Chap, iil] the Movements of Plants. 537 



the movements from irritability and the daily periodical m 

 ments, as was done till recent times : the latter, he says, occur 

 not only in the leaves of Leguminosae, but in almost all similar 

 pinnate leaves, and with these periodical movements of leaves 

 he places also the periodical opening and closing of the flowers 

 of Calendula, Cichorium, Convolvulus, and others. That these 

 last movements are due to changes of temperature appeared to 

 him to be proved by an experiment of Jacob Comutus on 

 flowers of Anemone, which, when cut off and placed in a well- 

 closed box in a warm place, opened at an unusual time if the 

 flower stalk only was dipped in warm water. This fact, after- 

 wards forgotten and discovered again a few years ago, of the 

 dependence of the movements of flowers on changes of temper- 

 ature, was applied by Ray to explain the periodical movements 

 of leaves, which, to use his own expression, fold themselves 

 together as the cold of night draws on, and open again with the 

 day, and as he thought that these movements are of the same 

 kind as the movements of irritability in Mimoseae, he tries 

 to explain how cooling has the same effect as a touch. It was 

 natural in the existing state of science to assume that changes 

 of temperature were the first causes of various movements, for 

 a thrust was at that time almost the only recognised cause of 

 motion. Hence Ray explained the movements of growing 

 stems which are now called heliotropic by a difference of tem- 

 perature on the opposite sides. A certain Dr. Sharroc had 

 observed the stem of a plant on which he was experimenting 

 grow towards that part of a window, where the air found free 

 entrance through an opening ; from this circumstance, and 

 from the rapid elongation of the stems of plants growing under 

 cover, which he ascribed to the higher temperature, Ray con- 

 cluded that cold air hinders the growth of the side of a stem 

 on which it falls, and that this side must become concave. 

 Thus Ray used the etiolation of plants grown under cover 

 to explain their heliotropic curvatures, as De Candolle did one 

 hundred and forty years later, only with this difference, that he 



