3 52 ANATOMY AND PHYSIOLOGY OF 



stigmas of MaHynia and Mimulus, the style of Goldfussia anisophylla, 

 and tlie column of the style of Biylidium, 



Among the plants possessing irritable organs. Mimosa pudiea 

 especially lias been the subject of repeated investigations. The 

 common petiole of this plant is connected with the stem by a 

 much enlarged articulation, and similar articulations are met "with 

 at the bases of tbe secondary petioles and of tlie individual leaflets. 

 When strongly iriitated, as for example by shaking, the entire 

 leaf sinks by a bending of the articulation situated at the base of 

 the inner petiole, the secondary petioles approach together, and 

 the leaflets, curving forwards and upwards, apply themselves to- 

 gether, like tiles on a roof, upon the secondary petioles, so that 

 tlie whole leaf assumes the position of a sleeping leaf, which gave 

 rise to the idea formerly entertained generally, that this motion 

 18 the same as that which occurs when the leaf goes to sleep, an 

 opinion incorrect in many respects. 



The motion of the articulation of the petiole may be produced 

 by direct irritation of it, but the stimulus must act upon the 

 under side of the joint, that becoming concave in the motion ; 

 even a slight touch upon the joint in this part will cause a sink- 

 ing of the leaf, while strong stimulus, even wounding, of the 

 upper side of the joint has no efiect. But at the same time sti- 

 mulus afifecting other parts of the plant is propagated to the joint 

 and causes it to move, provided the irritation be strong enough. 



The articulation is composed of an accumulation of parenchy- 

 ,matous cells containing chlorophyll, each also exhibiting in its 

 [interior a lai'ger or smaller globular mass of a substance strongly 

 refracting light (oil ?). The latter substance, however, does not 

 appear to be essential since it is absent from the cells of other 

 irritable organs. Through the middle of the joint run the vas- 

 cular bundles entering the petiole, united into a comparatively 

 slender cord. There is nothing at all peculiar in these anatomical 

 conditions, they perfectly resemble what we find in many other 

 plants, not irritable. The only circumstance to be regarded as 

 essential is, that the parenchymatous tissue existing in compara- 

 tively large quantity, exhibits a considerable distention, so that it 

 strives to occupy a larger space than is allowed by the mechanical 

 conditions in which it m placed, If we cut a plate longitudinally 

 out of the middle of the joint, which of course will consist of the 

 woody bundle in . the middle, and of a layer of parenchymatous 

 cellular tissue at 'each side, and then cut this plate lengthways 

 into thin strips, the middle of which is composed of the vascular 

 bundle, and the two sides of cellular tissue, the latter pieces 

 immediately expand about l-5th longitudinally, whence it is evi- 

 dent that the vascular bundle is too short in proportion to the 

 tur<3-escent mass of cellular tissue of the aiticulation, and that the 

 latter is compressed in the direction of the longitudinal axis in 

 the uninjured joint. 



