30 A TEXTBOOK OF BOTANY [Ch. Ill, 4 



visible, — each a compartment inclosed by a wall and con- 

 taining various contents. 



The chlorenchyma cells are inclosed by thin walls, and 

 contain three kinds of contents. Most prominent of all 

 are the chlorophyll grains, or chloroplastids, discoid in 

 form, and uniformly dyed by the chlorophyll, which does not 

 occur outside them. These chloroplastids have this great 

 importance, that they are the actual seats of the photo- 

 synthetic process. Within the same cells occurs also an 

 inconspicuous, shadowy-grayish, thin-gelatinous material 

 (shown by a sparse dotting in our picture), the protoplasm, 

 the living material which builds all the rest. The proto- 

 plasm, which contains the chlorophyll grains embedded 

 within it, forms in these cells only a lining to the walls, 

 against which it is held tightly pressed by the cell sap. 

 This sap is water containing sugar and other substances 

 in solution ; and not only does it fill the whole cavity of the 

 cell, but is ordinarily under tense pressure, sufficient not 

 only to hold the lining of protoplasm against the wall, but 

 also to keep the elastic wall itself somewhat stretched. 

 The chlorenchyma cells are variously shaped, — spheroidal, 

 ellipsoidal, ovoid, cylindrical, — as our picture shows. 

 The cylindrical shape prevails towards the upper surface, 

 where the cells occur tightly packed together, forming the 

 so-called palisade (as distinct from the spongy) tissue ; and 

 thus the greater part of the chlorophyll grains are brought 

 towards the best-lighted surface. This is the reason for 

 the familiar fact that most leaves show a deeper green color 

 on their upper than on their lower faces. 



When a vein is cut squarely across, as shown in our picture, 

 its cells appear angular, compact, and colorless. Three kinds 

 of cells appear in each vein. First, is an outer or sheath 

 layer forming the bundle-sheath, large and thick-walled 

 with thin protoplasmic lining. When seen in lengthwise 

 section they are found to be several times longer than wide. 

 They are most developed on the largest veins, thinner on 



