THE PLANT CELL 73 



in the protoplasm, and when the latter has been 

 reduced to a layer in contact with the cell wall the 

 nucleus will be moved toward this wall. In very- 

 old cells, such as the wood in the center of a tree, all 

 traces of protoplasmic contents are lost and the cells 

 are in reality dead. The walls and cavities of these 

 dead cells perform important functions as supporting 

 and transporting elements. 



The non-protoplasmic contents or inclusions com- 

 prise a large number of substances and these show 

 great variation both as regards chemical composition 

 and structural character. A majority of plants con- 

 tain starch, a typical inclusion; but the granules 

 of starch in different plants show material differences 

 in structure. The non-protoplasmic contents include 

 alkaloids, enzymes, starch, glucosides, sugars and fats. 

 In dealing with foods and drugs one seldom encounters 

 the nuclei or other protoplasmic contents; therefore 

 in microanalysis the term cell contents is generally 

 applied to the non-protoplasmic cell structures. 



Structure of the Cell Wall. — The walls of growing 

 cells are mere membranes composed of the substance 

 cellulose, but after attaining full growth the cells require 

 a stronger covering, and this is obtained by a thicken- 

 ing of the original membrane. Thickening of a cell 

 wall may be accomplished by the deposition of suc- 

 cessive layers of cellulose or other materials outside 

 the original membrane, or by the introduction of 

 substances between layers of cell wall already formed. 

 Thickening by deposition is the usual method and 

 is termed growth by apposition. Introduction of sub- 

 stances between layers of wall is termed growth by 

 intussusception. In growth by apposition the different 



