64 PLANT CELLS 



vascular plants that are almost invariably present. Probably the most im- 

 portant of these is the almost universal presence of cellulose as the structural 

 framevv^ork of the wall. A second feature of plant cell walls is that the cellu- 

 lose seems invariably to possess a crystal-like structure. No plant cell wall, 

 however, is composed solely of cellulose. There is present in every wall 

 greater or lesser quantities of one or more other substances in addition to 

 cellulose. The most important of these are the pectic compounds, lignin, 

 hemicelluloses, cutin, and suberin. A third characteristic of most cell walls 

 is their lamellate structure. IVIost cell walls seem to be an aggregation of 

 numerous delicate lamellae of varying physical and chemical properties, all of 

 which are firmly welded together forming the superficially homogeneous wall. 



Origin and Development of the Cell Wall. — In the vascular plants the 

 division of a plant cell is preceded by the division of the nucleus. Nuclear 

 division is a complicated process involving organization of the nuclear reticu- 

 lum into unit chromosomes, splitting of those chromosomes, migration of one 

 of the chromosomes of each pair to opposite ends of the cell, and finally, re- 

 constitution of a daughter nucleus from each set of daughter chromosomes. 

 Just after the final stage ("telophase") of mitosis a membrane develops in the 

 center of the cell and extends its margins until it makes contact on all sides 

 with the existing cell wall, thus forming a septum which separates the proto- 

 plasts of the newly formed cells (Fig. 12). This first membrane, the 

 middle lamella, does not contain cellulose in quantities that permit its detec- 

 tion but seems largely, if not entirely, composed of colloidal pectic compounds. 

 During the enlargement of the cell a thin laj'er composed largely of cellulose 

 is deposited from the cytoplasm of each young cell on the two sides of the 

 middle lamella. This primary wall contains large amounts of pectic materials 

 in addition to cellulose and may vary in thickness and in physical properties 

 at different times of the year. Primary walls may become considerably thick- 

 ened as is the case in the collenchyma cells of young stems. As long as a cell 

 wall possesses a marked elasticity and exhibits reversible changes in thickness 

 or in other phj'sical properties it is classed as a primary wall. The walls of 

 cambium cells, parenchyma cells and collenchyma cells are all examples of 

 primary cell walls. 



After the enlargement of the cell ceases the deposition of cellulose on the 

 inner surface of the primary wall may continue until the wall becomes con- 

 spicuously thickened. When, as in most cases, this thickening is accompanied 

 by an almost complete loss of the elasticity of the wall the added material is 

 known as the secondary ivall. Secondary walls are further characterized by 

 the absence of reversible changes in thickness and by their low content of 

 pectic compounds. They are largely composed of cellulose which is frequently 



