Water-Content of Plants 



57 



It Avill be noticed that so soon as the amount of water 

 in ordinary soils becomes about three times the hygro- 

 scopic content it begins to assume physiological impor- 

 tance. A soil which contains merely hygroscopic moisture 

 is " air-dry " ; and if this amount only, or any amount 

 less than " available/' were present, the soil would actually 

 withdraw water from the plant, thus inducing drying-out 

 independent of transpiration. 



The following table, compiled from Hedgecock, includes 

 certain agricultural plants as well as species inhabiting 

 marshy and xerophytic conditions : - 



PLANTS GROWN IN LOAM, UNDER SIMILAR GREENHOUSE CONDITIONS 



PLANT 



UNAVAILABLE 



WATER 



Coleus (Coleus blumei Benth.) 



Morning glory (Ipomcea purpurea Roth.) . 



Cabbage (Brassica oleracea L.) 



Corn (Zea mays L.) 



Sugar-beet (Beta vulgar is L.) 



Wild rye (Elymus canadensis L.) .... 



Oats (Avena saliva L.} 



Asparagus (Asparagus officinalis L.) 



Lettuce (Lactuca sativa L.) 



Cucumber (Cucumis sativus L.) 

 Arrowhead (Sagittaria lati folia Wild) . 

 Pondweed (Potamogeton americanus C. and S.) 



3.0 



4.1 



5.8 



5.9 



5.9 



5.9 



6.2 



7.0 



8.5 



10.8 



15.6 



24.8 



It will be noted that plants normally inhabiting water or 

 swamp-land wilt first, and next to these are cucumber and 

 lettuce, both with high- water requirement and relatively 

 little structural protection against water-loss. 



