252 



THE MAINTENANCE OF THE INDIVIDUAL 



oak leaf of ordinary size, while four or five hundred thousand to a 

 leaf is a common estimate. Surrounding the opening of each stoma 

 are found two kidney-shaped cells, the guard cells, which can easily 

 change their shape under certain conditions. They are of great 

 importance in the life of the plant, since they control to a great 

 extent the amount of moisture that may be lost from the leaf's sur- 

 face. The guard cells are noticeably greener than the epidermal cells, 

 the color being due to many tiny green chloroplasts. 



If the leaf is cut in cross section and examined under the microscope, 

 it will be found to be made up largely of a tissue known as mesophyll. 



Lying close to the epi- 

 dermis are one or two 

 layers of elongated cells 

 with the long axis placed 

 at right angles to the sur- 

 face of the leaf. These 

 layers of cells are collec- 

 tively called the palisade 

 layer. Each cell of this 

 layer contains numerous 

 chloroplasts which are 

 found in the protoplasm 

 close to the cell wall. It 

 has been estimated that a 

 square inch of a sunflower 

 leaf contains as many as 

 thirty million of these 

 chloroplasts, which are 

 most important structures 

 in the plant so far as food 

 making is concerned. 

 Below the palisade layer 

 is a layer of numerous irregular cells containing fewer chloroplasts. 

 These cells are known collectively as the spongy parenchyma. Be- 

 tween them are found air spaces connected with the exterior of the 

 leaf through the stomata. We have already noted that the veins 

 form the framework of the leaf and in a cross section are often found 

 occupying part of the area of spongy parenchyma. These veins 

 connect the vascular tissue of the root and stem with the leaf. The 

 petiole, or leaf stalk, is made up largely of vascular and supporting 



Cross section through a leaf; e, upper epider- 

 mis, e', lower epidermis, showing stomata (s) ; 

 I, intercellular spaces in the spongy parenchyma. 

 Note the cross section of the vein (v). Why is 

 the palisade layer (p) so placed ? 



