1911.] New Method of Estimating the Aperture of Stomata. 149 



It will be seen that with N. glauca (fig. 6) there is a sudden fall in the 

 curve about sunset. Owing to the fact that no readings were taken between 

 6.45 and 7.47, it is impossible to know at what hour the fall began. In the 

 observations (F. Darwin ('98), p. 595) made with the horn hygroscope on 

 Tropaeolum and Pelargonium, there is a similar fact, i.e., a gradual fall during the 

 afternoon and a sudden fall about sunset. But in Experiment 67 the stomata 

 had nearly reached their utmost degree of closure 24 minutes after sunset, 

 whereas in Pelargonium this was clearly not the case, and probably not so in 

 Tropaeolum. On the other hand, in Narcissus (op. cit., p. 589) the horn 

 hygroscope was at zero 38 minutes before sunset. 



In the observations on the laurel in the present paper (fig. 7), the curve 

 falls from about 4 P.M. to 9.17, no very great effect of sunset being visible. 



The most interesting fact observable in figs. 6 and 7 is that the stomata 

 begin to open long before sunrise ; in fact, almost as soon as they have reached 

 the maximum of closure. This is especially striking in fig. 7, but is perfectly 

 clear with N. glauca, fig. 6. Further work will show whether the phenomenon 

 is a general one. 



In both plants a sudden opening of the stomata occurs about sunrise. 



There can be little doubt that the opening of the stomata during the early 

 hours of the night is due to periodicity : Lloyd ('08, p. 37) remarks under the 

 heading of " normal daily periodicity " that there seems " to be a tendency for 

 stomata to open a little during the night." He shows (p. 67) also the 

 existence of a rise in transpiration rate during the night and early morning in 

 complete darkness, but (p. 73) he speaks doubtfully as to the connections of 

 this fact with increased stomatal aperture. The whole question of periodicity 

 requires re-investigation with the porometer. 



§ 6. The Effect of Withering. 



In two previous papers* the curious fact has been recorded that the closure 

 of the stomata, produced by depriving a leaf of water-supply, is often preceded 

 by increased transpiration, which was assumed to be due to opening of the 

 stomata and was accordingly described as the " temporary opening." 



Lloydf has investigated the phenomenon in question and concludes that it 

 does not occur. Whether this is due to his relying on the microscopic method 

 or to the fact that he experimented on other plants we cannot say. 



The following experiments show very clearly that, at any rate in certain 

 plants, a striking temporary opening of the stomata occurs : — 



* F. Darwin ('98), p. 548, and ('04), p. 89. 

 t Lloyd ('08), p. 81. 



