PHYSIOLOGY 273 



leaf, in favourable conditions of temperature (25°-30° C.) and moisture, 

 all its parts perform rapid movements. The leaflets fold together, and, 

 at the same time, move forward, the secondary petioles lay themselves 

 laterally together, while the primary leaf-stalk sinks downwards (Fig. 

 213, on the right). Leaves thus affected, if left undisturbed, soon 

 resume their former position. 



The behaviour of the leaves is still more marvellous when only a few of the 

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

 burning match near the leaflets of one of the pinnfe. The leaflets directly affected 

 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 a 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 further through the stem, and 

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



The movements of the pulvini are due solely to differences in 

 turgidity. It has been observed that a sudden escape of water into 

 the intercellular spaces takes place out of the cells of the lower or 

 irritable side of the pulvinus of the primary leaf-stalk. According 

 to the recent investigations of Haberlandt, the conduction of the 

 stimulus does not appear to be accomplished by the movement of the 

 water thus discharged, but by the mucilaginous contents of sacs which 

 are situated in the phloem portion of the vascular bundles, and which 

 are easily affected by variations in the hydrostatic pressure. 



The position of an irritated leaf resembles externally its sleep or 

 nocturnal position, but in reality the turgor tension of the pulvinate 

 motory organ is different. In the nocturnal position the turgor 

 in the different sides of the pulvinus is unequally increased, 

 and its rigidity, as a whole, is therefore increased ; in the position 

 assumed after a shock the turgidity of the upper and lower sides is 

 unequally diminished, and as a result of this process the pulvinus 

 loses its rigidity. 



Bobinia, Oxalis acetosella and Biophytum (Oxalideae) exhibit similar, 

 although less active, movements, under the influence of mechanical 

 stimuli. 



The state of rigour sometimes occurring in motile organs may 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 degree, no movements take place and the whole plant 

 passes into a condition known as COLD rigour, while, on the other 

 hand, at a temperature of about 40°, heat rigour occurs. Drought 

 rigour is induced, just before wilting, by an insufficient supply of 

 water, and a DARK rigour by a prolonged retention in darkness. 



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