LOSS OF WATER 35 
known as cuticle. The mesophyll, or leaf-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 facilitated or retarded. 
37. Structure of the Petiole. The main function of 
the petiole is to hold the leaf-blade well exposed to light, 
while, at the same time, keeping it connected with the 
stem. It will have been noted already that the veins 
