152 ON THE POWER OF MOVEMENT IN PLANTS. 



heat, their contained water passes out from them, their turges- 

 cense is lessened, and their elastic cell-walls contract, the contrac- 

 tion affecting the side of the whole mass on the touched side of the 

 pulvinus ; the result is that the contraction of the touched side is 

 communicated to the stalk, and it is moved up or down, as the 

 case may be. In Mimosa^ both at the main joint and the secon- 

 dary ones, it is the imder surface of the pulvinus which is irritable. 

 You may touch the upper side and no result follows; but the 

 strumce are irritable only at the upper part. Hence, if the tinder 

 side of the pulvini be touched, the leaf-stalk and the scondary 

 stalk fall down^ depressing the leaf as a whole ; but the 7ipper side 

 of the strumas being touched induces an upward movement of the 

 ultimate leaflets, causing them to fold on one another. 



Touch a struma at the tip of a leaflet, and the folding of the 

 leaflets goes from tip to stalk-joint, the closing of one pair being 

 sufficient to communicate the disposition to close to the neigh- 

 bouring pair, this to the next and so on. Hence, if the lowermost 

 pair be made to close, the impulse travels from joint to tip, by the 

 same sympathetic influence. The movement is not hindered by 

 the vascular-bundles inside the pulvinus, owing to their being 

 extremely flexible. After depression has ensued, a fresh flow takes 

 place of water into the emptied cells, turgescence sets in afresh, 

 the leaf is raised, and the leaflets open again. 



All this is effected periodically in the Mimosa by the light and 

 heat of the atmosphere, which are the stimuli ; the sleep com- 

 mences just before sunset, the waking precedes the sunrise. So 

 you see the Mimosa closes its leaflets and drops its leaves gradually 

 during the day, and during the night is gradually raising the 

 leaves and opening the leaflets, whereas many sensitive plants raise 

 their leaves by day and droop them by night. Understand, the 

 sleep is not comparable to that of animals, there being no relaxa- 

 tion, but the rigidity of tissues is persistent, although the organs 

 may be in different positions. 



One thing more. Mimosa shows a curious tendency on a 

 succession of mechanical shakings of getting used to it. Desfon- 

 tai?ies proved this experimentally by carrying a Mijnosa on a stage- 

 coach journey, when after a time, although at first affected by the 

 jolting, the plant showed an admirable indifference to the inconve- 



