536 History of the Doctrine of [BOOK in. 



tioned the heliotropic movements of the stalks of many flowers, 

 which he says were for that reason called heliotropic flowers ; 

 in the following century Pliny says that the leaves of clover close 

 when bad weather is approaching ; Albertus Magnus in the 

 1 3th century, Valerius Cordus and Garcias del Huerto in the 

 1 6th, thought the daily periodical movements of the pinnate 

 leaves of some Leguminosae worth recording; Cesalpino 

 noticed the movements of tendrils and climbing plants, and 

 was surprised to see that the latter to some extent seek for 

 their supports. These were every-day phenomena, but the 

 striking sensitiveness of the leaves of Mimosa pudica intro- 

 duced from America could not fail to attract attention, and so 

 we find an essay on the causes of it in Robert Hooke's ' Micro- 

 graphia ' of 1667. The irritability of the stamens of Centaurea 

 had been already mentioned by Borelli in 1653. 



i. We meet with the first speculations on the subject at 

 the end of the iyth century. Ray in his ' Historia Plantarum J 

 (1693) commences his general considerations on the nature 

 of the plant with a succinct account of phytodynamical 

 phenomena, and introduces the whole by a sentence of 

 Jung : ' Planta est corpus vivens non sentiens,' etc. Though 

 Ray, like Cesalpino, seems to believe in the Aristotelian 

 soul of plants, yet he does on the whole endeavour to 

 explain the movements which he describes by physical and 

 mechanical laws ; he thinks that the irritability of Mimosa 

 in particular is not due to sensation, but to known physical 

 causes ; the movement of the leaf when it is touched is caused 

 by a contraction, which again is due to a withering or relaxation 

 of its parts. He endeavours to apply the knowledge of his 

 time to the explanation of the mechanical process : leaves, he 

 says, remain tense only because the loss by evaporation is kept 

 constantly supplied by the water that flows to them from the 

 stem ; if then in consequence of a touch the sap-passages of 

 the leaves are pressed together, the supply of water is not suffi- 

 cient to prevent their becoming relaxed. Ray mixes up together 



