CARBOHYDRATES 



277 



chief purpose of cell walls is to protect against loss of water and 

 the entrance of destructive organisms, then cutin or suberin, fatty 

 or wax-like substances, are formed and combined with cellulose. 

 Cutin is common in the outer walls of epidermal cells, being 

 especially noticeable in the rinds of fruits, such as apples. Suberin 

 is present in the walls of cork, as in the bark of trees and shrubs 

 and in the rinds of potatoes, turnips, etc. 



Very commonly pectose occurs in cell walls associated with 

 cellulose. The adjoining walls of adjacent cells are held together 

 by a thin layer, called middle lamella, which consists chiefly of a 

 mineral compound of pectose, the com- 

 pound being known as calcium pectate. 

 Pectose readily changes into pectin 

 which swells in water and becomes gela- 

 tinous, thus forcing cells apart and 

 bringing about the separation of cells as 

 in mealy ripe fruits. These gelatinous 

 pectins in the presence of sugar and 

 proper proportions of organic acids form 

 jelly and thus they have an important 

 connection with the making of jelly 

 from fruits. 



Other instances of mucilaginous sub- 

 stances occurring in connection with 

 cell walls are afforded by the Bacteria, 

 many Algae, some Fungi, the seed coats 

 of Flax and some Mustards, and the 

 endosperm of some Legumes. 



A form of cellulose, known as hemi- 

 cellulose, occurs in some seeds as a stor- 

 age form of food. It is readily changed 

 by enzymes to sugar in which form it is 

 transported and used as food. It is often called reserve cellulose. 

 It is stored as a thickening on cell walls, often the walls being so 

 much thickened with it that the cell cavities are almost closed. 

 (Fig. 249.) Usually it is an extremely hard substance and is 

 responsible for the hardness of Date seeds and Ivory nuts where 

 the cell walls are extremely thickened with it. It is present 

 in the seeds of Coffee, Nux Vomica, and a number of other 

 plants. 



FIG. 249. Cell walls of 

 Ivory nut, showing the ex- 

 treme thickening with hemi- 

 cellulose, which is deposited 

 in layers forming striations. 

 The walls are perforated by 

 the canals through which 

 the protoplasmic strends 

 pass. Highly magnified. 



