THE MECHANISMS OF ORIENTATION AND GROWTH 199 



no strain is thrown upon existing cellulose fibrils. The extreme apical 

 growth in Phycomyces which we have discussed above, and the similar 

 cases reported in the literature, suggest that in many cells this process 

 is strictly locaUzed. 



Some evidence for the localization of cellulose synthesis during wall 

 thickening has already been presented (42 {b) and {d)) but it is not, of 

 course, by any means certain that such considerations can be carried 

 over to the growing wall. 



There still remains to be considered the apparent independence, in 

 many cases, of the form of the cell from wall structure, and perhaps species 

 like Phycomyces give us a clue. If the making and breaking of bonds 

 occurs solely at one end of a cell then the cell will of necessity enlarge 

 into a cylinder. Whether or not the cylindrical form is adopted would 

 then depend on whether or not the hydrostatic pressure within the cell 

 is sufficient to cause the mutual displacement of the broken or separated 

 threads necessary for the insertion of new material. This would make 

 the deposition of cellulose, and its incorporation in the wall as part of 

 an organized structure, two rather independent processes. It is there- 

 fore with profound interest that we note the observation made recently 

 by Gorter(71) that treatment of root hairs with tri-iodobenzoic acid 

 inhibits elongation but does not prevent cellulose deposition at the tip 

 of the hair. 



It is not, however, possible to refer all cases to apical growth. To 

 take the two cell types studied mainly in this chapter, apical growth 

 sensu strictu has never been suggested for the parenchyma of oat coleop- 

 tiles* though a great deal of evidence has from time to time been brought 

 forward to show that in similar cells within the root the various parts 

 of the walls of any one cell do not grow simultaneously; and in cambial 

 cells, though apical growth has been adduced in order to explain features 

 of growth into which we cannot go here, there is no very clear evidence 

 on this point. Growth of this particular kind is not, however, essential. 

 It seems very unUkely indeed that all parts of the wall of a cell are under- 

 going the same processes simultaneously — for one thing, the ceU would 

 probably burst if that were so. It seems much more likely that the 

 breaking of bonds, the intercalation of new material, and the making 

 of bonds occur, in any one instant of time, only in isolated patches of 

 the wall. In the next instant, the processes will be taken up by other 

 patches, and so on. The form of a growing cell will then depend on the 

 spatial distribution of these patches, on the way in which the new 

 material is inserted (and therefore often, though not necessarily 

 * This suggestion has, however, now been made by Frey-Wyssling (73). 



