TRANSPIRATION BEHAVIOR OF RAIN-FOREST PLANTS. 101 



for all of the species excepting Peperomia turfosa. A comparison of 

 the averaged rates in the Ught with the average of the two sets of 

 darkness readings, nocturnal and diurnal, brings out the same behavior, 

 in which Peperomia turfosa is the only form showing a lowering of rate 

 due to darkness. 



In each of the experiments and in each plant investigated there was 

 a maximum of relative transpiration in the light which was well in 

 excess of the darkness rate. Rates which approach the daily maximum 

 in amount are, however, of infrequent occurrence in series of two-hour 

 readings, with the result that daily averages are low as compared with 

 the maximum rates. 



The influence of darkness on the aperture of stomata was briefly 

 investigated in Pilea nigrescens and Peperomia turfosa. Plants of these 

 species were placed in a dark chamber for three days, and at the end 

 of the period material for stomatal examination was taken in the usual 

 manner, without exposing the plants to any more light than was 

 necessary for the operation. The resulting measurements give for 

 Pilea: width 5.24^, leng th 14 .01/j; Peperomia: width 5.39^, length 

 20.17;u. The values for vTXw are respectively: 8.53 and 10.43. For 

 plants in the light, the figures given for stomatal aperture in Pilea 

 (table 37 and 38) show daily maxima of 7.62 and 9.01, readings with 

 the average of which the darkness aperture of 8.53 is in near agreement. 

 The daily maxima in the light, as determined for Peperomia (tables 36 

 and 38) are 12.25, 9.78, and 9.01, amounts which are also of the same 

 order of magnitude as the darkness determination of 10.43. 



Lloyd states' that in plants of Verbena ciliata placed in prolonged 

 darkness the stomata perform the usual nocturnal closure and remain 

 closed. Several earlier workers, using various and usually unreliable 

 methods, have stated that there is an opening of stomata in prolonged 

 darkness, usually following a closure during the first few hours. 



While I can not maintain from single readings on two plants that 

 the stomata are constantly as wide open in darkness as the above 

 figures indicate, nevertheless the probabiUty is extremely strong that 

 none of the possible fluctuations of aperture in darkness carry the 

 stomata to a degree of openness much below the possible normal daily 

 maximum under light conditions. It will be seen from the data in 

 table 28 that Pilea and Asplenium show a sharp increase of stomatal 

 area between 6 and 9 p. m., while Diplazium shows a slight increase — 

 behavior which is in accordance with the readings taken in darkness 

 and is indicative of a possible failure of the stomata to close on first 

 being placed in darkness. No other results were secured which throw 

 light on this matter. 



The fact that the stomata of plants placed in prolonged darkness 

 show a degree of openness similar to the somewhat transitory daily 



'Lloyd, F. E. The Physiology of Stomata, p. 115. Carnegie Inst. Wash. Pub. 82, 1908. 



