WATER-RELATION BETWEEN PLANT AND SOIL. 17 



two Cohus cultures, and from 10.9 and 15.5 to 7.3 and 9.1 for the two 

 Pelargonium cultures. Why the last-named pair of cultures should have 

 altered so markedly is not yet explainable, but it appears as though 

 the absorption rate of the plant must have exerted an influence. 

 Probably a larger number of irrigator cups (a larger irrigating surface 

 per unit of soil volume) would prevent this sort of occurrence. 



With such cultures, each supphed with a single irrigator cup and 

 grown under such temperature and Ught conditions as ours, it is safe 

 to suppose (see tables 2, 3, and 4) : that the soU-moisture content will 

 be found to be greater in the late night and early morning, and less in 

 the late afternoon and early evening; that the actual moisture content 

 will very closely approximate the mean for the entire day at two short 

 periods, one occurring before the maximum of water content and the 

 other after it; and that, for any day, the value of the maximum will 

 surpass that of the minimum by not over 2 per cent, the dry weight of 

 the soil being the basis for calculating the percentage of water content. 



This statement, that the plus and minus fluctuation in the soil- 

 moisture content of our cultures was always considerably less than 1 

 per cent of the dry weight of the soil, may be misleading, for the relative 

 magnitudes of such variations depend upon the actual mean water 

 content. It is therefore better to employ this mean as a basis for 

 calculating the percentage values. From the data thus derived (also 

 given in table 1), it appears that the moisture content of our soils 

 varied less than 5 per cent above and below the mean. 



In plant cultures where the maintenance of an approximately con- 

 stant soil-moisture content is requisite, this range of variation from the 

 mean of this condition, as here brought out, is probably practically 

 neghgible. There is no doubt that the auto-irrigator does maintain 

 soil-moisture conditions far more nearly constant than does any other 

 method of watering (aside from the growing of plants in saturated soil 

 or in water culture). Especially is this control of soil moisture satis- 

 factory when it is remembered what a short way has thus far been 

 traversed toward the control, or even measm-ement, of other environ- 

 mental conditions and of plant phenomena themselves. When plant 

 physiology and ecology become somewhat refined and when research 

 in these Unes becomes consciously and aggressively directed toward 

 permanent progress in scientific interpretation, then cultures may need 

 to be grown under more rigid control of soU moisture than that fur- 

 nished by the arrangement of the auto-irrigator as we have employed it. 

 For the present state of our science the accuracy of such an arrange- 

 ment is highly satisfactory. 



The nature of the device here under discussion makes it obvious that 

 it is susceptible of much greater refinement than has ever been put into 

 practice. There seems to be no doubt, for example, that our soil 

 masses woiild have been maintained much more constant in their water- 



