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. : 
Thissis a task afconsid: nace ag (200 ce. ecad of Ranunculus sceleratus, 
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 
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 
