LEAVES 29 
(7) Water reservoirs are common in leaves and other 
parts of plants of dry regions, and while they may not be 
regarded as a protection against loss of water, they ac- 
complish the same purpose in storing it. Usually the water 
reservoir is a definite tissue, and in many leaves it may be 
distinguished from the ordinary green working cells by 
being a group of colorless cells (Fig. 24). In plants of the 
drier regions leaves may become thick and fleshy through 
acting as water reservoirs, as in the agave. In the cactus 
the peculiar stems have become great reservoirs of moisture. 
The globular body may be taken to represent the form of 
body by which the least amount of surface may be exposed 
and the greatest amount of water storage secured (Tig. 25). 
In the case of these fleshy leaves and fleshy bodies it has 
long been noticed that they not only contain water, but also 
have great power of retaining it. Plant collectors have 
found much difficulty in drying these fleshy forms, some of 
which seem to be able to retain their moisture indefinitely, 
even in the driest conditions. 
(8) Profile leaves are those in which the margin is 
directed upward, in this way standing edgewise. This po- 
sition is developed in connection with intense light, and 
results in turning away the flat faces of the leaves from the 
intense rays of midday and exposing them to the morning 
and evening rays of less intensity. In the dry regions of 
Australia the leaves of many of the forest trees and shrubs 
have this characteristic edgewise position, giving to the 
foliage a peculiar appearance. The most famous illustra- 
tion in this country of a plant with profile leaves is the so- 
called compass plant, a rosinweed of the prairie region. 
Its name was given because its leaves were said to point 
north and south, serving the purpose of a compass. It is 
evident that the plane of a profile leaf, exposing its faces to 
the morning and evening sun, will lie in a general north 
and south direction. It is a significant fact that when 
