292 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



however, have to be explained why in one case the extensibility of 

 the primary wall of tubular texture is utilized, while the cylindrical 

 cells of the vascular members follow the unconventional course of 

 growing in girth instead of in length. It is difficult to avoid assuming 

 the existence of internal formative forces when the purposiveness 

 inherent in each individual cell development becomes apparent again 

 and again. 



Intercalation oj wax. The discovery by X-ray of the intercalation of 

 wax has enriched our knowledge of the structure of the primary cell 

 walls. In young cotton hairs, A.vena coleoptiles without epidermis and 

 many meristematic tissues, Hess and co-workers (1936) found X-ray 

 interferences corresponding to periods of 60 and 83 A. By extraction 

 these substances were isolated and identified as vegetable waxes 

 (GuNDERMANN, Wergin, and Hess, 1937). They are comparatively 

 short chains of the type QHgn+jCO-O-C^^Hoj^ + j, n and m amounting 

 to about 24 or 32, as established for other vegetable waxes by Chib- 

 NALL, Piper, and co-workers (1934). 



As these waxes produce far clearer interferences than cellulose, of 

 which often only the fibre period appears, they must be assumed to 

 be better crystallized than the cellulose chain molecules. The possi- 

 bility therefore exists that waxes of this kind are in part the source 

 of the birefringence of the primary cell walls. Pursuing this problem 

 as presented by the meristematic cell walls of A.vena coleoptile, K. and 

 M. Wuhrmann-Meyer (1939) established that the birefringence is 

 affected by the fatty wax component susceptible of extraction by 

 pyridine. Though this effect is, admittedly, lacking in the radial 

 sections through the cells, it appears in the tangential and cross- 

 sections. From this it may be inferred that the rod-shaped wax mole- 

 cules are orientated at right angles to the microfibrils of the tubular 

 texture; then there is isotropy on the radial section, whereas on the 

 tangential and cross-sections we have a birefringence which is the 

 reverse in character of that of cellulose, as will be clear from Fig. 144. 



The waxes being extremely hydrophobic and the cellulose chains 

 very hydrophilic, there can be no direct contact between these two 

 cell wall substances, so that an intermediate, polar substance is in- 

 terposed (Frey-Wyssling, 1 93 yd). Possible molecules with hydro- 

 philic and hydrophobic end groups are phosphatides (Hansteen- 

 Cranner, 1926). Seeing that Thimann and Bonner (1933) found no 



