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 CEdogonium, with numer- 

 ous chloroplasts. All highly magnified, but 

 not to the same scale. 



