CHAPTER X 



The Mechanisms of Orientation and Growth 



TTa^P 



As A result of observations such as those mentioned above it is 

 L frequently stated in the literature that in growing cells the mole- 

 cular chains of cellulose align themselves almost perpendicularly to the 

 direction of highest growth rate. This generalization is clearly pardon- 

 able since the bulk of the cells investigated have in fact been elongating. 

 In one instance, however, the cells of the cambium, the cells increase in 

 radial diameter much more rapidly than they do in length (though this 

 increase is masked by repeated divisions). Unless such a case can be 

 regarded as a complete exception — and such a view is most repulsive — 



then it follows that a cell can in- 

 crease equally well either longitud- 

 inally or transversely and that the 

 form of the cell might therefore be 

 after all controlled by some factor 

 other than the molecular orientation 

 within the wall. 



It is interesting that the more re- 

 cent attempts to explain orientation 

 in growing walls have centred 

 around the effects of the shape of 

 cells on the tensions in their walls 

 induced by internal hydrostatic 

 pressures. The earher suggestion 

 of Denham that protoplasmic 

 streaming was involved in the align- 

 ment of wall particles (much as the 

 chains in cellulose solutions are 

 aligned in the manufacture of arti- 

 ficial silk by extruding through fine 

 jets) was adequately disposed of by 

 Martens, and the later modification 

 by van Iterson, though ingenious, proved no more successful (see 

 references in 4{b) and 57). There is today no serious consideration 



182 



Fig. 61. For explanation, see text. 



