CHIEF ENVIRONMENTAL CONDITIONS. 119 



rapidly may moisture enter, providing, of course, that the maximum 

 rate of supply of soil-moisture to this surface is not surpassed. Also, 

 the rate of absorption must be markedly affected by the condition 

 of the absorbing membranes and the cells adjacent to them. If these 

 tissues are pathologically modified, as by the presence of poisons, 

 even a large extent of root surface may fail to allow as much water 

 entrance as might occur through a smaller root-system in a healthy 

 condition.^ 



The condition of the vessels in the stem, etc., whether well or poorly 

 developed, whether the lumina are large or small, and whether cross- 

 walls are frequent or not, is an important factor in determining the 

 maximum rate of water conduction with a given pressure gradient. 

 It will be remembered in this connection that the primary deleterious 

 effect of certain fungus growths within the vessels is due to a simple 

 stopping of these passages. In such cases the plant might suffer from 

 lack of water, although its roots possessed an adequate power of ab- 

 sorption and were in a soil of adequate water-supplying power. 



Of course, the causes of the internal conditions above mentioned 

 are to be sought in previously effective external and internal condi- 

 tions, as the effects of which any present status of affairs must be 

 considered; but this phase of environmental influence hes far beyond 

 the matters with which we are here concerned. 



3. RELATIONS BETWEEN WATER-REQUIREMENT AND WATER-SUPPLY. 



From the above consideration of the water-requirement and water- 

 supply of plants it is clear that growth and other activities are not 

 dependent upon either of these factors alone, but depend upon the 

 relation that holds between them. It is this relation which gives 

 the clue to all physiological and ecological problems concerning mois- 

 ture. So long as water moves into any tissue as rapidly as it is re- 

 moved, that tissue may maintain itself in a quiescent state; so long as 

 the possible rate of influx surpasses the actual rate of loss, the tissue 

 may increase in size and carry on any processes requiring the fixation 

 or destruction of water; and whenever the supply falls below the de- 

 mand ({. e., whenever the demand exceeds the supply), growth and 

 many other activities must cease. If the latter condition continues 

 long, partial or total death must follow, or at least the more or less 

 complete entrance of the organism into a state of dormancy. The 

 effect upon the plant is the same, whether the physiological lack of 

 water be brought about through an increase in the demand, through 

 a decrease in the supply, or through both of these acting together. 



* Livingston, B. E., Note on the relation between the growth of roots and of tops in wheat, 

 Bot. Gaz., 41: 139-143, 1906.— Livingston, B. E., J. C. Britton, and F. E. Reid, Studies on the 

 properties of an unproductive soil, U. S. Dept. Agric, Bur. Soils Bull. 28, 1905. — Livingston, 

 B. E., Further studies on the properties of unproductive soils, U. S. Dept. Agric, Bur. Soils 

 Bull. 36, 1907. 



