THE PHYSIOLOGICAL KOLE OF WATER IN PLANTS. 13 



(leteruiiuiiij^ moisture for tins purpose. The exact state of drought 

 whicli causes positive injury to the phiut should also be determiued. 



By watering- plants wilted to different degrees and at successive 

 l)eriods of vegetation it may be possible to establish morphnlogif^al 

 characteristics of the highest interest. It may thus be shown how .i 

 plant is affected under the influence of alternate drought and humidity, 

 and liow plants behave upon different soils, as well as what is the result 

 of permanent and excessive drought or humidity and of judiciously 

 combining drought and humidity. The author has conducted general 

 experiments along this line, and it may be of interest to discuss the 

 conclusions from this work in its agricultural l)eariugs. 



In order to apply these conclusions to agriculture it is necessary to 

 know what cultivated plant is to be especially studied — i. e., whether a 

 cereal, a forage plant, or a tuber-bearing plant. It is possible to deduce 

 a correct rule of irrigation based upon a curve showing the water 

 requirements of the plant under consideration for eacli period of its 

 growth, but there is a considerable difference between the necessary 

 water which a clay soil and a sandy soil must possess to ju-event plants 

 from drying out and dying from drought. In the author's experiments 

 it was found that clay soil requires more than 11 per cent of water to 

 prevent lupines from perishing, while 1 per cent suffices in a sandy soil. 

 The liygroscopicity of sand being practically nothing, the water which 

 is found in a sandy soil is all available for the plant. In other words, 

 in a clay soil there is from 10 to 11 per cent of moisture which is not 

 available to lupines, since about 10 per cent is retained by the hygro- 

 scopic action of the soil. For different i)hiiits this amount varies, being 

 11 per cent for lupines, 7.7 per cent for Erigeron, and 9.G per cent for 

 beans. These variations are as great for different soils as for different 

 plants. 



If K represents the capacity of the soil for water, a the hygroscopic 

 water of the soil, the maximum amount of water available to plants 

 per 100 gm. of soil will be Q=zK — <i. It is this value of Q whicli is of 

 grt at importance from a biological standpoint in experiments on the 

 role of water in the physiology of a given species of plant. It will be 

 found that a varies from to 1 5 and perhaps more. It is api)areiit 

 then that it is important to know the value of this factor for differeut 

 species of plants, since it determines so largely the value of Q. In 



general the soil should not be saturated, but ought to contain about 

 of K, and in rational irrigation the effort should be to realize the opti- 



nuim value of , which in this case is 0+«= — . As already observed, 



n n '' ^ 



the proportion of water in the soil which is not available to the ])lant 

 should be determined. 



In a locality which receives a certain average rainfall and is subject 

 to a i)eriod of drought, the same spe(;ies of plant will show different con- 

 ditions of growth according to the nature of the soil. In humid coun- 

 tries, such as Ireland, those soils which contain most silica are regarded 



