148 



CONSTRUCTION OF PLANT'S FOOD 



Fig. 8o. — Cross section of a 

 portion of a leaf of Indian corn. 

 a, upper, and b, lower epidermis; 

 c, c, palisade cells; n, border 

 parenchyma containing starch 

 within its chloroplasts ; e, vas- 

 cular bundle; d. d, stomata. 



but the parenchyma sheath on bright days is well filled with it 

 (Fig. 80). Of course the palisade cells manufacture soluble 



carbohydrate abundantly, and the 

 starch in the sheath cells doubtless 

 represents a surplus that comes to 

 the sheath cells from the palisade 

 tissue faster than it can be conducted 

 away. 



In the Agave, Codonanthe, etc., 

 the leaves are very thick and palisade 

 parenchyma sometimes stands against 

 the epidermis on both sides, while 

 the mesophyll cells making up. the 

 bulk of the leaf serve mainly as 

 water-storage cells (Fig. 81). Leaves 

 of this sort with large amounts of water stored against time of 

 need are not infrequent in desert regions or where a rainy season 

 is succeeded by a dry one. 



Conditions Affecting Photosyn- 

 thesis. Light. — The amount of photo- 

 synthesis varies with the light intensity 

 up to and even beyond full sunlight. 

 Nevertheless a very feeble light is suffi- 

 cient to sustain photosynthesis to a 

 slight extent, and it may be that in 

 nature plants get as much usable energy 

 for this function out of the diffuse light 

 from all quarters of the sky as from the 

 direct sunlight itself. In dwellings 

 where plants stand before a window they 

 are lighted by only a small part of the sky, 

 and this rapidly diminishes the farther 

 back plants are placed, so that only a few 

 feet from a window they may be actually 

 losing in weight for lack of sufficient food construction, although 

 they may be growing and having the appearance of some thrift. 



Fig. 81. — Cross section of 

 a portion of leaf of Codo- 

 nanthe, showing the water- 

 storage tissue at /, and the 

 chlorophyll-bearing tissues at 

 e. (After Schimper.) 



