58 THE PHYSIOLOGY OF STOMATA. 



DISCUSSION. 



According to the data above presented, the inference may be drawn that the 

 movements of the stomata are not the causes of the recorded whole differ- 

 ences in the rate of transpiration. Such part of each change of rate in partic- 

 ular as might be referred to the change in stomatal dimensions is so small 

 that it becomes negligible in its effect on the economy of water by the plant, 

 and a regulatory effect on the transpiration by stomata in Fouquieria and 

 Verbena amounts to nothing. 



One of the commonest and at the same time least understood phenomena of 

 transpiration is the fluctuation in the rate in successive short periods. They 

 may be observed only when the periods are brief enough ; they are, However, 

 readily seen to occur when hourly observations are made. When the periods 

 are reduced in length to a few minutes, this constant variation in rate, though 

 involving only small absolute amounts, becomes very striking. When the 

 periods between the hours of observation become longer, the fluctuations are 

 gradually lost sight of, until only the general diurnal rate values are encoun- 

 tered, which give the data of daily maximum and minimum. 



It is further to be noted that it appears to make no difference how con- 

 stant the conditions surrounding the. plant may be, or whether light be present 

 or absent, the fluctuations above mentioned still occur, though it may be in 

 less amplitude. Curtis (1902) has conducted very carefully guarded experi- 

 ments, which displayed this variation in transpiration rate. Some of these 

 experiments were placed under continual constant illumination for 1 2 hours ; 

 others in the absence of light. Nearly all of the plants used in my own experi- 

 ments exhibit the same behavior. In the absence of synchronous variations 

 in relative humidity and temperature, it seems impossible to avoid referring 

 such minor fluctuations in transpiration rate to the internal economy of the 

 plant, but what phase of this it may be is quite obscure. Among the more 

 obvious explanations, and one which has been invoked as the cause, is the 

 expansion and contraction of the stomata. 



Thus, speaking of the transpiration of plants in the dark, Curtis (loc. tit.) 

 expressed surprise at the amount of water-vapor transpired in view of the 

 fact that we usually consider the stomata as closed under this condition, but 

 later on, in reference to the behavior of Ficus elastica, he regards the evidence 

 offered by him as warranting the conclusion "that the stomata are suffi- 

 ciently open to permit transpiration during the night," and, still again, sug- 

 gests that the variation of minor amplitude in the rate of transpiration may 

 be due to the activity of stomata. He says, ' 'It is possible that, owing to the 

 activity of the secreting cells and the accumulation of intracellular water, 

 the stomata may open at varying periods, and, after the escape of the vapor, 

 close again." 



