. Botany for agricultural students. Plants. Fig. 107. — Cells with protoplasm (p) shrunken to show the cell mem- brane, which is represented by the dotted line surrounding the proto- plasm. 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 p
. Botany for agricultural students. Plants. Fig. 107. — Cells with protoplasm (p) shrunken to show the cell mem- brane, which is represented by the dotted line surrounding the proto- plasm. 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 pigments at all, are called leuco- plasts. Starch grains and other 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 hast 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 cellulos
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectplants, bookyear1919
