THE CELL WALLS OF PLANTS 77 



lateral primary wall until it unites ^^^th the lateral middle lamella, 

 the small cavity in its margin enlarging as an intercellular space in the 

 midst of the intercellular substance. Deposition of cellulose meanwhile 

 continues on all sides of the two protoplasts, including the new middle 

 lamella, so that each new cell has its own continuous primary wall. It is 

 only after cell divisions have ceased that thick secondar}^ wall layers are 

 added. 



Although cellulose and pectin are the chief constituents of cell walls, 

 other materials are commonly associated with them in mature tissues. 

 Some of these, notably lignin and suberin, as well as the main constituents 

 themselves, vary greatlj^ in relative amount in the various layers of the 

 wall and in the walls of different tissues. They have pronounced effects 

 upon reactions to stains. Lignified cellulose, which ^ ^^ ^.^ 



contains a pentosan {e.g., xylan) and an organic sub- ^„___^^ — -^li^ 

 stance with an aromatic nucleus, stains vigorously • • I • 



with safranine or crystal violet, whereas unlignified ^' ""*" ^^1^; i^^^ 

 cellulose walls do not. One should not, however, / ;"~\\ ^ 



rely too strongly upon staining reactions as criteria — ("t^JPr; 1 



of chemical composition. Suberin, formed in special vT "l^ ?! 



abundance in corky tissues, is an aggregate of various '— — - --"5 



anhydrides and glycerides of certain organic acids. ^^*^- , 53. Dia- 



. ' . . gram of bordered pit 



Cutin is similar in composition and occurs mainly as in wall of conifer 

 an external coating on epidermal cells. Other or- ^°?,'^" ^'^•'^'"' ^" ^ " 

 ganic compounds, such as tannins, oils, and resins, 

 are also deposited in old cell walls and are largely responsible for the char- 

 acters of heartwood in trees. Mineral matter, including certain salts of 

 silicon and calcium, may also occur in wood, and the location of ash in 

 incinerated tissues indicates that such matter is restricted largely to the 

 primary wall layers. Among certain algae and fungi, chitin and keratin 

 occur as wall constituents. 



Minute Structure of the Cell Wall. — The results of chemical studies 

 and physical researches with X rays on the cellulose wall have shown 

 that it has a crystalline structure, i.e., it is composed mainly of units 

 arranged in a regular three-dimensional pattern. The primary unit Is 

 the anhydrous glucose residue, CeHioOs. Such residues are united by 

 primary valencies into long cellulose chains, and these are linked lat(M-ally 

 by secondary forces to form a regular space lattice. The intermolecular 

 cohesive forces result in the formation of larger groupings, or micelles. 

 The presence and the nature of amorphous materials between the groups 

 of cellulose chains are debated points. 



As a result of the regular parallel arrangement of the submicroscopic 

 chains in the pnmary and secondary cellulose wall layers, these layers are 

 anisotropic in contrast to the isotropic intercellular substance. This 



