THE PRIMARY WALL OF GROWING CELLS 171 



of any new structural principle. On the contrary, the species of mole- 

 cule built into the wall are precisely those found in secondary walls, 

 with one outstanding omission — lignin. It is largely the relative propor- 

 tion only of these substances which varies; and the structure of the 

 component molecules remains as far as one can tell the same, though 

 the relative disposition and the degree and type of orientation varies. 

 This is therefore no new realm into which we are venturing. It is rather 

 the same realm in a somewhat different guise and fraught with even 

 greater difficulties. 



The major obstacle to be overcome at the outset is to prepare the 

 tissues in a state in which they can be studied by methods described here, 

 without serious modification from the normal fresh condition. The 

 material actually observed is preferably, or even more frequently 

 inevitably, air-dried (for observation in the X-ray spectrometer) or 

 dried in alcohol (for observation under the polarizing microscope). 

 This latter is never absolutely essential, but since the precise interpre- 

 tation of observations made with a polarizing microscope often depends 

 on parallel observations made by the X-ray method, it is clear that most 

 of the weight of interpretation falls on dried tissue. With secondary 

 wall material this is no serious matter; one would not expect such rigid 

 and endurable bodies to undergo much extensive change upon drying, 

 and it has in fact been shown that such changes as desiccation produces 

 are relatively minor. With primary walls, on the other hand, it is con- 

 ceivable that drying will produce the most serious modification, and 

 until such effects are very fully appreciated it is obviously dangerous in 

 the extreme to be positive about any but the most general statements of 

 structure. 



With a mere chemical analysis, on the other hand, we are upon some- 

 what safer ground. The act of killing the cell by drying must, of course, 

 change profoundly the inter-relationships of the various types of mole- 

 cule present. Their amount and relative proportion, however, and to 

 some extent their relative disposition within the wall, can hardly be 

 materially affected. This is a point we might well take up before pro- 

 ceeding to a discussion of structure. 



The chemical nature of the primary wall 



It now seems without doubt that the bulk at least of all plants whose 

 secondary walls contain cellulose also develop this polysaccharide in 

 the growing cell, and that again this substance forms the framework 

 around which the other substances are laid. Nevertheless there remain 

 some peculiarities in this cellulose matrix. Some of these are structural 



