Studies in Soil Physics. Ill 



elusions of the previous papers it is necessary only to recall 

 that in normal soils under usual conditions most of the water 

 exists in the form of films aiound the soil grains and in the 

 capillary spaces. It is this film water which forms the source 

 of supply for most of the useful plants. The cases in which the 

 roots of a crop actually reach the ground water are comparatively 

 rare. Nearly always this ground water must be raised by capil- 

 lary action before it can be useful to the plant. 



The Water K'^tiation of the Plant. Water is, of course, an 

 important material of plant food but in all ordinary land plants 

 the total water used is far in excess of that required for mere 

 nutrition. Water passes not only into but through the plant — 

 is both absorbed by the roots and "transpired" by the leaves. 

 We know fully neither the mechanism of absorption nor the 

 mechanism of transpiration and we know very little of the 

 mechanism of transfer from roots to leaves — the much discussed 

 problem of the' ' ascent of sap". But these gaps in our knowledge, 

 however regrettable, matter little to the present "discussion. We 

 need to know only that there w a water stream through the plant — ■ 

 and here is the important point — that its rate of flowjmay be limited 

 or restricted at two points: the point of exit and the point of 

 entry. The water equation of the plant is determined by two 

 rates, the rate of transpiration and the rate of absorption. 



Now these rates are not necessarily nor directly i elated. 

 They ma> vary almost independently and either may be larger 

 than the other. Obviously when transpiration is greater than absorp- 

 tion the plant is losing water and drying, while, when absorption 

 is greater than transpiration it is gaining water and swelling. 

 Obviously, also, neither of these unbalanced conditions can long 

 endure if there is to continue to be a plant. In one case it 

 will dry up and die and in the other it will swell up and burst. 

 Over extended periods of time, then, the summation of ab- 

 sorption must equal the summation of transpiration, plus 

 what water has actually been built into plant tissue. The 

 plant might be compared to a water reservoir, into which water 

 is running and from which it is being diawn. If the rate of 

 exit be greater than the rate of entry the reservoir will run dry; 

 if it be less it will run over. Either condition may be as- 

 sumed fatal to the proper functioning of the system To 



