l(y PLANT GROWTH 



(transpiration, Chapter 13). Plants must have the carbon 

 dioxide and discharge oxygen but the loss of water seems to 

 be unnecessary, and when the soil is dry may be very danger- 

 ous. The stomata may close under such conditions, prevent- 

 ing the diffusion of all molecules. In this way the lack of 

 water in the plant prevents its food-making, and during a 

 prolonged dry period, may reduce materially the plant's 

 growth. 



A knowledge of stomatal distribution and action will be 

 seen to be important in a number of chapters but particu- 

 larly in Chapter 15 on spraying for protection against disease. 



The mesophyll of the leaf includes all the cells between 

 the upper and the lower epidermis, except the cells of the 

 veins. Most leaves have one or more layers of elongated 

 palisade cells next to and at right angles to the upper epi- 

 dermis. The remainder of the mesophyll consists of more 

 nearly spherical cells and is called the spongy tissue. 



The chloroplasts are small portions of the protoplasm 

 containing the complex green pigment called chlorophyll. 

 They are more abundant in the palisade cells than in the 

 spongy cells, but are always in the periphery of the proto- 

 plasm. Many leaves have more of them than the one in the 

 illustration. Although they are fewer in shade plants, they 

 are so numerous that the leaf appears to be a solid green, but 

 by looking at small sections with a microscope the individual 

 chloroplasts may be clearly seen. All the photosynthesis 

 takes place in the chloroplasts. 



Starch grains are the storage form of the sugar made in 

 photosynthesis and are abundant in the leaf while the sun 

 shines. At night much of the starch is changed back to sugar 

 and translocated to other parts of the plant. A leaf may be 

 tested for starch by removing the chlorophyll by dissolving 

 it in hot alcohol, and putting iodine solution on the colorless 

 leaf. If it has starch it will turn a dark purple color. 



The intercellular or air spaces of the leaf are very exten- 



