THE LEAF 63 
atmosphere is likely to be clear and the intensity of light to 
be very great. Under such conditions light sufficient for photo- 
synthesis would penetrate through a much thicker layer of 
tissue than would be the case with plants exposed to lower 
intensities of light. 
Number and arrangement of stomata. The rate of transpira- 
tion will naturally vary with the number of stomata. Plants 
growing in moist situations are likely to have more stomata per 
unit of area than those in dry 
regions. Some plants which . 
are natives of arid localities 
have stomata that are sunk 
in pits (Figs. 45, 46), while 
some which are found in moist 
regions have their stomata 
raised above the general level 
of the epidermis. More water 
will naturally be transpired 
through exposed than through 
protected stomata. 
Thickness and cutinization 
of epidermal walls. The chief 
function of the epidermis is to 
restrict the amount of tran- 
_ spiration by means of its thick 
and cutinized outer walls. The amount of transpiration will, 
of course, decrease as the thickness and the cutinization of the 
wall increase. Plants growing in dry situations have thicker 
epidermal walls than those in moist regions. Even on the same 
plant, leaves exposed to the sun have thicker epidermal walls 
than those in the shade (Fig. 35). Xerophytic plants may have 
not only thick epidermal walls but also an epidermis composed 
of more than one layer of cells (Fig. 45). 
Development of trichomes. Many leaves have hairs on their 
surfaces. These hairs are outgrowths from the epidermal cells 
and are frequently dead. If such dead hairs are numerous and 

Fig. 46. Cross section of a portion of a 
leaf of a century plant (A gave cantula), 
showing a sunken stoma. (x 170) 
