THE DESMIDS 33 



which are tiny proteid bodies, are found in the chromato- 

 phores, and during assimilation, starch granules are 

 formed round them. The chromatophores are enclosed 

 in transparent protoplasm, which appears under the 

 microscope between the coloured area and the cell-wall. 

 The pores in the cell-wall, already spoken of, are exits 

 for fine threads of protoplasm, which gather on the 

 outside of the cell and envelop it in a gelatinous sheath. 

 Even groups of cells become enveloped in this way. 

 Some forms exhibit a slow, creeping movement, while 

 others fix themselves at one extremity to a substratum, 

 allowing the free end to describe circles. Movement 

 seems to have distinct relation to light. On a bright 

 day one may secure certain species on the surface of a 

 pool, but on a dull day one must seek them on or near 

 the bottom. In the genus Closterium (Plate II., Fig. 20), 

 which embraces crescent - shaped forms of singular 

 beauty, a small vacuole occurs at each end of the cell, 

 in which tiny granules of gypsum circle, as if chasing 

 one another, in quite an energetic fashion. 



Multiplication in the Desmids (Plate II., Figs. 12-19) 

 is vegetative, or asexual, and sexual. The vegetative 

 method is that of fission or division. The two valves 

 of the cell-wall are pushed apart by the growing cell- 

 contents, the nucleus divides into two, each daughter- 

 nucleus to become the nucleus of a new individual. 

 When the nuclear division is achieved, a septum or 

 partition - wall is extended between the two valves, 

 which shortly break apart, each retaining a layer of 

 the septum. Of course, immediately on division, each 

 of the parts appears like half a Desmid ; but it is 

 not long before the new cells assume the normal 



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