218 



THE STUDY OF LEAVES. 



Having examined a collection of leaves and noted the 

 foregoing points, we may now ask some questions. Why 

 are leaves flattened in form ? Bearing in mind the fact 

 that the leaf is the part of the plant whose special function 

 it is to react to sunlight and air, we may readily suggest 

 the answer : that the expanded leaf is a means of obtaining 

 a large amount of these. 



The limits to success with such an arrangement will be 

 clear when it is noted that a large leaf intercepting light 

 casts a large shadow which may cut off the light from 

 another part of the same plant. The arrangements of 



leaves upon plants to avoid 

 this should therefore be 

 considered, clear cases only 

 being taken in the first 

 instance. Some of these 

 arrangements have already 

 been incidentally noticed, 

 e.g. on the same buttercup 

 we saw that the leaves 

 lower down the stem have 

 longer stalks than those 

 above ; where vegetation is 

 thicker, as in a meadow, 

 the leaves are divided up 

 so that they are better 

 able to spread themselves 



out in spaces which are much divided. The lobing of 

 leaves in general and the existence of compound leaves 

 may be explained with reference to the same point (Fig. 86) . 

 On plants where the leaves are opposite the successive 

 pairs on the stem stand at right angles to each other so 

 as not to intercept the light, e.g. stitchwort, nettle, etc. 

 (Decussate). A. common arrangement of the leaves upon 

 the stem is a spiral one, so that there is ensured some 

 distance between leaves which stand directly over each 

 other (Alternate). Verify this, and note different types of 

 spiral, relatively close and open. 



An examination of trees and shrubs as well as herbaceous 

 plants from this point of view will reveal many instances 



PINNATE 



Fig. 86. Formation of Palmate and 

 Pinnate Compound Leaves by the 

 gradually increasing incision of 

 Lammas with Palmate and Pinnate 

 Venation respectively. 



