320 VEGETABLE IRRITABILITY. 



respects from animal contractility. They are usually hinge movements, 

 and may be referred to certain changes in the organs, causing dis- 

 tention or contraction of these tissues. Dutrochet and Morren refer 

 them to alterations in the circulation of fluids and air in the vessels 

 and cells. In plants with irritable leaves, there are frequently 

 swellings where the leaflets join the stalk, as well as where the stalk 

 joins the stem. These swellings contain cells which differ in their 

 dimensions and their contents, and the movements are considered as 

 being produced by changes in the contents of the cells, some of which 

 become more distended than others, and thus cause incurvation or 

 folding. In these swellings the vascular bundles are disposed in a 

 circle near the periphery, and may be concerned in the movements. 

 Mechanical and chemical stimuli are supposed to act by inducing 

 alterations in the contents of the vessels and cells. 



662. In the case of the sensitive plant, if the swelling at the base of 

 the common petiole is touched even slightly on its lower side, it is 

 followed by instant depression of the whole leaf, but no such effect is 

 produced if the upper portion of the swelling is lightly touched. 

 Again, touching the little swelling at the base of each leaflet on its 

 upper side, causes the upward movement of the leaflet, but no such 

 effect follows cautious touching of the lower part of the swelling only. 

 If a pair of leaflets is touched at the extremity of a petiole, the irritation 

 is usually continued downwards from apex to base ; but if a pair at the 

 base are touched, the progress of folding is reversed. Clear warm 

 weather, with a certain degree of moisture, seem to be the conditions 

 most favourable for these movements. They are seen best in young 

 plants. The leaves of the sensitive plant contract under the action of 

 electricity and galvanism. Some suppose, that in the sensitive plant 

 there are two kinds of cells connected with the upper and lower sides 

 of the leaves and petioles; the one set being contractile, and causing 

 the closing of the leaflet and the fall of the petiole, the other being 

 acted on chiefly through the circulation. In the case of the petiole, 

 it is conceived that the tissue on the lower side of the swellings is 

 contractile, while that in the upper is distensible. The turgescence 

 of the latter, which is kept up by light, counteracts the contractility of 

 the former, and maintains an equilibrium, so as to keep the petiole 

 erect; but when acted on by cold, mechanical irritation, &c., the 

 equilibrium is disturbed, and the contractility operates in depressing 

 the petiole. A careful microscopic dissection of the swelngs, shows 

 peculiar cells in some pars, which seem to differ in theircontents 

 from others in their vicinity. 



663. In the sensitive species of the Desmodium and Oxalis, the 

 movements are not so evidently influenced by mechanical irritation. 

 In the former, the little leaflets are supported on swollen petiolules, 

 and it is to the curvation and twisting of these in different directions, 



