220 PROCEEDINGS OF THE ACADEMY OF [1899. 



days are bright and crisply cold, with a breeze stirring, the plant 

 with broadly expanded leaves would transpire itself to death. 

 Kihlraan^ has clearly shown the action of a dry -wind in frosty 

 weather on vegetation to be destructive in the extreme. Secondly, 

 the folding of the leaves and downward curvature facilitates the 

 rapid shedding of snow and ice, which in the mountains where this 

 plant grows cover the evergreen trees and bushes to a considerable 

 extent. During the recent zero weather, even in the most pro- 

 tected places where exposed to the bright sunlight, the drooping 

 condition of the leaves was constantly maintained. In the shade 

 of one of the University buildings, the infolding was even more 

 accentuated than in the sun. as one would naturally expect, and in 

 the mountains of Pennsylvania, in the shadow of the leafless trees 

 and evergreen hemlocks, the cold rigor of the plant was very 

 noticeable at a hundred yards' distance, viewed from the windows 

 of a rapidly moving train. 



If a branch of a plant with its leaves in the hanging position be 

 carried into a warm room at about the temperature of 75°-80° F., 

 in response to this thermal stimulus the leaves will begin to unfold 

 and assume the diaheliotropic, or dorsiventral, position. The 



following observations made upon the reactions to heat and cold 

 stimuli show that the movements are made quite rapidly: 

 Branch 1. — Intervals of time given in minutes. 



1.30 miu. — Visible response to heat stimulus. 



2.00 min. — Unfolding of leaf began. 



3.00 min. — Leaf almost wholly unfolded and petiole erected through 

 an angle of about 40°. 



4.00 miu. — Upper leaves become dorsiventral. 



o.OO miu. — Blade of leaves fully expanded, but the petiole not 

 yet entirely straight. 

 Branch 2. — This branch was held over a radiator. It had been 



cut for twenty-four hours and placed out of doors in the cold. 



The record is as follows: 



f-1 min. — Response to stimulus. 



1.30 min. — Somewhat jerkv upward movement of the petiole 

 apparent. 



^ Kihlman, A. O. : Pflanzenbiologische Studieu aus Russisch-Lappland, 

 Acta Soc. pro Fauna et Flora fenuiea (1890). 



Scliiiuper, A. F. W. : Pflinzea-geographie auf physiologischer Griindlage 

 (1898), p. 183. 



