194 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



Finally, the genus Cladophora presents an even more striking case. We 

 have seen that in the large majority of species the wall structure re- 

 sembles closely that of Valonia. Whereas, however, Valonia develops 

 into a large bubble-like cell, the Cladophoras have adopted the fila- 

 mentous habit. Even more striking, in Spongomorpha, closely related to 

 Cladophora, the chains of cellulose are oriented at random. Here then 

 we have two almost identical growth habits with entirely diflFerent wall 

 structure. Incidentally, this difference in wall structure in two species 



Fig. 66. For explanation, see text. 



which were once allocated to the same genus raises problems of its own, 

 but this is not the place in which these can be discussed. 



Osmotic forces in growth 



Nor are the anomalies in current ideas confined to the structural 

 aspects of the growth process. If dimensional changes in cells were 

 effected through the mechanical strains in the wall induced by the 

 internal hydrostatic pressures, then we might expect the hydrostatic 

 pressure to be greater in a cell when growing than it would be during 

 resting periods. That this is not always so is at least indicated in the 

 work of Burstr6m(63(fl)). His results show most clearly that when a 

 cell begins to grow the internal pressure diminishes, and while we must 

 await confirmation of these results before expressing a definite opinion, 

 this observation tends to throw even more doubt on the conception of 

 dimensional changes in cells as a reaction to mere mechanical influences. 

 It would, of course, be natural to conclude that if the pressure does 

 not increase, then the resistance of the wall might decrease. We 



