112 The Plant World. 



be useful the reservoir must be kept filled to a level which must 

 be, if not absolutely constant, at least variable only within 

 definite and lelatively narrow limits. Similarily in the plant, 

 transient excesses of transpiration over absorption or vice 

 versa are peimissable and harmless, long maintained differ- 

 ences are fatal. * 



Since, then, these rates of absorption and transpiration 

 are so important to the life and the health of the plant it is 

 expedient to review a little the factors which control them 

 even though, in the case of transpiration at least, these factors 

 are fairly well known and appreciated. The factors which 

 control the rate of transpiration are partly internal and de- 

 pendent u] on the plant, and ] artlv external and dependent 

 upon its environment. Of the former there are prob- 

 ably two, the mechanism of exposure (including the amount and 

 character of the surface of the internal spaces of the leaf, the 

 character of the cell membranes the nature and number of the 

 stomata, etc.) and the concentration (acting here through its 

 effect on vapor pressure) of dissolved substances in the cell 

 solution from which the evaporation is taking place. The 

 former or mechanical factors are probably pretty nearly con- 

 stant for any one plant. 



The external or environmental factors are the tempera- 

 ture and the humidity of the air and its rate of move- 

 ment — the wind. All t]:ese may be included in the one ex 

 pression "the evaporating power of the air." This 



it is which practicallv controls the transpiration of any given 

 plant. An increase in the concentration of the cell solution 

 does, it is true, lower the rate of evaporation (here transpira- 

 tion) but this lowering is small and seldom comparable with 

 changes produced by variations in the evaporating power of 

 the air. If this evaporating power be greater than that to which 

 the plant is adapted or to which it has been accustomed the 

 plant dies. 



The rate of absorption is also controlled by factors which 

 may be divided into those determined by the plant itself and 



*Tn actual fact these is usually a difference between transpiration and absorption and it is 

 usualy diurnaly reversed. The plant looses water during the day and gains it at night. 

 There are some few plants also which have considerable capacity for storing water and 

 are able to withstand long periods of excess transpiration by drawing on water which had 

 ))een stored during previous periods pf excess absorption, 



