THE PRIMARY WALL OF GROWING CELLS 181 



US immediately (a) what causal factors underlie this ubiquitous trans- 

 verse orientation, (b) what happens to the orientation during growth? 

 We have already a partial answer to this last question since in the work 

 described above no attempt was made to choose any particular stage 

 in growth, so that it appears that the orientation is invariate. It is 

 nevertheless worth while to take up each point in turn since a good deal 

 of interest will emerge from the discussion. This discussion will, how- 

 ever, take us somewhat beyond the bounds of established knowledge 

 and is therefore best made in a separate chapter. 



