THE DAILY PERIODICITY OF TRANSPIRATION 205 



entirely cuticular. It is not justifiable to assume that absolutely no stomatal 

 transpiration occurs at night in any species unless this is definitely known 

 to be true. In some species of plants, as we have already seen, night 

 stomatal opening is a common occurrence, while in others it occurs under cer- 

 tain environmental conditions. Even in those species in which the stomates 

 are normally closed during the hours of darkness, a few may remain open. 



The transpiration rate shows a rapid rise during the early morning hours, 

 followed by a slower but consistent increase which culminates in a point of 

 maximum transpiration which occurs sometime during the mid-day hours 

 (11 A.M. - 2 P.M.). Following this peak of transpiration, the rate decreases, 

 usually consistently although often with an accelerated rate in the late after- 



.3.5 



Fig. so. 



Dail}' periodicity of transpiration in mullein {Ferbascum tliapsus). 



of Sayre (1920). 



Data 



noon, until the low and virtually steady night rate is attained at approxi- 

 mately the termination of the daylight period. 



The daily periodicity in the rate of stomatal transpiration in most species 

 of plants can be interpreted almost entirely in terms of two variable factors: 

 (i) the diffusive capacity of the stomates, and (2) the vapor pressure gra- 

 dient between the intercellular spaces of the leaf and the outside atmosphere. 

 In analyzing the daily periodicity of transpiration it is the diffusive capacity 

 of the stomates in the aggregate {i.e. all of the stomates on a plant) which 

 must be considered. The aggregate diffusive capacity of the stomates on a 

 plant on which all of the stomates were half open, for example, would be much 

 greater than that for a similar plant on which half of the stomates were fully 

 open and half fully closed. 



With the advent of daylight the stomates open over a period of time which 

 varies in length according to the species and with environmental conditions. 

 Some open earlier or more rapidly than others. The initial rise in the rate oi 



