SECT. xiv. VEGETABLE IRRITABILITY. 4.31 



the vegetable as in the animal kingdom. To be irritable, 

 to change its normal form as a vessel of excitation, and 

 to revert to the normal form after a while by its internal 

 elasticity, are characteristics of the living cell. In 

 plants these properties are met with only when the vital 

 processes are in full activity, and therefore are par- 

 ticularly noticed during the period of flowering, when 

 the processes are at the maximum. And it may be re- 

 marked that the stamens, in which irritability is most 

 frequently noticed, are the only organs in which an 

 elevation of temperature measurable by the thermometer 

 occurs, although a certain degree of heat is generated in 

 all plant cells by the chemical process going on within 

 them. It is to be supposed that irritable properties 

 belong to all parts of plants, but that they exist in an 

 intensified degree, and for a certain epoch, in those parts 

 where their results arrest attention, as in the stamens of 

 the Centaurea, berberry, cactus, Cistus, nettle, &c., and in 

 the anthers of the Stylidese, the leaves of Dionsea musci- 

 pula, and many others, all of which are more or less 

 affected by the external action of mechanical force and 

 electricity ; for it is scarcely possible that plants should 

 not be under the influence of atmospheric electricity, since 

 every shower of rain forms a perfect conductor between 

 the clouds and the earth. The motion does not always 

 immediately follow the excitement ; plants often require 

 to be rudely shaken before the movement begins. M. 

 Hofmeister has observed that all young shoots and leaves 

 become curved by mechanical shaking. 



Light is the most universal and important exciting 

 cause in the vegetable world. The mouths of the sto- 

 mata are opened by the influence of light. The leaves, 

 young shoots, and tendrils turn to the light ; it regu- 

 lates the sleep of plants, as well as the diurnal motions 

 of the daisy and sunflower. The opening of blossoms 

 and of folded leaves which had been closed in sleep 

 during the night, shows the susceptibility of their tis- 



