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 bo 

 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 (Fig. 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 rosin weed 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 



