410 



SCIENCE 



[X. S. Vol. XXVIII. No. 717 



consequently the exact area of the stomatal 

 openings could be determined at any moment 

 desired. The experiments revealed no correla- 

 tion between the daily periodicity of tran- 

 spiration and stomatal movement. On the 

 other haJid, it was first of all found that the 

 rate of transpiration increased for a consider- 

 able time after the maximum stomatal open- 

 ing in the early morning and that finally the 

 rate may undergo siidden and wide changes 

 without the accompaniment of a sufficient 

 change in the dimensions of the stomata to 

 account for them on the theory of stomatal 

 regulation of transpiration. This latter re- 

 sult is in accord with the conclusions of 

 Brown and Escombe who have found that the 

 diffusion capacity of the stomata are quite 

 generally greatly in excess of the actual masi- 

 mnm rate observed. 



The experiments conducted iinder constant 

 conditions demonstrated that transpiration is 

 a physiological process, and not a physical one 

 and that it is not to be looked upon as a 

 necessary evil with which the iilant has to 

 contend. The rhythm of transpiration under 

 constant conditions could not be correlated 

 with stomatal movements and indeed it was 

 later found in the case of wilting leaves that 

 the beginning of the closure of the stomata 

 occurs somewhat later than the initial wilting 

 of the leaf and this movement appears rather 

 as a result of the loss of water by the leaf as 

 a whole than as a response in anticipation of 

 wilting. The results of all the experiments 

 indicate that the stomata are not adaptive 

 structures in the active sense and if a regula- 

 tion of transpiration exists it is effected by 

 other means. 



A valuable portion of the study is to be 

 found in the second part of the work, dealing 

 with the physiology of the guaid-cells. By 

 comparisons of the contents of the guard-cells 

 taken at periods corresponding to the stomatal 

 movements observed in connection with the 

 work on transpiration, the writer found that 

 these movements are correlated with marked 

 changes in the nature of the cell contents. Thus 

 it was found that staieh begins to accumiilate 

 in the guard-cells in the afternoon, the maxi- 



mum amount being observed in the night, 

 while during the earlier hours of the morning 

 the starch largely disappears, globules of oil, 

 frequently one in each guard cell, taking its 

 place. The movements and periods of stacis 

 of the stomata were closely correlated with 

 these fluctuations of the starch contents and 

 it is inferred that the disappearance of the 

 starch and the openings of the stomata are 

 connected with the action of some unknovsm 

 ferment. This conclusion necessitated the 

 hypothesis that the metabolism of the guard- 

 cells is radically different from that of the 

 mesophyll cells and evidence was found to 

 warrant this conclusion. The guard-cells 

 were seen to accumulate starch at a time when 

 it was disappearing from the ordinary 

 chlorenchyma and on the other hand they were 

 quite free of starch when photosynthesis was 

 most active. This difference in function was 

 further emphasized by experiments in which 

 the leaves were exposed to the blue end of the 

 spectrum, to darkness and to air devoid of 

 carbon dioxide — under all of these conditions 

 the plastids of the guard cells continued to 

 accumulate starch though photosynthesis was 

 impossible. It is maintained that the starch 

 occurring in the guard cells is derived from 

 the carbohydrates in the mesophyll and that 

 the function of the chlorophyll in the guard- 

 cells is in part, probably largely, secretory. 

 It becomes necessary in accepting this hypo- 

 thesis on the role of the guard-cells to assume 

 that the ferment operative in the transforma- 

 tion of starch is of a radically different nature 

 from other amylases since it is absent or inac- 

 tive during the night and because of its 

 marked activity during the earlier morning 

 hours. 



In conclusion the author finds no evidence 

 in the behavior of the stomata studied to 

 jiistify the conclusion that they in any way 

 adapt these plants to the unfavorable condi- 

 tions of the desert. He holds that the preva- 

 lence and magnitude of the devices that char- 

 acterize xerophytes does not indicate that these 

 plants have become fitted to their environ- 

 ment, but being fitted, they have survived. It 

 must be conceded, however, that practically 



