WATER RELATIONS OF PLANTS 215 



dition when we consider the phenomena of growth at the expense 

 of this water. The water in the interior of a cactus which puts 

 forth flowers and produces fruit without any advent of moisture 

 from the outside, stands in the same relation to the transpiring 

 and growing tissues as does the actually external soil moisture 

 to the similar activities in a non-succulent form. In the scheme 

 below, internal conditions are denoted by an asterisk. 



The study of this whole subject of water relations forms an 

 excellent exercise in applying to organic phenomena the principles 

 of physical equilibrium and the general theory of physical causa- 

 tion . 



The requisite water content for the activities of any tissue 

 depends in general upon the following condition which may be 

 stated as a ratio, I = II : 



I. The rate of renewal of free water in the active parts must, 

 in general, be equal to 



II. The rate of removal of free water from the active parts. 



I. The rate of water renewal depends upon 



A.* The rate of liberation of water within the plant, which 

 depends upon: 

 1.* Rate of chemical formation of water, as by respi- 

 ration, depending largely upon temperature 

 (usually negligible) . 

 2.* Rate of physical liberation of water, as by cell 

 shrinkage, death, etc. (usually negligible). 

 B. The rate of water entrance from without, which depends 

 upon: 

 1. Power of the soil (or other water-supplying me- 

 dium) to deliver water, per unit of its cross 

 sectional area. This depends upon : 

 (1) Force of adhesion manifested between soil 

 particles and soil water (resistance to 

 water movement) , which depends upon : 



a. Size and form of soil particles. 



b. Nature of soil particles. 



c. Arrangement of soil particles. 



