82 Introduction to Botany. 



the lilac and horse-chestnut two leaves are borne at the 

 opposite sides of each node, each successive node having 

 its leaves at right angles to those of the node next below. 

 In corn and other grasses the leaves are in two opposite 

 rows, only one leaf occurring at each node. In the sun- 

 flower there are five rows of leaves, the rows being 72° 

 apart, and each leaf 144° from the one next below or above 

 it. Numberless examples could be brought forward to show 

 that leaves are arranged in a definite order — a fact which of 

 itself suggests that they are important members of the plant 

 body. Not only do they have definite places of origin on 

 the stem, but they grow outward from the stem at some- 

 what uniform angles with the horizontal — a fact which may 

 suggest to the student that they sustain important relations 

 with external forces. 



62. Light Relation of Leaves. — When we stand beneath 

 a tree we can see that the shaded portion of its crown does 

 not bear leaves, but only the better-lighted peripheral por- 

 tion. In carrying out Observation 79 we have noticed that 

 the direction assumed by leaves appears to have some direct 

 relation to the Hght ; for when the plant has been illuminated 

 more on one side than on another, the direction of those 

 leaves which are still capable of growth becomes changed 

 so as to expose their broad surfaces more nearly at right 

 angles to the greatest incident light. Our experiments 

 have taught us further that starch is formed in leaves ex- 

 posed to the light, but not in those kept in the dark. Finally, 

 when the leaves were stripped from a plant it attempted to 

 produce others, but, being prevented in this, it died. 



Such observations lead us to the conclusion that the defi- 

 nite arrangement of leaves on the stem and the more or 

 less constant angle which they make with the horizontal are 

 for the purpose of insuring that the light shall be impeded 



