ADAPTATION TO LIGHT 



181 



ever the cause may be, it is evident that the elongation of the 

 stem is an advantage to plants that grow in diffuse Ught. This 

 holds for submerged as well as for shade plants. Upon a stem 

 wdth elongated internodes, the leaves interfere less with the illu- 

 mination of those belov/ them. This is also true of the branches, 

 which serve further to carry the leaves away from the stem and 

 from each other in such a way that the plant obtains the greatest 

 possible exposure of its leaf surface. 



Fig. 63. Shade and sun plants of Gaura parviflora, the former produced in 



a shade tent. 



Experiment 56. The production of adaptations to light. Construct a 

 series of shade tents 1-2 meters square and high. This may be done 

 in the greenhouse or out-of-doors by using wooden strips of 3-5 cm. to 

 make a framework of the size desired. The latter is divided into three 

 parts, and each is covered with cloth of the proper texture to give the 

 light intensity sought. It is desirable to have a scries of tents with light 

 intensities 0.1, 0.05, and 0.01 of the normal sunshine out of doors. These 

 values may be approximated by cheese-cloth, muslin, and duck, and after 

 a few trials may be secured almost exactly. Each tent is covered liy 

 tacking the proper cloth upon it. The interior walls are usually made 

 of the cloth belonging respectively to the second and the third lent. 

 On one side the cloth is not tacked, but is arranged to button closely 



