CONCLUSION AND SUMMARY 63 



ation are capillarity, surface tension , and varying concen- 

 tration of the soil solution. 



The per cent of water in dying plants varies not unly with 

 the species, but also with the individual. Seedlings die 

 with a greater per cent of water in their tissue than older 

 plants, and they usually leave an increased amount of non- 

 available .water in the soil. Mature plants when dying con- 

 tain the minimum per cent of water. Shade plants as well 

 as those grown in humid air die with a higheT per cent of 

 water in both soil and plant. Dryness of atmosphere 

 modifies the growing plant so that it may live on a slightly 

 lower per cent of water in the soil. Rosette plants of xero- 

 phytic tendencies contain less water in their tissues when 

 dying than rosettes of hydrophytic tondencies, and also 

 may die at a lower per cent of soil moisture. The same 

 observation was made with regard to plants of both groups 

 with immature stems. 



B. NON-AVAILABLE WATER IN SOILS. 



Heat (and light) below or above the optimum for the plant 

 affect it unfavorably, decreasing its ability to withdraw 

 water from the soil during drought, increasing the non- 

 available water. 



Increased relative humidity of the air, when high, in- 

 creases the size of the leaves and retards the development 

 of the root system. Transferring a plant from a humid or 

 moist atmosphere suddenly into a comparatively dry one 

 hastens the death of the plant, especially when the soil at 

 the same time is deprived of its water supply. In both the 

 former and the latter case the plant dies at a higher limit 

 of non-available water. Excessive soil moisture during 

 the growth of a mesophytic plant tends to retard develop- 

 ment of the root system and to raise the limit of non-avail- 

 able water. Where mesophytes of strong hydrophytic ten- 

 dencies or where hydrophytes are placed in both dry and 

 wet soils the plants seem able to adapt themselves partially 

 to their new soil conditions and die at a lower limit of soil 



