Chapter VI — 63 — Intracellular Distribution 



2) The primary wall, the first wall layer containing cellulose. This 

 wall is infiltrated by pectic materials, and is characterized by its ability to 

 undergo increase in surface area, and to exhibit elasticity, plasticity, and 

 reversible changes in thickness. It may also contain lignin. 



3) The secondary wall, a thickening layer deposited on the primary 

 wall. It is composed of cellulose, lignin, and sometimes other materials. 

 It is not capable of reversible changes in thickness. 



Both primary and secondary walls are believed to be composed of a 

 skeletal framework or matrix of cellulose forming a continuous system, the 

 interstices of which form another continuous system, of pores or micro- 

 capillaries (Bailey, 1938). Filling these may be found a variety of sub- 

 stances, namely water, pectic materials, lignin, hemicelluloses, cutin, suberin, 

 and sometimes small amounts of other organic or inorganic substances. 

 Resins, gums, tannins, callose, fats, oils, pigments, ethereal oils, proteins, 

 phospholipids, and salts of sodium, potassium, and silicon have been identi- 

 fied. Very young primary walls may contain protoplasm in the intermicel- 

 lar system (Frey-Wyssling, 1939). 



Most of the important advances made in recent years toward an under- 

 standing of cell wall properties have dealt with fibers of various origins — 

 wood, cotton, ramie and flax, materials with thick secondary walls and of 

 high cellulose content. Hence much of the information does not directly 

 relate to the problems of primary cell walls. 



Agreement has been reached that the ultimate units composing cellulose 

 are long chains of glucose residues, the so-called "molecules" of cellulose. 

 However, the number of residues in any chain has not been established with 

 certainty. Estimates vary from 50 to several thousand, or possibly the 

 number is indefinite. Due to unavoidable degradation of the cellulose in 

 preparing samples for particle size analysis, the more likely figure is doubt- 

 less in the thousands. 



It is further agreed that these chains or molecules are aggregated 

 parallel to one another into groups called micelles, but the manner of group- 

 ing is in question. One theory holds that the glucose chains form elongated 

 submicroscopic micelles. Another theory states that the chains are aggre- 

 gated into definite microscopically visible cellulose particles, separated by 

 infiltrating substances (Farr, 1944). Frey-Wyssling has proposed (see 

 1939) that the micelles are made up of very long slender cellulose chains, 

 which are arranged in a manner such that there are alternately crystalline 

 and amorphous regions. In the latter, the chains are not sufficiently close 

 or parallel to form a crystal lattice. Aggregates of micelles, termed fibrils, 

 may be observed under the microscope in many fibers. These are the small- 

 est visible structures in the cell wall. Their orientation within the wall may 

 explain many of the physical properties of cellulose. 



The forces binding water to cellulose have been considered as residing 

 in the amorphous regions of the micelle and in the intermicellar amorphous 

 material. The forces holding the glucose residues together in the chain 

 are strong primary valence (covalent) linkages between carbon and oxygen. 

 The forces binding the chains together in the micelles are hydrogen bonds, 

 the weaker attractions of OH dipoles, and the permanent electric moment 

 of the C-O-C groups (Mark, 1944). In the crystalline regions, the lattice 

 energy is such that water does not cause a separation of the chains. Swell- 

 ing consequently would be due entirely to the increase of the distance be- 

 tween the micelles resulting from the absorption of water by the amorphous 



