. Elementary biology; an introduction to the science of life. Biology. THE LEAF AS STARCH FACTORY yj 102. Work of the leaf. The work of food-making in the leaf has already been described (see p. 53), and we may now show the relation between the structure of the leaf and the details of this work. The cells containing the chlorophyl must get their income from the sur- rounding cells or from the sur- rounding air spaces. The water is brought up through the ves- sels of the wood (Fig. 23), and it passes through the cell walls by osmosis. The carbon dioxid is absorbed by osmosis from the air inside
. Elementary biology; an introduction to the science of life. Biology. THE LEAF AS STARCH FACTORY yj 102. Work of the leaf. The work of food-making in the leaf has already been described (see p. 53), and we may now show the relation between the structure of the leaf and the details of this work. The cells containing the chlorophyl must get their income from the sur- rounding cells or from the sur- rounding air spaces. The water is brought up through the ves- sels of the wood (Fig. 23), and it passes through the cell walls by osmosis. The carbon dioxid is absorbed by osmosis from the air inside the leaf, and this air is in direct communi- cation with the outer air by way of the stomates. The oxy- gen given off by the cells passes into the air spaces and diffuses from these to the ex- terior by way of the stomates. The skin cells are not di- rectly concerned in the work of starch-making; their func- tion may be described as protec- tive. In addition to protecting the delicate pulp cells against mechanical injury, they are even more useful in protecting X^. Fig. 24. Breathing holes of plants 1, stomates, or breathing pores, on the sur- face of a leaf, inclosed by the " guard ; 2, section through a leaf, showing an air space just inside the guard cells. Stomates are found in the epidermis of twigs as well as on leaves. As the stem grows tougher the breathing holes become larger and more irregular patches connecting the spaces be- tween the cells and the outside atmosphere. The roughened breathing spaces on the bark are lentkels. 3, lenticels on the bark of birch. (Microscopic views about x 200) the plant against the loss of water. That a great deal of water is lost by the plant through evaporation may be inferred from what we know about the evaporation of water from other wet surfaces (Fig. 24). 103. Transpiration. The loss of water is perhaps the most serious danger to which most plants are exposed, since more plants die from the results of wilting than
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