14 



PLANT PHYSIOLOGY AND ECOLOGY 



plete wilting, since the younger parts are able to draw upon t 

 watery tissues of stem and root for some time after the pla 

 is unable to obtain water from the soil. Three samples are aga 

 taken from the soil, and the average water content determine 

 as before. This is the non-available water or echard. This 

 likewise computed for 100 grams of dry soil, and the result 

 subtracted from the holard. The final result is the availab. 

 water, or chresard, expressed in the number of grams to 100 grarr 

 of dry soil. 



19. Chresard of habitats. In order to find the amount c 

 water available for a plant in its own habitat, it is necessary t 

 produce wilting by cutting off the water-supply. This is accom 

 plished by digging up a plant in its own soil and transferring 

 it to a pot of good size. The pot is placed in the hole from which 

 the plant is taken, and a canvas awning arranged to prevent 

 wetting by the rain. Where the distance of the habitat makes 

 this method difficult or impracticable, the plant is transferred 

 to the plant-house. In either case the soil samples are taker 

 as indicated above, and the chresard is arrived at in exactly th 

 same way. The available and non-available water of six rep 

 resentative soils, together with the amount necessary for satu 

 ration, are indicated in the following table. The figures serv( 

 equally well to indicate percentages and the number of grams ol 

 water for each 100 grams of dry soil. It must be clearly recog- 

 nized that these figures will not be exactly the same for every 

 kind of sand, clay, etc.: 



Experiment 2. Determination of available water. Germinate sun- 

 flower seeds in sand and in rich garden soil. Transplant the seedlings 

 from time to time so that they will be from 1 to 2 feet in height when 

 they reach a 6-inch pot. Find the holard and echard in the way in- 

 dicated, and from these compute the available water content of each 

 soil. 



