DIV. ii PHYSIOLOGY 361 



reverse order ; finally, the secondary petioles themselves draw together. 

 Suddenly, when the whole process seems apparently finished, the 

 main leaf -stalk in turn makes a downward movement. From this 

 leaf the stimulus is able to travel still farther through the stem, and 

 it may thus induce movement in leaves 50 cm. distant. The stimulus 

 can also be conducted from the roots to the leaves. In this case we 

 are dealing with a wound -stimulus which has far-reaching effects. 

 On otherwise disturbing the plant we also find a conduction of the 

 stimulus which, it is true, is not so extensive. ' 



The rate of conduction of the stimulus ( 118 ) may attain after wounding 10 cm. 

 and after contact 3 cm. per second, and thus be of considerable rapidity. It is, 

 however, greatly below the conduction of tlie stimulus along human nerves. 

 While it is not yq known with certainty how the stimulus is conducted in 

 Mimosa, it is clear that the process differs both from the conduction along nerves 

 and from that in other cases in plants. The stimulus can certainly be carried 

 across killed regions ; it probably passes along the tracheides of the xylem and 

 depends on the movement of water. Mimosa, thus reacts not only to the stimulus 

 of shock but to that of wounding, and the same movements of the leaves follow on 

 electric shocks, sudden changes of temperature, and chemical stimuli. 



The position of a disturbed leaf is externally similar to its sleep- or night-position, 

 but the conditions of tension in the pulvinus which lead to the two positions differ. 

 The seismonastic, like the sleep-position, is caused by variations in turgor, but 

 depends on a diminution of the osmotic pressure and a flaccid condition of the half 

 of the pulvinus that becomes concave. This condition can be most clearly recog- 

 nised in the irritable under side of the main pulvinus of the leaf ; it is connected 

 with an escape of liquid from the cells into the adjoining intercellular spaces. 



Many Leguminosae and Oxalideae are similar but less irritable. Thus Robinia 

 pseudaeacia and Oxalis acetosella exhibit slight movements on strong mechanical 

 stimuli. These are much less considerable than in Mimosa. Movements of the 

 leaves in response to wounding also are not confined to Mimosa. 



The power of reaction to stimuli in Mimosa evidently depends 

 on external factors, and each of these when in excess or lacking may 

 lead to a state of rigor. Whenever the temperature of the surround- 

 ing air falls below a certain level (15), no movements take place, and 

 the whole plant passes into a condition known as COLD RIGOR, while, 

 on the other hand, at a temperature of about 40, HEAT RIGOR occurs. 

 DROUGHT RIGOR is induced, just before withering, by an insufficient 

 supply of water, and a DARK RIGOR by a prolonged retention in 

 darkness. In a vacuum, or on exposure to hydrogen and other gases 

 chloroform vapour, coal gas, etc. movement also ceases, partly on 

 account of insufficient oxygen, and partly from the actual poisonous 

 action of the gases themselves. If the state of rigor is not continued 

 too long, the original irritability will again return on the restoration 

 of normal conditions. Similar conditions of rigor are met with in 

 other cases of irritability. 



The variation-movements exhibited by the staminal leaves of some Berberi- 

 daceae (Berberis, Mahonia} and Compositae, especially beautifully by Centaurea 



