32 THE PLANT CELL WALL 



In Lantana most of the silica is like that in the other species, 

 but also contains a form of silica showing positive double refraction 

 (to = 1.54, e = 1.55). The crystallographic character of the two 

 forms of silica has been established by X-ray diffraction analysis. 

 The common type of pattern is that of the amorphous Si0 2 . opal, 

 and is identical with geological deposits of diatom (biogenetic) 

 opal. The second component of the Lantana trichome gives the 

 X-ray diffraction pattern of a-quartz. Petrographic and X-ray 

 diffraction analysis are in complete agreement. 



Some comparative aspects of cell wall mineral substance are 

 yet to be considered, but it should be noted that a great deal remains 

 to be understood about the chemistry, physics, and physiological 

 significance of inorganic wall components. 



Although lipids are often present only in small amounts, they 

 are nonetheless important as a cell wall component. Their hydro- 

 phobic character is associated with water-proofing and perhaps 

 other protective functions. The lipophilic wall substances associated 

 with the cuticle constitute a quite diverse group from the structural 

 standpoint. Their location and the common properties which are 

 conferred by the long hydrocarbon chains and low proportions 

 of polar functions serve to place them together. 



Cuticular substances may be located by their affinity for lipid 

 dyes or basic lipid dyes. Micromelting tests may be used in conjunc- 

 tion with optical analysis in the differentation of cuticular substan- 

 ces. 



Chemically, the cuticle consists of substances soluble in benzene, 

 ether, pyridine and the like, together with more refractory materials 

 which may be solubilized only by saponification. The extractable 

 cuticular substance has sometimes been designated collectively as 

 "waxes". These substances actually consist of paraffins, aliphatic 

 acids, aliphatic alcohols and the monesters which are, properly, 

 the waxes. Both the alcohols and acids consist of the even-numbered 

 members of series lying between Ci 6 and C34. The paraffins consist 

 of odd-numbered unbranched members of the series C 2 7 to C 31 . 

 The saturated acidic components include palmitic C 15 H^COOH, 

 stearic, CnH 35 COOH, arachic C 19 H 39 COOH, cerotic C 25 H S1 

 COOH acids, and others up to C33H37COOH. The unsaturated 



