WALL STRUCTURE IN THICK CELL WALLS 



93 



glance, they have fundamentally the same type of cellular organization. 

 The cells are large with very firm cell walls, a thin lining of protoplasm 

 and a large central vacuole. They differ from normal cells in higher 

 plants not only in their very much greater size but also because the 

 cytoplasm contains many nuclei — they are said to be coenocytes. In a 

 strict sense, therefore, they are perhaps not to be called cells at all, 

 though from the present point of view this is a matter of httle impor- 

 tance. In V. ventricosa, which has been the more common object of 



Fig. 34. Three species of Valonla. 



(a) V. ventricosa. Large single vesicles with basal holdfasts. 



{b) V. utriciilaris. Smaller vesicles; many attached laterally giving the impression 



of a palisade of cells, 

 (c) V. macrophysa. Vesicles, intermediate in size, with basal holdfasts (from which 



other vesicles occasionally grow out laterally). 



investigation in this genus, the cells are very large indeed, ranging in 

 size up to that of a pigeon's egg or even greater, and these giant cells 

 occur singly. At the base of each cell, i.e. the end nearer the point of 

 attachment to the rocky substrate, small pieces of protoplasm are cut 

 off by lenticular walls and the outer, parent wall "blows out" into a 

 cylindrical protuberance which serves to anchor the cell (Fig. 34A). 

 There are several such holdfasts or rhizoids to each cell, but in this species 

 they all occur close together at the base and no "watch-glass" cells are 

 developed anywhere else over the cell surface. These small cells con- 

 stitute the only signs of cellular differentiation found in this species. 

 In the other two species the structures are essentially the same except 

 that here watch-glass cells can develop anywhere. At the base these 

 again produce holdfasts, but those situated farther away from this 

 region produce new cells like the parent and these remain attached. 

 This leads to the development of a coherent mass of cells each of which 

 is, however, fundamentally solitary. In the main we shall deal here 

 with V. ventricosa though it may be taken for granted that the descrip- 

 tions refer also to the other two species. 



