LEAVES. 63 
shrubs constantly turned to the light, and is said to be 
destitute of pores or stomata, or very scantily supplied with 
them. The other set of vessels proceeds from the lower 
surface of the leaf to the bark, and conveys to it the sap, 
now rendered fit for the nutrition of the plant. If the 
_ leaves of a branch be placed so that the upper surface be 
_ turned towards the earth, they will gradually resume their 
natural position: and if prevented from doing this, they 
will wither and die. 
218. The under surface abounds with stomata, through 
which the watery part of the sap passes off in vapour. In 
aquatic plants the upper surfaces of those leaves which float 
on water are provided with most stomata, for the purpose 
_ of evaporation, as this process cannot take place in water. 
- - 219. The upper surface of the leaf is smoother and,more 
_ shining than the lower, has seldom any down or hair, and 
few or no stomata. The lower surface is often downy or 
_ covered with hair, provided plentifully with stomata, of a 
220. The leaves of succulent plants, such as the Cactus 
and the Aloe, are provided with few pores, and evaporate 
very slowly, and may be considered as reservoirs of nutriti- 
_ ous matter for the use of the plant; in great heats, and in 
a dry soil, preserving its existence by their power of retain- 
ing the fluids which are absorbed. Plants which grow in 
_ dry and parched situations, where supplies of rain are very 
_ Searce, have leaves of this kind. The thin small leaves of 
_ the Fir are of an opposite nature, being provided with nu- 
_ Merous pores, and evaporating freely. 
221. The leaves, it has been supposed, are good radiators — 
of caloric, and hence become lower in temperature during 
_ the night, by which they condense the watery vapour inthe = 
atmosphere ; it is then absorbed in the form 6fdew. In 
_ grasses and other herbaceous plants, the leaves of which 
* 
