ELEMENTARY TISSUES 23 



thus permitting numerous intercellular spaces. It is the most 

 primitive and most abundant tissue in the plant kingdom. The 

 lower forms of plant life are composed entirely of parenchyma 

 tissue. The young embryo of the higher plants is composed 

 entirely of parenchyma cells, which undergo differentiation re- 

 sulting in the formation of the tissues found in the various parts 

 of the mature plant and described in this chapter. The growing 

 parts of a plant are always composed of parenchyma cells and 

 are known as the meristematic tissue or meristem (see page 

 27). The parenchyma tissue is found in many parts of the 

 plant, especially in the cortical region, the pith, the leaf, the 

 fleshy roots, the fruits and seeds and many other parts of the 

 plant. The edible parts of the plant are composed largely of 

 parenchyma tissue, their value as food depending on the thin 

 walls of the cells, the food stored in the cells and the absence of 

 injurious or objectionable substances. 



Collenchyma (Fig. 7). The cells of this tissue are usually 

 called the " thick-angled cells." They are derived from the par- 

 enchyma cells but differ from them by having somewhat thicker 

 walls, especially at the angles, somewhat smaller cavities, and 

 fewer and smaller intercellular spaces. They are frequently 

 filled with protoplasm and starch. They are much more regular 

 in size and arrangement than the parenchyma cells and are most 

 abundant in the stem and petioles of herbaceous plants, espe- 

 cially those with angular stems. They are found just beneath 

 the epidermal structures and are of some commercial import- 

 ance in certain medicinal plants. 



The collenchyma is a strengthening tissue and is very abun- 

 dant in some of the herbaceous plants. In many cases the paren- 

 chyma does not appear to serve any very important function ; but 



