204 FACTORS AFFECTING TRANSPIRATION 



tube. In leaves in which the stomates are not sunken the diffusion column 

 is relatively short, amounting only to the diameter of the guard cells as seen 

 in cross section, while in sunken stomates the column of gas may be longer 

 by several times. The diffusion of gases through such stomates is undoubtedly 

 slower than through stomates of the ordinary type. This retarding effect of 

 sunken stomates upon the rate of transpiration as compared with the stomates 

 of the non-sunken type has been estimated by Renner (1910) to range from 

 30 to 70 per cent. 



5. Structure and Distribution of the Root System. — Under some en- 

 vironmental conditions an individual plant of a given species will develop 

 a greater leaf surface in proportion to the extent of its root system than 

 under others. The latter type of structural development will, other con- 

 ditions being equal, favor the maintenance of higher transpiration rates than 

 the former. 



The distribution of a root sj'stem in the soil sometimes influences rates 

 of water absorption and transpiration. In a habitat where deep-rooted and 

 shallow-rooted species are growing side by side the former may transpire 

 more rapidly during drought periods than the latter because their roots pene- 

 trate to a soil horizon which still contains available water, while roots of the 

 latter are in a moisture deficient soil. 



The completeness with which the soil mass is interpenetrated by roots 

 may also influence rates of absorption and transpiration. Thus Miller (1916) 

 found that sorghum plants have almost twice as many fibrous roots as maize 

 plants. Rates of absorption and transpiration can be better maintained, espe- 

 cially when the soil is relatively dry, by plants with the sorghum type of root 

 system than by plants with the maize type of root system. 



The Daily Periodicity of Transpiration. — All plants exhibit a daily 

 periodicity of transpiration rate which varies somewhat with the species and 

 is greatly influenced by the environmental conditions to which the plant is 

 exposed. We shall first consider the transpiration periodicity upon a "stand- 

 ard day" as defined in the preceding chapter, since most experiments upon 

 daily variations in transpiration rate have been conducted upon plants growing 

 under approximately such conditions. It should not be assumed, however, that 

 under natural conditions the daily periodicity of transpiration usually or "nor- 

 mally" follows the trend shown in Fig. 50. The daily periodicity of trans- 

 piration under these roughly defined conditions does, however, afford a con- 

 venient reference standard with which to compare the many variations which 

 are manifested by this phenomenon. 



The transpiration rate during the hours of darkness is generally low, and 

 in most species water loss during this period may be regarded as almost 



