WATER RELATIONS OF PLANTS 217 



(2)* Extent of root system (amount of root 



surface) . 

 (3)* Structure of root system (kind of root 

 surface) . 

 3.* Rate of advance of roots through the soil (root 

 growth) . 



II. The rate of water removal depends upon: 



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

 depends upon: 

 1.* Rate of chemical fixation, as by photosynthesis 



(usually negligible). 

 2* Rate of physical fixation, as by growth (usually 

 negligible) . 

 B. The rate of water outgo from the plant. This depends 

 upon : 

 1.* Rate of water-loss by external secretion, as of 

 guttation, nectar secretion, etc. (usually neg- 

 ligible). 

 2.* Rate of loss of water vapor from aerially exposed 

 surfaces (transpiration) . This depends upon : 

 (1) The water extracting power of the aerial 

 environment, depending upon the cli- 

 matic, conditions of the habitat (see 

 I,B,1, (2), d), comprising: 



a. Evaporating power of the air, which 



depends upon : 

 a 1 Temperature. 

 a 2 Humidity. 

 a 3 Air movement. 



b. Influx of radiant heat to transpiring 



surfaces, which depends upon: 

 b 1 Intensity of sunshine. 

 b 2 Quality of sunshine. 

 b 3 * Radio-absorptive power of trans- 

 piring parts (color, etc.). 



