50 



PLANTS AND SUNLIGHT 



85. Day and night positions of the 

 clover leaf. 



cause their internal structure is such that light is most 



efficient when it strikes this surface, as we shall learn later 



on. Some plants, how- 

 ever, expose both surfaces 



to the light, and their leaves 



stand vertical. Others 



avoid the intense light of 



midday and turn in the 



direction of least light. 



Leaves standing edgewise 



are said to exhibit -polarity. 



They are "compass plants" if they point north and south. 

 The famous compass plant or silphium of 

 the prairies, and the wild lettuce (Fig. 86), 

 are examples of plants having polar leaves. 

 Every leaf has a story to tell of the value of 

 sunlight. 



117. Winter Buds Show What Has Been 

 the Effect of Sunlight. — Buds are borne in 

 the axils of the leaves (87), and the size or 

 vigor of the leaf determines to a large extent 

 the size of the bud. Notice that, in most 

 instances, the largest buds are nearest the tip. 

 (Fig. 87.) If the largest ones are not near the 

 tip, there is some special reason for it. Ex- 

 amine the shoots on trees and bushes. 



118. The largest buds usually start first 

 in spring, and the branches that arise from 

 them have the advantage in the struggle 

 for existence. Plants tend to grow most vig- 

 orously from their ends. Observe that only 

 the terminal bud grew in the hickory twig 



86 ieuuc d e?a r weed k ^ in Fig. 64. If the side buds or lower buds 

 wMte ld pia a ce^ grew more vigorously than the end buds, the 

 l^ss Tert^aL 6 ° r plant would become exceedingly branched 



