CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 39 



sion. Hence, the combination of frame and matrix is reminiscent of 

 a reinforced concrete wherein comparatively slender and flexible 

 steel rods of great tensile strength are buttressed by an amorphous 

 imbedding medium which has little resistance to extension forces, 

 but great resistance to compression. 



Among the imbedding substances, lignin is distinctive in many 

 ways, as we have seen. One physical property, a consequence of 

 a predominatly hydrocarbon constitution is its hydrophobic 

 character. Cellulose undergoes moderate to appreciable swelling 

 and shrinkage as its hydration water is varied. The interstitial 

 polysaccharides are less ordered than cellulose and can become 

 more hydrated. Consequently, these substances must undergo 

 extreme volume changes according to their state of hydration. 



The incrustation and interpolation of hydrophobic lignins 

 into the hydrophilic gel and onto the surfaces of cellulose micro- 

 fibrils introduces a structural element of comparatively constant 

 volume into the reticulum thereby providing the cell wall with 

 a dimensional stability to a degree otherwise impossible. This 

 property may be of greatest moment relative to the loss of water 

 from plant tissues during maturation and senescence. The effect of 

 lignins on staining, on interstitial free space and shrinkability and 

 its marked form double refraction all support the concept that 

 it provides a stable element of bulk in the wall. 



Although lignins are, quantitatively, the major hydrophobic 

 substances of the cell wall, young unlignified tissues and non- 

 mechanical superficial cells may contain cuticular substances or 

 other lipoidal materials. 



In the young primary wall with tubular texture, X-ray inter- 

 ferences corresponding to periods of 60 and 83 A have been iden- 

 tified with waxes intercalated into the microfibillar system. Rod- 

 -shaped wax molecules are oriented at right angles to the micellar 

 strands of the wall. As in the case of lignin deposition, these ex- 

 tremely hydrophobic molecules mask the polysaccharides, particular- 

 ly the individual cellulose strands. In cutinized walls, the outermost 

 material is cutin alone, but most of the cuticular layer consists of 

 hydrophilic lamellae of cellulose and pectin, layers of radially 

 arranged wax, and between them, randomly arranged cutin. Cutin 



