. Botany for agricultural students . Botany. CELLULAR STRUCTURE OF LEAVES 251 places the chloroplasts around the cell wall where they are well exposed to light, and provides a large central vacuole which accommodates a large quantity of cell sap consisting of water in which sugar, carbon dioxide, oxygen, mineral salts, and other substances related to the activities of the cell are dissolved. (Fig. 233.) Through the layer of protoplasm, the outer border of which behaves as an osmotic membrane, the cell sap osmoti- cally pulls in water from the veins or surrounding cells, and in this way develop
. Botany for agricultural students . Botany. CELLULAR STRUCTURE OF LEAVES 251 places the chloroplasts around the cell wall where they are well exposed to light, and provides a large central vacuole which accommodates a large quantity of cell sap consisting of water in which sugar, carbon dioxide, oxygen, mineral salts, and other substances related to the activities of the cell are dissolved. (Fig. 233.) Through the layer of protoplasm, the outer border of which behaves as an osmotic membrane, the cell sap osmoti- cally pulls in water from the veins or surrounding cells, and in this way develops a pressme which dis- tends and gives rigidity to the cell. Its cells being rigid, the leaf is rigid and ex- panded to the light. That this pressure or turgor within the cells gives rigidity to the leaf is shown by the fact that leaves wilt when water is so scarce that the cells can not maintain their internal pressure. The chloroplasts, usually oval in shape in Flowering Plants, consist of two sub- stances. First, the chloroplast has a body which consists of cytoplasm denser than ordinary cytoplasm and known as a plastid. Plastids multiply by constrict- ing into two equal parts, and are as color- less as cytoplasm unless they develop pigments. Second, there is the chloro- phyll which is the green pigment that protoplasm (p) containing saturates the plastid, which is then known the nucleus (n) andchloro- as a hloroplastid or by the shorter term P^^^*^ ^"^^ \'^ ^^^ ^^'^^ 7 7 7 J T j-i. 1, 1 J. central vacuole. chloroplast. in the higher plants the chlorophyll is developed by the plastids and does not occur except in connection with these bodies. Plastids are common in all parts of the plant. In regions where they develop no pigments, the formation of starch from the sugar present is their chief function. They are even abundant in underground organs, such as fleshy stems and roots, which store starch. The presence of chlorophyll depends mainly upon exposure to ligh
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
