2 70 BOTANICAL GAZETTE [october 



closely interrelated. Accompanying the opening of the stomata 

 in the morning, there is a rise in the transpiration indices, and 

 their curves follow closely those of the factors just mentioned 

 until about ii:oo a.m,, when a marked divergence occurs. The 

 transpiration curve declines rapidly from a maximum at this 

 time, while the curves representing relative humidity, tempera- 

 ture, and evaporation continue to rise until about 2:00 p.m., 

 following which there is a gradual decline. This feature is little 

 in evidence in the graph of station A, but becomes more so in 

 those of B and C; that is, the divergence becomes more and more 

 conspicuous with an increase in exposure of habitat. This same 

 feature will be found to recur with greater prominence when we 

 consider the data of stations D and E on the open sand. 



There has been considerable discussion concerning the inter- 

 pretation of this sudden dechne: Lloyd (ii), following his inves- 

 tigation of stomatal action in Fouquieria and Verbena, concluded 

 that the earlier view that stomatal movement controlled the 

 transpiration rate was not entirely true, for there may be an 

 increase in the transpirational loss for some time after the maxi- 

 mum opening; and there was found to be frequent decrease 

 and subsequent increase in the afternoon, with Uttle or no 

 accompanying stomatal movement. Renner (13) advanced the 

 concept of "saturation deficit" to explain this behavior in transpi- 

 ration, and Livingston and Brown (9) in the following year 

 discussed this behavior under the "incipient drying" theory. 

 These are essentially the same concept, namely, that following 

 the rise in the transpiration rate which accompanies the stomatal 

 opening in the morning a point may be reached when the tur- 

 gidity of the mesophyll cells of the leaf becomes sufficiently 

 reduced to increase the surface tension of the water films in the 

 walls. A check in the molecular diffusion of water from the cells 

 follows. The increased concentration of the cell contents would 

 then exert additional pull on the water in the translocating system, 

 and a restoration of turgor may then follow without visible wilting 

 occurring in the leaf. The length of time elapsing before the 

 restoration of turgor depends largely upon the evaporating power 

 and temperature of the atmosphere, also upon the relative humid- 



