64 THE PHYSIOLOGY OF STOMATA. 



INDUCED RHYTHM IN TRANSPIRATION.* 



Although it was no part of the contemplated scope of the present paper 

 to pay attention to transpiration as such, it has happened that a not incon- 

 siderable amount of evidence bearing upon the phenomenon of induced 

 rhythm has been gathered, especially from experiments with Fouquieria 

 splendens. By "induced rhythm" is meant an autogenic fluctuation in the 

 rate of transpiration corresponding to the daily fluctuation in the normal rate, 

 or periodicity, as ordinarily conceived, or as Kohl (1886, p. 48) puts it "eine 

 von ausseren Einflussen unabhangige Periodicitat." A resume of the litera- 

 ture pertinent to the question may be found in Kohl (loc. cit.) and in Burger- 

 stein (1887, pp. 53, 449; 1904, p. 150). Briefly summarized, the positive 

 view of Unger, as formulated in 1852, has been supported by Sachs (1859), 

 Sorauer (1880), Kohl (loc. cit.), and Curtis (1902), and has been denied, on 

 apparently slender grounds, by Barenetsky (1872) and Eder (1875)^ But 

 little unequivocal evidence has, however, been offered, that supplied by 

 Curtis being, in the light of my own work, by far the most important. For 

 my purpose I need not dwell upon any of the data furnished by previous 

 students, except those of Curtis, and, in view of the support offered by him 

 to certain conclusions advanced by Francis Darwin (1898), upon the work 

 of this investigator also. 



Curtis's views on this point are based upon two series of experiments carried 

 out upon (a) several plants kept under constant conditions in darkness, 

 their hourly rate of transpiration being determined by weighing; and (b) 

 plants kept under the same conditions, but exposed to given periods of 

 illumination in the morning and in the afternoon. 



Of the latter series of experiments (loc. cit., p. 371) all but two must be 

 regarded as insufficiently critical for the present purpose. These two con- 

 sisted of exposing two plants (Aucuba japonica and Justicia elegans) to 

 electric illumination of constant value for the period of one hour in the 

 morning and in the afternoon. Otherwise, the plants were kept under the 

 constant conditions above mentioned. It was found that the response in 

 the morning hour was absolutely and relatively greater for both plants than 

 in the afternoon hour. 



Curtis prefers to refer this behavior to "a pronounced periodicity in the 

 stomata" and comes in this regard to the support of Francis Darwin (1898). 

 Of this phase of the question I speak elsewhere (p. 74)- Similarly, I believe 

 that Francis Darwin's own experiments must be interpreted as bearing rather 



♦Presented in brief form before the Botanical Society of America, Dec, 1904. 

 tOther authors have found induced rhythm in absorption of water and bleeding (see 

 Kohl's work, loc. cit.) but their results have no direct bearing on the present problems. 



