THE MECHANISMS OF ORIENTATION AND GROWTH 193 



zone) the growth zone is completely unstained. This type of observa- 

 tion implies that the production of new wall occurs only at the very tip 

 of the sporangiophore, in Stage IV therefore immediately below the 

 sporangium itself (62). Here, indeed, is the only locality in which the 

 "transverse" chitin chains are not associated with longitudinal chains, 

 as judged by the X-ray diagram (62). It is at the moment impossible to 

 be more specific, but it does seem highly probable that the insertion of 

 new wall material does not occur uniformly over the whole growing 

 region. 



Wall structure and cell shape 



An examination of the inter-relationship of growth and structure 

 from another point of view is even more striking. It has always been a 

 puzzle why cells which are approximately isodiametric develop into 

 long cylindrical forms. The advent of the new knowledge of structure 

 derived by the physical methods described in the previous chapters soon 

 pointed the way to a ready explanation of this phenomenon, an explana- 

 tion which is no doubt widely held today. We can see now that this 

 cannot be accepted except in a very general sort of way. With cells like 

 those in coleoptiles it is dangerously easy to conclude that, since the 

 chains of cellulose are oriented transversely, then the cell will naturally 

 enlarge more readily longitudinally, since the resistance to elongation 

 of cellulose is much less at right angles to the chains than parallel to 

 them. Even here a warning note is sounded, for in the cambium, which 

 has almost precisely the same structure, the bulk of the dimension 

 change is transverse, leading to the frequent longitudinal divisions. It 

 is very instructive to repeat an observation made some years ago by 

 Tupper-Carey and Priestley. If the bark of a tree is removed as shown 

 in Fig. 66 in such a way that the upper and lower parts are joined only 

 by horizontal strips, then the behaviour of the cambium in these strips is 

 most peculiar. The cambial cells, which are originally elongated in the 

 vertical direction, undergo first repeated transverse divisions cutting each 

 cell into a number of isodiametric cells. These then begin to extend in a 

 horizontal tangential direction so that after a time the cambial cells are 

 reoriented in a horizontal direction; they continue to produce tracheids 

 of the wood, but these are at first isodiametric and then elongated 

 horizontally. We have repeated and confirmed these observations on 

 several occasions, and it is obviously very difficult indeed to harmonize 

 such behaviour with the idea of cell form as a reaction to wall structure. 



The story in the algae is even clearer. With Hydrodictyon it is 

 difficult to see why the cell with such a wall fails to grow into a balloon. 

 13 



