156 



GREEN PLANTS AS FOOD MAKERS 



sawdust received their light by an indirect course. The young 

 seedling in this case responded to the influence of the stimulus 

 of light so that it grew out finally through the hole in the box into 

 the open air. This growth of the stem to the light is of very 

 great importance to a growing plant, because food making depends 

 largely on the amount of sunlight the leaves receive. 



Practical Exercise 6. Why do the leaves of lettuce or cabbage when 

 "headed" turn white? 



Effect of light on leaf arrangement. It is a matter of common 

 knowledge that green leaves turn toward the light. Place growing 



1 



1^* 



4f 



1 



Brootclyn Botanical Garden, N. Y. 

 Why are the leaves of these plants well arranged for obtaining sunlight ? Why do they need a 



great deal of sunlight ? 



pea seedlings, oxalis, or any other plants of rapid growth near 

 a window which receives full sunlight. Within a short time the 

 leaves will be found in positions to receive the most sunlight 

 possible. Careful observation of any plants growing outdoors 

 shows us that in almost every case the leaves are so arranged as 

 to get much sunlight. The ivy climbing up a wall, the morning- 

 glory, the dandelion, and the burdock, all show different arrange- 

 ment of leaves, each presenting a large surface to the light. Leaves 

 are often definitely arranged, each fitting in between others so as 

 to present their upper surface to the sun. Such an arrangement 

 is known as a leaf mosaic. Examples of such mosaics are seen on 

 trees having leaves that come out from the branch alternately, first 



