THE CELLULAR STRUCTURE OF PLANTS 49 



infiltration of lignin or a similar substance made by the living 

 protoplasts of these cells. In these highly modified cells the pro- 

 toplast usually disappears after the cell wall is fully formed, and 

 they become dead cells in which the cell cavities are filled with air. 



The skeleton, or supporting framework, of plants is thus com- 

 posed of the cell walls of the individual cells making up the 

 plant body. In the case of the higher plants these cell walls be- 

 come thickened and hardened, as indicated above, and form a firm 

 supporting skeleton for trees and other plants of large size. This 

 skeleton of cell walls divides the living substance (protoplasm) 

 of the entire plant body into separate units, or protoplasts, each 

 protoplast being surrounded by its own skeletonlike cell wall. 

 This is quite unlike the condition found in animals, where the 

 cells often have no definite cell walls and are supported by an 

 internal body skeleton, as in higher animals, or by a crustlike 

 external skeleton, as in insects and crayfish. 



The plastids are minute granules of denser protoplasm which 

 occur in most living plant cells. They occur in the cell cytoplasm 

 and vary in form from minute granules to very elaborate bands 

 and disks, such as are found in some algae. Some plastids 

 are green, others are composed of colorless cytoplasm, while 

 still others are tinted with yellow and orange pigments. They 

 all agree in being composed of living protoplasm. They are 

 classified according to their color into chloroplastids, which are 

 green ; leucoplastids, which are colorless ; chromoplastids, which 

 are variously tinted. 



Chloroplastids are found in all parts of plants which have a 

 green color, such as leaves and the outer parts of some stems. 

 The green color in such organs is due to a green pigment, chlo- 

 rophyll, which is secreted within the numerous plastids con- 

 tained within the cells of such structures (Fig. 28, a and 5). 

 Each chloroplastid (5) is composed of a cytoplasmic granule, or 

 disk, and the green pigment which gives it its color. In such thin 

 leaves as those of mosses or of the water weed Elodea the plas- 

 tids are plainly visible under low powers of the microscope, 

 embedded in the cell cytoplasm. In Elodea the plastids are often 

 carried around the cells in the moving cytoplasm, like boats in 



