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 chlorophyil 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 — py¢. 107. — Cells with protoplasm 
which are colorless, having no (p) shrunken to show the cell mem- 
pigments at all, are called leuco- brane, which is represented by the 
plasts. Starch grains and other ‘otted line 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 
Fic. 108.—Cellfrom © the plant and which we use in the form 
a leaf. w, cell wall; Of lumber. In the bark of trees, Potato 
n, nucleus; v, a large skins, and other structures for protection, 
vacuole in the cyto- fat-like substances are deposited in the 
plasm; ch, chloroplasts. 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 celiulose to be used as a food during germination. 
