10 THE TELL WALL 



logues in terminal meristems and other thin-walled tissue (e.g. 

 parenchyma ) . In contrast to the amorphous and ' ' structureless ' ' 

 intercellular substance, the primary wall "is also characterized 

 by possessing plasmodesmata which may be uniformly distributed 

 or aggregated in more or less conspicuous primary pit fields." 

 Primary walls are capable of readjustments and reversible 

 changes in thickness during tissue development, lailess special 

 technique is adopted (e.g. polarized light) the intercellular sub- 

 stance and its adjoining primary walls usually appear as a single 

 non-lamellated partition, especially in tracheary elements. Un- 

 der such circumstances, the term "compound tniddlc lamella" 

 may be applied to this complex of lignified layers. 



3. The sccoHdary wall, which is often extremely complex, both 

 chemicallv and phvsicallv, and which is normally the most mas- 

 sive of all the main layers of the cell Avail. Bailey and Kerr 

 (1935) use the term "secondary wall" to designate "the strongly 

 anisotropic layers of secondary thickening which are formed after 

 a cell has attained its final size and sliape." These investigators 

 strongly emphasize that in conti-ast to tiie primary or "cambial" 

 wall, the true secondary wall is incapable of undergoing rever- 

 sible changes in thickness. This is very often the case because of 

 the ultimate disintegration of the protoi)last of many cell types 

 which possess definite secondary walls, e.g. tracheids and fibers. 

 According to Bailey and Kerr, most tracheids, fiber-tracheids, and 

 libriform fibers in gymnosperms and angiosperms possess a three- 

 layered type of secondary wall. The inner and outer layers, 

 M'hich are of relatively constant thickness "exhibit strong double 

 refraction and are brilliant" when examined in transverse sec- 

 tion in j)olai-ized light l)etween crossed nicols. The middle layer, 

 on the conti-ary. "is dai-k or noticeably less bii-efi-ingent" and 

 fluctuates very widely in tiiickness. Not all secondary walls, 

 however, possess the above type of stratification, exceptions being- 

 furnished by cei-tain fibers and by the wall of the cotton hair. 

 Prom a physical-chemical standpoint, Bailey and Kerr conclude 

 1li;it the cen1f;il hiyei- of the secondary wall consists of an "ex- 

 tremely eomph^x and firmly cohei"ent matrix of cellulose" within 

 which "lignin" and a wide variety of other organic and inor- 

 ganic substances ina>- l)e deposited. The secondary wall of plant 



