GROWTH, IRRITABILITY AND MOVEMENT 163 



or joints at the base of each leaflet in Clover and Wood-sorrel, while 

 in Mimosa they are also present at the base of the pinnae and of 

 the leaf stalk. Externally the pulvini can be recognised as swollen, 

 dark green regions, while internally the vascular tissues are found to 

 be contracted to a compact central strand, surrounded by a broad 

 cortical band of parenchyma which forms the motor tissue. The 

 movements of the leaves are brought about by differential changes 

 of turgor in the motor tissues on the upper and lower sides of the 

 pulvini, induced chiefly by changes in light intensity. An increase 

 in turgor on one side will lead to an expansion of that half of the 

 pulvinus, a decrease to a contraction. A corresponding movement 

 of the leaflet or other part governed by the pulvinus will follow. 



The point which leads to Mimosa being commonly called the 

 Sensitive Plant is that in response to mechanical shock the leaves 

 rapidly assume the same appearance as they take up more gradually 

 in the evening (Fig. 98). The disturbance of walking roughly through 

 a patch of Mimosa pudica results in a broad track of completely 

 transformed vegetation : it must, however, be admitted that the 

 value of this sensitiveness to the plant is far from obvious. If the 

 stimulus be applied gently, steps in the leaf movement can be ob- 

 served. A soft touch at the sensitive lower surface of the hinge 

 at the base of the petiole makes the whole leaf fall, and the response 

 may then be extended outwards to the pinnae and successive pinnules. 

 Or if the distal pair of pinnules be pinched, or stimulated with the 

 hot head of an extinguished match, the stimulus received distally 

 will extend downwards. The leaflets will fold in successive pairs, 

 and finally the leaf will fall. If the stimulus be sufficiently strong 

 its effect may extend along the stem to other leaves. It thus 

 appears that the state of excitation set up at the point of stimula- 

 tion may be conducted through the plant. 



The method of conveyance of excitation through the plant is still the sub- 

 ject of investigation. It has been found that if a stem of Mimosa is cut right 

 across and the two parts then joined together by a glass tube filled with 

 water, application of a stimulus below the interruption will result in closure 

 of leaves above it. The inference from such experiments is that a chemical 

 substance of hormone-type is produced at the point of stimulation, and 

 that it is conducted through the plant, affecting the pulvini in turn as they 

 are reached. The path of conduction of the stimulus is still in doubt. An 

 earlier suggestion was that the conduction of excitation is effected in a 

 mechanical fashion as a wave of pressure running through peculiar elongated 

 elements that are present in the phloem of Mimosa, in addition to the sieve 

 tubes. It was suggested that the pressure waves are set up as the result of 

 the movement in the pulvinus where the stimulus was first received, and 



