SENSITIVE PLANTS. 379 



stimuli are supposed t6 act by inducing alterations in the contents of 

 the vessels and cells. 



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 irrita- 

 tion 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 wfeather, 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 con- 

 tractile, 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 disten- 

 sible. The turgescence of the latter, which is kept up by light, 

 counteracts the contractility of the former, and maintains an equili- 

 brium, so as to keep the petiole erect ; but when acted on by cold, 

 mechanical irritation, etc., the equilibrium is disturbed, and the 

 contractility operates in depressing the petiole. A careful microscopic 

 dissection of the sweUings, shows peculiar cells in some parts, which 

 seem to differ in their contents from others in their vicinity. 



In the sensitive species of the Desmodium and Oxalis, the move- 

 ments 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 that 

 the movements seem to be owing. The leaflets remain flat and do 

 not fold on themselves. It is said that by arresting the vital actions 

 going on in the leaflets, by giving them a coating of gum, and thus 

 preventing transpiration and respiration, the movements are stopped, 

 and that they recommence when the gum'is removed by water. Cutting 

 a leaflet across, and only leaving a small portion of its lamina attached 

 to the petiolule, does not immediately stop the movement of gyration. 

 In such a case, however, the motion ultimately ceases, while it con- 

 tinues in the uncut leaflet. So also, if a leaflet is divided longitudi- 

 nally into two parts, each of them continues to move for a time, but 

 the motions cease as the process of desiccation goes on. 



The leaves of plants belonging to the natural order Droseracese 



