. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. mesophyll cells making up thebulk of the leaf serve mainly aswater-storage cells (Fig. 81). Leavesof this sort with large amounts of water stored against time ofneed are not infrequent in desert regions or where a rainy seasonis succeeded by a dry one. Conditions Affecting Photosyn-thesis. Light.—The amount of photo-synthesis varies with the light intensityup to and even beyond full a very feeble light is suffi-cient to sustain photosynthesis to aslight exte
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. mesophyll cells making up thebulk of the leaf serve mainly aswater-storage cells (Fig. 81). Leavesof this sort with large amounts of water stored against time ofneed are not infrequent in desert regions or where a rainy seasonis succeeded by a dry one. Conditions Affecting Photosyn-thesis. Light.—The amount of photo-synthesis varies with the light intensityup to and even beyond full a very feeble light is suffi-cient to sustain photosynthesis to aslight extent, and it may be that innature plants get as much usable energyfor this function out of the diffuse lightfrom all quarters of the sky as from thedirect sunlight itself. In dwellingswhere plants stand before a window theyare lighted by only a small part of the sky,and this rapidly diminishes the fartherback plants are placed, so that only a fewfeet from a window they may be actuallylosing in weight for lack of sufficient food construction, althoughthey may be growing and having the appearance of some FIG. 81.—Cross section ofa portion of leaf of Codo-nanthe, showing the water-storage tissue at /, and thechlorophyll-bearing tissues ate. (After Schimper.). CONDITIONS AFFECTING PHOTOSYNTHESIS 149 Time Required.—Engelmanns bacterium method shows thatphotosynthesis begins instantly on exposure to light. It is found,however, that it takes several minutes, and in some cases anhour or more before starch appears in the chloroplasts; and thisis evidence that this starch represents carbohydrate that is formedfaster than it is being carried away—a surplus that would hinderthe constructive process if allowed to remain in solution in thecell-sap. And the starch in the chloroplasts might be of directbenefit in increasing the chloroplastic surface and in refractingand reflecting the light so that more of it would be retained withinthe body of the chloroplast. Carbon Dioxide.
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