92 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



know something about the relative amounts of the cellulose which is 

 in the crystalline ("miCellar") form. 



It is not, of course, always easy, or even possible to make all these 

 determinations, and attention will often be confined to (1) and (2). 

 This is perhaps more particularly the case when investigating specifically 

 the inter-relationships of wall structure and growth processes. Even 

 here, however, observations of angular dispersion and micelle size also 

 yield information of very considerable value. 



Although chronologically the elongated cells of the higher plant have 

 priority, it is in many ways more convenient to begin our survey of wall 

 structure with the algae. As we have seen already, one algal cell — that 

 of Valonia — was in fact used early in the development of modern ideas 

 on cellulose structure, but it was not until 1937 that any serious attempt 

 was made to investigate the algae for their own sake and for the light 

 they may throw on problems arising in the higher plants. The advan- 

 tages of studying the algae first are obvious. Here we have a wide 

 variety of cell types, ranging from the unicellular forms with almost 

 spherical cells, like Protococcus, through simple filamentous forms like 

 Spirogyra and Chaetomorpha and branched filaments as in Cladophora, 

 to the more complicated structures found in, for example, Codium. 

 Many of the species available must, however, be ruled out here since 

 cellulose is not present in the walls; we shall concentrate only on those 

 which do carry this skeletal substance. 



All these cells grow with little, if any, interference from neighbouring 

 cells, and studies of the interconnection between growth and wall 

 organization can be made here without the complexities involved with 

 cells growing in tissues. This in the main forms the reason why the 

 algae will prove of importance; not that they are without interest of 

 themselves. Far from this, for they are a group of plants with extra- 

 ordinary fascination, with a beauty of structure, even as revealed up to 

 date, equal at least to any found in higher plants; and with intrinsic 

 problems at least as interesting. It is clear that unless the growth 

 forms in algae can be explained, studies of higher plants rest on a very 

 infirm foundation. Since the group includes the largest single cell 

 known, and this has been studied in great detail, it will be as well to 

 consider this first. 



Valonia 



This alga is a native of the warmer seas and three species have been 

 studied more or less intensively — V. ventricosa, V. macrophysa and 

 V. utricularis (Fig. 34). Although these look rather different at a casual 



