XEKOPHYTE ASSOCIATIONS. 197 



periods, there are other habits by which a temporary re- 

 duction of surface may be secured. For instance, at the 

 approach of a period of drouth, it is very easy to observe 

 certain leaves rolling up in various ways. As a leaf be- 

 comes rolled up, it is evident that its exposed surface is 

 reduced. The behavior of grass leaves, under such cir- 

 cumstances, is very easily noted. A comparison of the grass 

 blades upon a well-watered lawn with those upon a dried-up 

 lawn will show that in the former case the leaves are flat, 

 and in the latter more or less rolled up. The same habit 

 is also very easily observed in connection with the larger- 

 leaved mosses, which are very apt to encounter drouth 

 periods. 



148. Fixed light position. — In general, when leaves have 

 reached maturity, they are unable to change their position 

 in reference to light, having obtained what is known as a 

 fixed light position. During the growth of the leaf, how- 

 ever, there may be changes iu direction so that the fixed 

 light position will depend upon the light direction during 

 growth. The position finally attained is an expression of 

 the attempt to secure sufficient, but not too much light 

 (see §13). The most noteworthy fixed positions of leaves 

 are those which have been developed in intense light. 

 A very common position in such cases is the profile posi- 

 tion, in which the leaf apex or margin is directed upwards, 

 and the two surfaces are more freely exposed to the morn- 

 ing and evening rays — that is, the rays of low intensity — 

 than to those of midday. 



Illustrations of leaves with one edge directed upwards 

 can be obtained from the so-called compass plants. Prob- 

 ably most common among these are the rosin-weed of the 

 prairie region, and the prickly lettuce, which is an intro- 

 duced plant very common in waste ground (see Fig. 170). 

 Such plants received their popular name from the fact that 

 many of the leaves, when edgewise, point approximately 

 north and south, but this direction is very indefinite. It is 



