THE CASTOR BEAN 117 



floats on the surface of the water. The shape of the stomata 

 and guard cells varies slightly in different plants, but their 

 structure is always essentially like that seen in the Figures 

 58 and 59. 



In the middle of the leaf may be seen cross sections of the 

 veins, which are typical fibrovascular bundles (Fig. 57/6), 

 composed of essentially the same kind of cells that we have 

 found in the bundles of the stem. The rest of the substance 

 of the leaf is filled with a loose mass of cells which are the 

 active cells of the plant. Immediately under the upper epi- 

 dermis is a layer of slightly cylindrical cells forming a fairly 

 definite row. These are called the palisade cells; Fig. 57 p. 

 They contain minute granules (chloroplasts) of green coloring 

 matter called chlorophyll (Gr. chloros = green + phyllon = 

 leaf), cl, and each contains protoplasm and a nucleus. Below 

 the palisade cells are other cells more irregular in shape and 

 more loosely packed. In this part of the leaf these cells are 

 called mesophyll cells (Gr. mesos = middle + phyllon = leaf), w, 

 and their shape is so irregular and they are so loosely packed 

 that many air spaces communicating with the exterior through 

 the stomata are left between them. These mesophyll cells 

 are filled with active protoplasm and crowded with chloro- 

 plasts (Gr. chloros = green + plastos = molded) . The intimate 

 connection which these chlorophyll-bearing cells have with 

 the air that enters through the stomata is evident from Figure 

 57, and is a matter of extreme significance, since these cells 

 extract from the air the food from which the plant manufac- 

 tures starch, the first step in the production of food for all 

 animals and plants; see page 129. 



The epidermis of the leaves of some plants has various 

 other structures. Not infrequently it is prolonged into hairs 

 of various shapes and sizes; sometimes these hairs have a 

 little poison at their ends and then they constitute nettle hairs. 

 The general function of the hairs is to protect the plant from 

 injury by small insects and other animals. 



