SECT, ii PHYSIOLOGY 295 



considered irritable plants, so long as only mechanical irritations, 

 from which visible movements resulted, were regarded as stimuli. 



Of irritable plants in this sense, mention has already been made of Dionaea 

 muscipula (p. 237), whose leaves, when touched on the upper side, especially if 

 the bristles are disturbed, fold together. The most familiar example of this 

 irritability to mechanical stimuli is furnished by Mimosa pudica, a tropical 

 leguminous shrubby plant, which owes its name of sensitive plant to its extreme 

 sensitiveness to contact. The leaves of this plant are doubly compound (Fig. 237). 

 The four secondary kaf-stalks, to which closely crowded leaflets are attached left 

 and right, are articulated by well-developed pulvini with the primary leaf-stalks ; 

 while they, in turn, as well as the leaflets, are similarly provided with motile 

 organs. Thus all these different parts are capable of independent movement, and 

 the appearance of the entire leaf becomes, in consequence, greatly modified. In 

 their unirritated, light position (Fig. 237, on the left) the leaf-stalk is directed 

 obliquely upwards, while the secondary petioles with their leaflets are extended 

 almost in one plane. Upon any vibration of the 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. 237, on the right). Leaves thus affected, if left undisturbed, 

 soon resume their former position. 



The behaviour of the leaves is still more remarkable 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 pinnse, 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 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. 



The movements of the pulvini are due solely to differences in turgidity which, 

 as in the case of nyctitropic movements, occur antagonistically in the halves of 

 the pulvinus. It has been observed that a sudden escape of water into the inter- 

 cellular spaces takes place out of the cells of the lower or irritable side of the 

 pulvinus of the primary leaf-stalk ; the lower surface is that provided with tactile 

 hairs. According to the 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 tubular cells which are situated in 

 the phloem portion of the vascular bundles ; while FITTING regards the conduction 

 as taking place through living cells. MAC'DouGAL was unable to induce the move- 

 ments by causing differences in the hydrostatic pressure. Tiie position of an 

 irritated leaf resembles externally its sleep or nocturnal position, but in reality the 

 turgor tension of the pulvinus is different ("). 



Neptunia oleracea, Desmanthus plenus, and Biophytum sensitivum are similar 

 though less sensitive. Robinia psevdacacia and Omlis acetosella respond to violent 

 mechanical stimuli by slight but similar movements. 



The state of rigor sometimes occurring in motile organs may also 



