192 PLANT STUDIES 
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. 
137. 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 in 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 
