CELL WALL 



115 



known as plastids, which are masses of cytoplasm but denser than 



ordinary cytoplasm. (Fig. 108.) They often develop pigments 



as in case of leaves, stems, and other green organs where they 



develop chlorophyll, the pigment upon which the green color of 



these organs depends. Plastids 



containing chlorophyll are called 



chloroplasts and are very impor- 



tant structures because they have 



so much to do with making plant 



food. Plastids which occur in 



the petals of some flowers have 



yellow or red pigments. Plastids 



which are colorless, having no 



F IG< 107. Cells with protoplasm 

 (p) shrunken to show the cell mem- 



pigments at all, are called leuco- brane, which is represented by the 

 plasts. Starch grains and other d ^ Une surrounding the proto- 

 small bodies (chondriosomes) not ] 



shown in our figure are also commonly present in the cytoplasm. 

 Cell Wall. The cell wall is formed by the protoplasm and 

 may be variously modified by it. In actively growing cells the 

 wall is thin and composed of cellulose a substance which allows 

 the wall to stretch as the protoplasm ex- 

 pands in growth. As the cell develops, the 

 protoplasm in many cases thickens the cell 

 wall by depositing new layers of material, 

 which may be of cellulose or of some other 

 substance better adapted to the function 

 which the cell is to perform. In nearly all 

 plants but in trees more especially some 

 cells deposit lignin in their walls, thus be- 

 coming the wood cells which give rigidity 

 to the plant and which we use in the form 

 of lumber. In the bark of trees, Potato 

 skins, and other structures for protection, 

 fat-like substances are deposited in the 

 walls of the cells which then are known as 

 cork. Sometimes, as in the so-called bast fibers, which are the 

 strengthening fibers especially prominent in Flax and Hemp, the 

 walls are extremely thickened with cellulose. The same is true 

 in Date seeds and Ivory Nuts where the walls are extremely 

 thickened with cellulose to be used as a food during germination. 



FIG. 108. Cell from 

 a leaf, w, cell wall; 

 n, nucleus; v, a large 

 vacuole in the cyto- 

 plasm; ch, chloroplasts. 



