46 BOTANY OF CROP PLANTS 



described above is typical of dicot leaves growing in situa- 

 tions with a moderate water supply. Water leaves are 

 thin and have no palisade tissue. Palisade tissue is also 

 absent in the leaves of grasses. The leaves of plants grow- 

 ing in arid situations are usually thick. The increased 

 thickness is commonly due to an increase in the number of 

 rows of palisade cells. Palisade may develop on both upper 

 and lower surfaces. Some leaves have palisade tissue from 

 epidermis to epidermis. The thickness of leaves growing in 

 arid conditions may also be, in part, the result of the de- 

 velopment of a very thick cuticle. 



The Work of Foliage Leaves. — ^Leaves are very important 

 organs of the plant. We are all familiar with the injury to 

 a plant that results from defoHation through any cause, or 

 from disease of leaves, or from their meager development. 

 We have learned to associate an abundance of bright green 

 leaves with plant vigor, just as we associate a rosy com- 

 plexion with health in people. And, with but few excep- 

 tions, we may judge of the health of a plant by its leaf 

 development and color. 



Carbohydrates are made by green plants only, and only 

 by those cells of green plants that possess chlorophyll. The 

 cells of roots and other underground plant parts, and all 

 those cells of the plant so far removed from the surface as to 

 be beyond the influence of light, do not have chlorophyll, 

 and hence, have no power of making carbohydrates. Other 

 than that in the relatively small amount of green tissue in 

 young stems and in the sepals of flowers, all the carbohydrate 

 of the plant is made in the chlorophyll-bearing cells of leaves. 

 The manufacture of carbohydrate by green tissue is called 

 carbohydrate synthesis or photosynthesis. When we realize 

 that carbohydrates form the basis of all the other more com- 

 plex foods of the plant body, such as fats, amides and 



