EXPERIMENTAL EVOLUTION 



157 



In sunny habitats, the light intensity is most easily reduced by means of 

 cloth awnings, which can be put in place conveniently. It is not a difficult 

 matter to produce effective shade by using shrubs or small trees for this pur- 

 pose. This plan is especially advantageous in habitats too remote to make 

 frequent visits feasible. When a shrub or tree is used, the experiment nec- 

 essarily requires a longer time, though this disadvantage is partly compen- 

 sated by the fact that 

 the shelter requires 

 practically no attention 

 after the shrub is once 

 established. Forest 

 plantations furnish ex- 

 cellent examples of this 

 kind of experiment. On 

 the other hand, clear- 

 ings afford the only ex- 

 amples of habitats mod- 

 ified in such manner as 

 to increase the light. In 

 nature, the diffuse light 

 in which shade plants 

 grow is due to the pres- 

 ence of tall plants, 

 chiefly shrubs and trees, 

 and an increase in the 

 light intensity is possi- 

 ble only through the 

 thinning-out or removal 

 of the plant screen. 

 This is a task of consid- 

 erable magnitude in for- 

 ests, but it can be readily accomplished in thickets and at the edges of wood- 

 lands. It is quite practicable to establish a series of awnings or clearings of 

 various light values, but the labor required is hardly worth while when it is 

 recalled that the method of transfer makes it possible to take advantage of 

 the various intensities already found in nature. 



METHOD OF CONTROL CULTURES 



Fig. 48. Control ecad of Ranunculus sccleratus, 

 holard40^ (200 cc). 



195. Scope and procedure. Control experiments are necessarily 

 carried on in the planthouse, since factors can be controlled in the field 

 only with great difficulty. Their greatest value is in connection with 



