70 IJKITISH DESMIDIACEJK. 



Sub-family II. PLACODERMvE. 



In this sub-family the cell-wall is segmented and is 

 differentiated into an inner and an outer layer. The 

 inner layer is structureless and consists of cellulose. 

 The outer layer is firmer and thicker, and in most 

 Desmids of this sub-family it consists of a ground- 

 substance of cellulose through which pass numerous 

 tube-like structures known as " pore-organs." The 

 latter do not consist of cellulose, and through each one 

 runs a minute canal. The contents of the canals also 

 pass through the inner layer of the cell- wall and usually 

 terminate on its internal surface in a slight lens-shaped 

 thickening or bulbous swelling. From the external end 

 of the canal delicate club-shaped or flower-shaped 

 structures frequently arise, and these curious structures, 

 which are of a tough gelatinous consistency, haye fre- 

 quently given rise to erroneous conceptions of the 

 nature of the firmer part of the cell-wall. They are 

 sometimes prominent features of living specimens of 

 Cosmarium, Arthrodesmus, Staurastrum, and other 

 genera. The large mass of enveloping jelly which is 

 often present in the placoderm Desmids is the result of 

 a secretion of the cell-protoplasm, the products passing 

 outwards through the pores. This external gelatinous 

 coat commonly exhibits a radiating structure which has 

 sometimes given rise to grave misconceptions.* 



This group includes the vast majority of Desmids, 

 and there is almost always a distinct demarcation 

 between the new and the old semicells, the younger 



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part of the cell-wall being joined to the older part by 

 a narrow, bevelled surface. 



Cell-division is of a fixed type, following strictly after 



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the manner of previous cell-divisions, and the younger 

 semicells are always interpolated between the old ones. 



A. Point of division of cells variable or sometimes 

 fixed (at the isthmus). 



"' Such was the case with XantJtidiv.m spinulosum Bennett. 



