DesmidiacesB 



357 



In a few Desmids those belonging to the Spirotsenieae among Saccoderm 

 Desmids the cell-wall consists of a single layer of cellulose, all in one piece, 

 and of a homogeneous structure. In the remaining Saccoderm Desmids, of 

 the Gonatozygae, the cell-wall is also quite continuous in one piece, but there 

 is a differentiated outer layer. The great majority of Desmids belong to the 

 sub-family Placodermoe, in which the cell-wall is composed of two well 

 differentiated layers of varying thickness. Micro-chemical tests show that 

 the inner layer is practically structureless and consists of pure cellulose. The 

 outer layer is stronger and thicker, consisting of a ground substance of cellulose 



Fig. 221. Various species of Stauraxtrum. A, St. pelagicum W. & G. S. West, x 520 ; B, St. 

 turgescens De Not. , x 520; C, St. monticulosum Breb. var. pulchrum W. & G. S. West, x 520; 

 D, St. pungens Breb., x520; E, St. saltans Joshua, x 520; F, St. cyclacanthum W. & Gr. S. 

 West, x 520. a, front view of cell ; b, vertical or end view ; d, basal view of semicell. 



impregnated with various other substances, amongst which compounds of iron 

 have been demonstrated. It is in some species of Closterium and Penium 

 that the iron compounds are most prominent, and this is presumably directly 

 concerned with the yellow-brown colour of the cell-wall so often seen in these 

 genera. 



The pores pass through both layers of the cell-wall and in the outer layer 

 each pore is surrounded by a cylindrical tube-like structure which does not 

 consist of cellulose. To these parts of the cell-wall Ltitkemiiller gave the 



