112 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



largest, however, the cells are too small to be examined singly by X-rays 

 without the use of a microcamera. Bundles of them can, however, be 

 pressed flat and photographed with the X-ray beam in any chosen 

 direction to the flattened faces and therefore to the wall surface, and 

 the resulting diagrams are exactly like those of Halicystis. The wall 

 structure must therefore be similar. How, then, does it come about 

 that these cells are cylindrical whereas those of Halicystis are spherical? 

 Here we meet the same difficulty as in the first group of algae, and again 

 must leave the matter until later. One point may, however, be noted 

 here. As mentioned above, when a piece of wall is stretched, a reorienta- 

 tion of the micelles is observed. When Hydrodictyon is growing, on 

 the other hand, no reorientation occurs although the cells increase in 

 length much faster than in girth. Here again therefore we have the 

 clearest evidence that growth in a cell does not cause a strain in the 

 wall such as to cause a reorientation. Indeed, this particular evidence 

 makes it very doubtful if the wall undergoes any passive strain at all. 

 Either these cylindrical cells, which are attached at each end to similar 

 cells, must grow by localized insertion of new wall material (com- 

 parable with the apical growth of Cladophora, for example) or, if the 

 whole wall is involved uniformly in the process, then growth is not just 

 a question of passive extension by internal pressure. These questions 

 will be considered further in a later chapter. 



