LOSS OF WATER 35 



stance, known as cuticle. The mesophyll, or parenchyma, 

 has two well-defined regions. In the portion next the 

 upper epidermis the cells are elongated, and arranged close 

 together at right angles to the epidermis. This portion is 

 the palisade layer. In the other portion of the mesophyll 

 the cells are of irregular shapes, and loosely arranged, with 

 intercellular spaces. Commonly, also, the palisade cells 

 contain more, and more deeply colored, chlorophyll grains 

 than do the cells of the spongy parenchyma. The above 

 facts make it clear why the upper surface of leaves is 

 darker green than the lower surface. Cross-sections of 

 veins are also seen, imbedded in the spongy parenchyma. 

 Details of their structure need not be considered here. 

 The absence of chlorophyll from the epidermal cells 

 (except the guard-cells) may also be noted. 



36. Stomata and Guard-cells. — In the lower epidermis, 

 sections of the stomata are found, and it is readily seen 

 that the stomata are tiny holes or pores through the epi- 

 dermis, connecting the intercellular spaces with the out- 

 side air. The guard-cells are so constructed^that under 

 changing conditions of light and moisture they may be- 

 come more or less turgid. When they become more 

 turgid, they are more convex, and thus enlarge the diam- 

 eter of the stoma; when less turgid, they become less 

 convex and this diminishes the size of the opening, and 

 in certain cases may even close it completely. By these 

 changes the passage of water-vapor or other gases through 

 the stomata may be either facihtated or retarded. 



37. Structiure of the Petiole. — The main function of 

 the petiole is to hold the leaf-blade well exposed to hght, 

 while, at the same time, keeping it connected with the 

 stem. It will have been noted already that the veins 



