STOMATAL RESPONSE TO SUDDEN CHANGE. 59 



The following sentence from Haberlandt (1904, p. 404) may be regarded 

 as a summary of a rather generally entertained opinion on this point : 



Die Spaltoffnungen sind zufolge ihrer Fahigkeit, sich zu ciffnen und zu schliessen, nicht 

 blosse Ausfuhrungsgiinge des Durchluftungssystems, sondern zugleich auch Regulatoren 

 des Gasaustausches, in erster Linie der Abgabe von Wasserdampf, der Transpiration. 



But what sort of evidence do we find to underlie this generalization ? We 

 need not recapitulate the arguments based upon comparative and genetic 

 grounds, since conclusions drawn therefrom are subject to examination by 

 experimental methods. 



It has been shown by blocking the stomata with appropriate agents, the sur- 

 faces of leaves, by means of papers sensitive to water- vapor (Stahl, Merget), 

 and by direct measurement of collected water (Garreau), that the amount 

 of water-vapor passed off from the stomatal surface of a leaf is greater than 

 that from the astomatal, and that large differences in the numbers of stomata 

 on the two faces are related to observable differences in the amount of trans- 

 piration. Upon similar evidence also, low and high rates of transpiration 

 have been coupled with a "closed" or "open" condition of the stomata, 

 though the possible accompaniment of differences in the rates by constant 

 stomatal condition has, of necessity, escaped observation by these means. 



The change in rate of transpiration following reversal of the normal position 

 of a bifacial leaf, maintained to occur by Meissner (1894) but denied by 

 Francis Darwin (1898), was ascribed by Meissner to the closing of the under 

 stomata upon exposure to direct sunlight. Similarly, I/idforss (1896) referred 

 the low rate of transpiration in evergreens in winter to the closed condition 

 of the stomata, a conclusion believed by Francis Darwin to need "further 

 research." But the studies of this author, based upon the readings of a 

 hygrometer, provided data which are often open to two interpretations, 

 since, in the absence of direct observation of the stomata, the horn hygro- 

 scope may be depended upon to record transpiration rate, and not the condi- 

 tion of the stomata. Thus, this instrument remained at zero on the red, 

 chlorophyll -free leaves of Poinsettia of the greenhouses. "The difference 

 in the green and non-green parts seems to depend upon the aperture rather 

 than the number of stomata, but on this point more observations are needed," 

 Darwin concluded, for he also observed, "the stomata in the white and in 

 the yellow part of at least some variegated leaves close in darkness and as the 

 leaf withers." The behavior of the hygroscope on the Poinsettia bract may 

 be taken to mean a low transpiration rate merely, or a closed condition of the 

 stomata, and hence a low transpiration rate. But there is no direct evidence 

 that the stomata of the non-green parts are at all different from those found 

 elsewhere (Weiss, 1898). I myself find the stomata of the red bracts of 

 Poinsettia provided with chlorophyll and open in daylight, though I have not 

 made careful quantitative measurements of transpiration nor measurements 



