. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. 15° CONSTRUCTION OF PLANT'S FOOD the temperature must fall almost or quite to o° C; and cool- temperate, arctic and alpine plants continue photosynthesizing until they become frozen. In regard to the maximum tem- perature, plants in general cease photosynthesizing after long exposure to approximately 38° C. Photosynthesis in the Lower Plants.—In the simpler Algae where each individual consists of a single cell, a chain of cells forming a filament, or a thin lamina
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. 15° CONSTRUCTION OF PLANT'S FOOD the temperature must fall almost or quite to o° C; and cool- temperate, arctic and alpine plants continue photosynthesizing until they become frozen. In regard to the maximum tem- perature, plants in general cease photosynthesizing after long exposure to approximately 38° C. Photosynthesis in the Lower Plants.—In the simpler Algae where each individual consists of a single cell, a chain of cells forming a filament, or a thin lamina one cell thick, each cell contains one or more chloroplasts and carries on food synthesis. In Pleuro- coccus the chloroplasts are large in comparison with the size of the cell and seem nearly to fill the cell cavity. In CEdogonium and Nitella the chloroplasts are numerous and relatively small. In Spirogyra each cell has one to few chloro- plasts each in the form of a spiral band (Fig. 82). In these as well as in the higher plants the chloroplasts lie embedded in the cytoplasmic layer surrounding the vacuole and lining the cell-wall. In liverworts and mosses the photosynthetic tissue reaches a fair degree of differentiation. The thallus of Marchantia polymorpha has beneath its upper epidermis groups of chloro- plast-bearing cells that correspond to the palisade cells in the leaves of higher plants (Fig. 83). Each of these groups is con- tained in a diamond-shaped compartment as seen from the sur- face, the partitions at I being a single cell in breadth, and each compartment communicates with the outer air through an un- usually large stoma. Each group of photosynthesizing cells is. Fig. 82.—chloroplasts from different sources. A, Pleurococcus, with chloroplast at c; B, cell of Spirogyra, with spiral chloro- plast at c, nucleus at «, and pyrenoid at e. C, cross section of Spirogyra cell, with nucleus at n, and section of chloroplast and pyrenoid below. D, cell of CEdogoni
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