THE MATERIAL INCOME OF PLANTS 299 



Removal and alteratioivof the wall. The cell wall, formed by the 

 protoplast, is subject to partial or complete removal by it. In green 

 plants it is usually composed at first of cellulose; but pectic substances 

 early appear in it, and with increasing age it is subject to various modi- 

 fications, which alter its relation to water and thus profoundly affect 

 the conditions of life of the protoplast within. 



One alteration to which the wall is subject is known as cutinization, 

 because cutin is deposited or formed within it. Sometimes, as on the 

 outer face of superficial cells, this takes place to such an extent as to 

 form the cuticle, a layer which may be loosened and removed entire 

 from the rest of the wall. Parts of the outer wall adjacent to the cuticle 

 may also become impregnated with cutin to varying degrees. The 

 cuticle and these cutinized layers repel water, so that a minimum only 

 is found in the wall and little can pass through. 



By another modification portions of the wall may become gelatinous. 

 When wetted, they take up great quantises of water (sometimes as 

 much as 98 per cent of their wet weight) and swell so enormously as to 

 lose altogether their usual firmness. 



Again, the wall may become lignified, a condition characteristic of the 

 walls of woody tissues, whence the name. Lignified walls do not swell 

 so remarkably as gelatinized ones, but they allow water to pass through 

 them with comparatively little resistance. 



Water of the plant. From what has been said it is evident that water 

 forms an important part of the cell ; but it is necessary to comprehend its 

 intimate relations to every part in order to understand its full significance. 

 In ordinary land plants water constitutes always over one half and usu- 

 ally about three fourths of their weight. Of the least watery parts, such 

 as wood, it forms one half, and of the most watery parts, such as the pulp 

 of juicy fruits, as much as 95 per cent. In ordinary speech it is common 

 to indicate the general character of an object by naming its most abun- 

 dant component; as, a wooden table, a brick wall, wood and brick 

 being respectively the dominant but not the only material in the struc- 

 ture. If the water of the plant were visible to the eye, distinct from 

 the other constituent materials, on the same principle a plant might be 

 spoken of justly as water, held in form by other substances mingled with 

 it. This is quite the reverse of the ordinary conception, but its essential 

 truth becomes evident when we consider not merely the quantity of 

 water relative- to other constituents, but attempt to picture the relations 

 of water to the va.riou$ parts of the cell. 



