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PLANT PHYSIOLOGY AND ECOLOGY 



plants. The effect of diffuse light in causing stems to stretch 

 upward has been known for a long time, but the way in which 

 it is brought about is still unexplained. It is often stated that 

 the stem elongates to obtain more light, but the only basis for 

 this is that the stem grows in the direction from which the light 

 comes. This elongation is much more marked in the case of 

 plants grown in the dark, in which the height of the plant has 

 no bearing upon the amount of light that it can obtain. It has 

 been shown that the stretching of the stem is due to the excessive 



Fig. 62. Modifications of the stem in several forms of Androsace diffusa. 

 The rosette is an alpine plant growing at 3800 m.; the middle plant 

 grows in the open gravel at 2600 m. and the branched plant in the shade 

 at 2600 m. 



elongation of the parenchyma cells, but the cause of the latter 

 is in doubt. It is generally thought to be the absence of the usual 

 action of sunlight, which is assumed to be a retarding of growth 

 in sun plants. The evidence in favor of such a view is far from 

 conclusive. It seems probable that the elongation of the paren- 

 chyma cells takes place under conditions which greatly promote 

 the mechanical stretching of the cell wall, but prevent the normal 

 growth of the latter by intussusception. The fact that photo- 

 synthesis, and hence the amount of constructive material, is 

 greatly reduced in shade plants favors such an explanation. What- 



