TRANSPIEATION AND LEAF- WATER 13 



each hour of observation, namely, midnight, 4 and 8 a.m., 12:30, 

 4 and 8 p.m. The average length of the pore is constant (20 

 microns). The average width is indicated for each hour in the 

 curve. If the probable errors were calculated, the shape of this 

 curve would be slightly altered, but not sufficiently to change it 

 materially. 



It is to be noted that there was an opening of the stomata 

 between midnight and sunrise, from 2.4 to 4.4 microns. This 

 agrees with my previous observations on the same plant 9 and with 

 the recently published data of Darwin and Pertz 10 on Nicotiana 

 and Prunus, obtained by means of the porometer. 



Between 4 a.m. and 8 a.m., the opening movement is continued 

 till an average maximum opening of 8.1 microns is attained. At 

 12:30 p.m. the stomata were found to have closed to about 5 

 microns, after which but little change was noted. The behavior 

 during the latter part of the day cannot with certainty be regarded 

 as wholly normal as it does not accord with my previous results. 

 For the sake of considering the question in hand, we may make the 

 assumption that there was a stead}? - closure between 1 and 4 p.m., 

 from 5 to 2 microns, and treat.it alternately with the apparent 

 fact of the case. 



One sees that during the period of stomatal opening, the plant 

 suffers a measurable net loss of water, and it seems in a high degree 

 probable that this loss takes place in the leaf. The diffusion 

 capacity of the stomata is therefore so great as to allow a condition 

 which may be regarded as preliminary to that of wilting. The 

 stomata during the morning hours are not effective in keeping up 

 the supply of leaf-water to a definite amount. The loss of water, 

 indeed, continued with the interruptions noted, during the whole 

 day till toward sunset, so that, even on the assumption made in 

 the preceding paragraph of stomatal closure during the afternoon, 

 the stomata, during closing, are at any time still not effective in 

 shutting off the loss to the plant. It would seem, therefore, that 

 the increase in water-content of the leaf in the afternoon depends 



9 Publ. 82, Cam. Inst. Wash. 



10 On a New Method of Estimating the Aperture of Stomata. Proe. Roy. Soc. 

 B. 84: 136-154. 1911. 



