WALL STRUCTURE IN THICK CELL WALLS 101 



are less satisfactory, and even now only the first steps have been taken in 

 an approach to the problem. These will be considered in the next section. 

 It is interesting from this point of view to note, however, the peculiar 

 appearance in the electron microscope, of the innermost lamella of 

 Valonia. This is the lamella which has just been deposited and is still 

 in contact with the cytoplasm of the cell. It presents roughly the same 

 appearance both in formalin-preserved (42(Zj)) and in freeze-dried 

 (42(c)) material. The wall is covered by an amorphous mass pierced 

 with holes of various diameter and this can be interpreted as the cyto- 

 plasm. Through the holes, or thinner places, the underlying fibrils are 

 seen to be arranged at random, in marked contrast to the beautiful 

 regularity of the lamellae deeper in the wall. The most interesting 

 feature, however, is this. When this cytoplasm is swept away, isolated 

 patches still remain firmly attached to the wall. These are most clearly 

 in intimate association with the microfibrils; and one interpretation 

 would therefore be that they are "islands" of cellulose synthesis. This 

 would be in harmony with modern trends in biochemistry, for the need 

 for islands of synthesis in the cytoplasm is turning up again and again. 

 Such observations do not as yet do anything more than make the 

 problem of microfibril orientation still more obscure. It does seem, 

 however, that the microfibrils are produced first and are then oriented 

 afterwards. 



The filamentous algae 



One general point does apparently emerge from the observations 

 made thus far on Valonia in comparison with the condition already 

 found in fibres. These latter will be discussed in more detail later on 

 (p. 113 et seq.). At the moment let us put side by side the facts that in 

 fibrous cells the bulk, at any rate, of the cellulose chains lie almost 

 parallel to cell length, whereas in the bulbous Valonia there are two sets 

 of chains running almost at right angles to each other. Then it is 

 immediately clear that the walls of fibrous cells are much stronger 

 parallel to their length than at right angles to it while in the walls of 

 the alga the anistropy must be much less marked. Is this why Valonia 

 cells are balloon-shaped while fibres are long and thin? This is a question 

 often put by the uninitiated and we should perhaps pause a moment to 

 consider its implications. Notice that the question involves almost a 

 non sequitur. The Valonia cell is growing while clothed in a wall exactly 

 as described here; the fibrous wall we have looked at so far is the 

 secondary wall laid down after change in cell dimensions had ceased. 

 The comparison is therefore invalid and discussion must wait until 



