoids are commonly subject to variation in number and dis- 

 position 1 , but in other genera they are remarkably constant. 



Under normal conditions Desmids frequently exhibit very 

 active movements. Klebs described four phases of motion in 

 certain elongated forms, all the movements being due to an 

 exudation of mucilage, the nature and extent of which has been 

 recently demonstrated in a few species by Schroder 2 . The net 

 result is always a movement in the direction of the strongest 

 light, the longest axis of the Desmid being placed in the direction 

 of the incident rays of strong light and at right angles to those of 

 feeble light. 

 \ Vegetative multiplication takes place by simple cell-division, 



1 Liitkemiiller, 'Beobacht. tiber die Chloroph. einig. Desmid.,' Oesterr. botan. 

 Zeitsdirift, xliii, 1893, no. 1 ; West & G. S. West, in Ann. Bot. xii, 1898, pp. 51, 

 52, t. iv, f. 3036 ; G. S. West in Journ. Linn. Soc. Bot. xxxiv, 1899, pp. 399, 400. 



2 Schroder in Verhandl. d. Heidelb. Naturbist.-Med. Vereins, Bd vii, 1902. 



