324 BOTANY part i 



leaflets are acted upon by the stimulus. This is easily demoustrated by holding a 

 burning match near the leaflets of one of the pinnae. The leaflets directly aff"ected 

 by the flame fold quickly upwards, and this movement is performed successively 

 by each pair of leaflets of the pinna until the articulation with the primary leaf- 

 stalk is reached. The stimulation is then conveyed to the other pinnae, the 

 leaflets of which go through the same movement in the 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 dowmward move- 

 ment. 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. 



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. 



Tlie processes in the conduction of tlie stimulus are not fully .known, but it is prob- 

 able that they are efl"ected diff"erently in the case of wounding and of shock. After 

 wounding the conduction may take place with a rapidity of about 10 cm. in a 

 second (^"). 



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 difl"er. 

 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 recognised 

 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 Rohinia 

 pseitdacacia and Oxalis acetosella exhibit slight movements on strong mechanical 

 stimuli. 



The state of rigor sometimes occurring in motile organs mav also be best observed 

 in Mimosa, for, although so sensitive to the action of external influences, it does not 

 exhibit its irritable movements at all times. Whenever the temperature of the 

 surrounding 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 Q^'^). 



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

 daceae {Berberis, MaJioniu) and Compositae, especially beautifully by Ccataurea 

 americana, bear a certain relation to those of foliage leaves. The bow-shaped fila- 

 ments of the stamens of the Compositae straighten upon mechanical irritation. As 

 they fi-equently contract 10-20 per cent of their length, the style becomes extended 

 beyond the anther-tube (Fig. 241). The reduction in the length of the filaments is 

 accompanied by a moderate increase in tlieir thickness, due to the elastic contraction 

 of the cell-walls, and the consequent expulsion of water into the intercellular spaces. 

 The stamens of Berberis and Mahonia are only sensitive to contact on the inner side 

 near the base, and as their contraction occurs only on the inner side, the anthers 

 are thus brought into contact with the stigma. Examples of variation movements 



