LEAVES AND THEIR WORK 



117 



Heliotropism. We saw that the stems of the plants kept in 

 the darkness did not always lift themselves erect, as in the case of 

 stems in the light. If seedlings have been growing on a window 

 sill, or where the light comes in from one side, you have doubtless 

 noticed that the stem and leaves of the seedlings incline in the 

 direction* from which the light comes. The tendency of young stems 

 and leaves to grow toward sunlight is called positive heliotropism. 



The experiment pictured on the preceding page shows this effect 

 of light very plainly. A hole was cut in one end of a cigar box 

 and barriers were erected in the interior of the box so that the seeds 

 planted in the sawdust received their light by an indirect course. 

 The young seedling in this case responded to the influence of the 

 stimulus of light so as to grow 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, as we shall 

 see later, food-making 

 depends largely on the 

 amount of sunlight the 

 leaves receive. 



Effect of Light. - 

 We have already found 

 that seedlings grown 

 in total darkness are 

 almost yellow-white in 

 color, that the leaves 

 are but slightly de- 

 veloped, and that the 

 stem has developed far 

 more than the leaves. 

 We have also seen that 

 a green plant will grow 

 toward the source of 

 light, even against 

 great odds. It is a matter of common knowledge that green 

 leaves turn toward the light. Place growing pea seedlings, oxalis, 

 or any other plants of rapid growth near a window which receives 



Tall straight stems of the hemlock ; the trees reach 

 up toward the source of light. 



