THE MECHANISMS OF ORIENTATION AND GROWTH 185 



remains cylindrical. Again, the alga Valonia never was other than 

 approximately spherical in shape and nevertheless it lays down cellulose, 

 with great facility, transversely oriented to the (somewhat vague) axis 

 of symmetry. 



Finally, under the third heading, we may note that in the elongated 

 cells discussed in Chapters VII and VIII, the secondary wall was 

 deposited also during the continued presence of hydrostatic pressure 

 in the cell; and nevertheless the orientation is far from transverse. 



60 



50 



40 



30 



20- 



10- 



. =i. 



Fig, 62. For explanation, see text (after Maas Geesteranus). 



Attractive though it is to attempt to correlate growth form with known 

 physical principles, this particular aspect must at the moment be aban- 

 doned, and the explanation of transverse orientation in growing walls 

 — and indeed of orientation in walls generally — must be sought along 

 other lines. Discussion of these may well be postponed until the impact 

 of growth on wall organization has been explored. 



The invariate orientation during growth 



The above discussion already makes it rather clear, and it has already 

 been mentioned briefly, that growth has remarkably little effect on 

 orientation. -This point has been investigated specifically by several 



