PROTECTION 249 



opposite side remain turgid, the petiole will tend to bend 

 toward the flaccid cells. This is a striking example of the 

 way in which the condition of the protoplast affects turgor. 

 It is the protoplast, of course, which is the sensitive part of 

 the cell and receives the stimulus. The stimulus appears 

 to cause quick alterations in the protoplast, and these in 

 turn cause those physical changes which result in the 

 movement. 



C. The Edgewise Position. — Leaves which are cross- 

 wise to the light are so much more conspicuous than those 

 which are edgewise that it is natural to think of all leaves 

 as exposing themselves to light as much as possible. One 

 has only to visit a swamp, however, to realize that very 

 many plants have their leaves set edgewise to the sun, 

 edgewise at least when the sun is high in the heavens. The 

 cat- tails and flags which are common in swamps have their 

 sword-hke leaves set with the points straight up. Even in 

 a meadow or on lawns you may note that' the blades of 

 grass grow so close together that it is usually impossible 

 for them to be crosswise to the light. 



It appears that, where there is an abundance of Hght, 

 leaves tend to avoid the intense light of midday. Intense 

 light often seems to be too much of a good thing. It 

 has been observed that excessive light produces injury to 

 the chlorophyll. A thing which is absolutely essential for 

 photosynthesis may thus sometimes interfere with it. In 

 the open plants get more hght than they need, and the 

 supply of carbon dioxide and the temperature are the 

 things which, under this condition, chiefly limit photo- 

 synthesis. 



The edgewise position results in the loss of less water by 



