1923] Density of the Cell Sap of Plants 67 



A close correlation of plant succession with the underlying causal 

 factors shows that the succession is from xerophytism and drought- 

 resistant species toward greater mesophytism and species of lesser 

 drought resistance as the atmospheric and soil moisture increases and 

 produces greater stability and lower sap densities. It is evident that 

 the existence of a high osmotic concentration of the soil solution tend- 

 ing toward a physiological dryness of the soil would prove most detri- 

 mental to species of low sap concentration by cutting off the moisture 

 supply. Shallow-rooted species having low sap concentrations would 

 be the first to be adversely affected by periodic droughts. The depth 

 of penetration, lateral spread, general character of the root system 

 and volume of soil penetrated by the roots, as well as the capacity of 

 the species to absorb moisture, are all important in determining the 

 adaptability of the species to a given habitat. Likewise, these factors 

 influence the amount of moisture available to the plant. 



The zonation of plant associations and their alternation on oppos- 

 ing north and south aspects can be logically explained on the basis of 

 the effect of environmental conditions on the known physical proper- 

 ties of the cell sap of their component species, although occasionally 

 other factors may cause more or less sporadic occurrences. Such cases 

 of distribution can be traced often to those factors which offset un- 

 desirable environmental conditions during the seedling and sapling 

 stages, such as shade or other means of protection. 



These studies indicate that osmotic pressure also influences growth, 

 which is accelerated by weak soil solutions and retarded by concen- 

 trated ones. Both age and the conditions of growth influence the os- 

 motic pressure of the cell sap. Newly developed leaves showed lower 

 concentrations in midsummer than one year old leaves taken from 

 the same plants, all of which were growing in the same habitat under 

 identical exposure. 



In the course of a year the plant passes through a definite cycle of 

 changes directly dependent upon the moisture content of the soil and 

 atmospheric conditions which may materially influence sap concen- 

 trations. In the spring the concentration is uniformly low. With 

 the advance of the season and the advent of midsummer the supply of 

 available soil moisture is usually' very much depleted and transpiration 

 has been materiallj^ increased by the occurrence of the highest temper- 

 atures of the season and the greatest sunlight intensities. All of these 

 factors, together with a minimum of precipitation, unite to cause a 



