128 



BOTANY 



tend to droop or fold after a much shorter exposure to the dark 

 ness. These changes in position have been called sleep move- 

 ments. Charles Darwin suggested that the leaves of a plant which 

 take this position secure protection from injury by frost. In 

 most cases the movement may be accounted for when we remem- 

 ber that leaf blades naturally turn their upper surfaces toward the 

 light. In the absence of light the leaf blade might easily be in- 

 fluenced to droop or even fold by the counter stimulus of gravity. 

 Sensitiveness to Contact. — Leaves of some plants are also sensi- 

 tive to the stimulus of contact. The sensitive plant (Mimosa 

 pudica) is the best-known example. Here the leaflets of the com- 

 pound leaf respond to the slightest touch by folding and drooping. 

 Under normal conditions the leaf soon returns to its original posi- 

 tion. Changes in temperature and moisture may totally prevent 

 this movement, showing that the living matter in the plant is 



most delicately attuned to the 

 influences of its immediate 

 surroundings. 



The Sun a Source of En- 

 ergy. — We all know the sun 

 is a source of most of the 

 energy that is released on this 

 earth in the form of heat or 

 light. Solar engines have not 

 come into any great use as 

 yet because fuel is cheaper. 

 Actual experiments have 

 shown that vast amounts of 

 energy are given to the earth. 

 When the sun is in the zenith, 

 energy equivalent to one hun- 

 dred horse power is received 

 by a plot of land twenty-five by one hundred, the size of a city lot. 

 Plants receive and use much of this energy by means of the leaves. 



In which Part of a Variegated Leaf does Starch Exist P — Leave two plants, 

 such as the Coleus (variety with red-colored leaves) or Tradescantia (the 

 white-striped leaf variety) and another Coleus or Tradescantia with the 



A hydrangea plant, upon the leaves of which 

 disks of cork have been pinned in order to 

 exclude sunlight from the leaf. 



