1<)2 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 droutli, it is very easy to observe 

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

 comes rolled np, 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 niwn 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 

 jieriods. 



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

 reached maturity, they are unaljlc to change tlieir 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 attenqit to secure sufficient, Ijut not too much light 

 (see S13). Tlic most noteworthy fixed positions of leaves 

 are tlKjse wliicli liave l)een developed in intense light. 

 A vei-y c(immon ])osition in such cases is the profile jiosi- 

 tiou, in wliich the leaf apex or margin isdirccteil upwards, 

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

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

 than to those of ntidday. 



Illustrations of leaves with one edge directed ujnrards 

 can be obtained from the so-called compass jjlants. I'rob- 

 al)ly most common among these are the rosiu-wced of the 

 |ii'airic region, and the prickly lettuce, which is an intro- 

 duced plant very common in waste ground (see I'ig. 170). 

 Sufdi ]ilants received theii' jioiiular name from tlu' fact that 

 many of tin; leaves, when edgewise, point approximately 

 north and .south, l)ut this direction is verv indefinite. It is 



