206 Irritability. 



responds with the increase of volume in the centre. At last 

 growth ceases, but the circulation still continues ; and then the 

 tissue that lines the walls of the central cell is pressed upon 

 forcibly by the pulp that it encloses, until this pressure becomes 

 so violent that rupture must take place somewhere. The pe- 

 duncle being articulated with the fruit, at length gives way, and 

 is expelled with violence ; at the same time the cellules of tissue 

 lining the cavity all simultaneously recover their form, the press- 

 ure upon them being removed, and instantly contract the space 

 occupied by the mucous pulp ; the consequence of which is that 

 it also is forced outwards at the same time as the peduncle. It 

 has been found by measurement that the diameter of the central 

 cavity is less after the bursting of the fruit than before. Move- 

 ments produced by touch or external violence are very frequent. 

 The sensitive plant, Mimosa Pudica, which will rapidly fold up 

 its leaves as if in a state of sleep, is perhaps the most familiar 

 instance, but many others also exist. Roth is recorded to have 

 seen something of the same kind in the Drosera Rotundifolia. 

 If the bottom of the stamens of the common berberry is touched 

 on the inside with the point of a needle, they spring up against 

 the pistillum. The valves of the Impatiens possess this pro- 

 perty of bursting open, which is explained on the same princi- 

 ples as the squirting cucumber. M. Dutrochet, by throwing fresh 

 valves of Impatiens into sugar and water, which gradually 

 emptied the external tissue, and after at length rendering the 

 valves straight, curved them backwards. The column of the 

 genus Stigledium, which in its quiescent position is bent over 

 on one side the corolla, if slightly irritated instantly springs with 

 a jerk over to the opposite side of the flower. In Kalmia the 

 anthers are retained in little niches of the corolla, and as soon as 

 they are by any cause extricated, the filaments which had been 

 bent back, recover themselves with a spring. In certain orchide- 

 ous plants of the tribe called Vandea?, the caudicula to which 

 the pollen masses are attached, will often, upon the removal of 

 the anthers, disengage themselves with a sudden jerk. At pre- 

 sent we know scarcely anything of the causes of the diseases of 

 the vegetable kingdom ; but it can scarcely be doubted that to 

 ascertain what are the specific effects of deleterious matters upon 

 the vital powers of plants, is to lay the foundation of an acquaint- 

 ance with their pathology. Marcet's experiments proved that 



