CARBOHYDRATES, CHITIN AND CUTIN 



2QI 



protoplasm likewise circulates in an axial direction and the nascence 

 of the fibrous texture of the secondary wall could be explained as 

 being causally mechanistic. Van Iterson now goes so far as to suggest 

 as an explanation for the crosswise layers of the Valonia cell wall 

 (brought into prominence by X-ray investigation) that the proto- 



^) 



Fig. 144. Diagram of intercalation of wax in meristematic cell walls of tubular texture. 

 a) Radial section; U) tangential and cross-sections. 



plasm is forced to change its direction of flow by about 90° after the 

 deposition of every layer; for the tendency of the cell to expand is 

 always perpendicular to the direction of the iibrillae of the newly 

 formed layer, for which reason the flow of plasm is supposed to be 

 passively directed cross-wise over the youngest lamella. 



With all due admiration for Van Iterson's reasoning, and con- 

 ceding a certain contributory role to the forces he has discovered, it 

 can scarcely be said that mechanistic theories of this kind are at 

 present adequate to resolve the mysteries of morphogenesis. For there 

 are several facts of observation which do not come within the compass 

 of causation. For instance, out of similar cells near the cambium, to 

 the primary wall of which tubular texture is ascribed, are differentiated 

 on the one hand fibres with spiral texture, which may have been 

 generated as suggested above, but on the other hand, vessels with 

 tubular texture. This might be due to the fact that the fibres have tip 

 growth, whereas the vascular members have not; it would then, 



