156 



THE PLANT 



origin of polyphyletic species. In this connection, it hardly needs to be 

 pointed out that this simple transfer of a species to several separated areas 

 of a new habitat may often furnish complete proof that a new form may 

 arise at different times, and at different places. 



194. Modification of the habitat. Efficient changes in the habitat are 

 brought about by increasing or decreasing the water-content, or by varying 

 the light intensity between sunshine and the diffuse light of deep forests. 

 Humidity can not well be regulated except in so far as it is connected with 

 water-content. Since its effects merge with those of the latter, its modifi- 

 cation is unnecessary. An increase in water-content is readily brought about 

 by irrigation. A stream may be dammed and its water allowed to spread 

 over the area to be studied, or the water may be carried to the proper place 



by deflecting the stream or 

 by digging a canal. The 

 construction of earth reser- 

 voirs makes it possible to 

 obtain almost any per cent 

 of soil water by varying the 

 size of the reservoir or the 

 height of the wall or bank. 

 Near a base station, such as 

 Minnehaha, where there is a 

 simple system of water- 

 ^vorks, the experimental area 

 may be watered whenever 

 desirable by means of a hose. 

 Water-content may be read- 

 ily decreased by drainage, or 

 by the deflection of a stream. 

 When such means are not 

 available, as in the case of 

 extensive marshes, hum- 

 mocks may be used or con- 

 structed, and the soil blocks, 

 containing plants placed up- 



Fig. 47. Control ecad of /Ranunculus scclcratus, 

 holard 10% (50 cc). 



on them. By the use of sand or gravel, the water-content of mcsophytic 

 areas can be reduced in a similar manner, or by surrounding the plant in situ 

 with either of these soils which hold little water. In meadows, especially, 

 the addition of a large quantity of alkaline salts decreases the amount of 

 available water, while the holard may be reduced by denuding the soil about 

 the plants concerned. 



