Puiicr of Movement in Plants. \i\. 



The author then proceeded to discuss the movements occur- 

 ring in living parts of plants during active growth, which he 

 characterised as the most intei'esting, though difficult of classi- 

 fication. These he grouped under two heads : — Periodic, which 

 occurred at regular times and under constantly similar circum- 

 stances ; and Induced, which were mainly brought about by 

 mechanical stimuli, such as touch, concussion, &c., and not 

 simply by heat or light. 



With regard to Periodic movements, the morphology both of 

 these and of the Induced kind consisted mostly of a folding-up, 

 or curvature of some sort. Usually', too, there was either some 

 peculiarity in tissue-constr\\ction, or in the union of the moving 

 organ to some other organ, such as the stem. Heat and light 

 greatly influenced Periodic movements ; for example, in the 

 Miiiios/i, or sensitive plant, there was no movement at a tem- 

 perature under 59° Fahr., while death occurred above 125°. 

 Prolonged darkness caused rigidity, and therefore loss of motile 

 power. The so-called sleep of plants, or of plant-organs, was a 

 notable kind of Periodic movement, which the aiithor illustrated 

 by the case of the leaves of Miinosa. These were bi-pinuate ; 

 the compound leaf was divided into four leaflets, arranged like 

 the sprays of a feather on the main axis ; and agam, each pinna 

 was divided into a series of pinnules similarly situated. At 

 night the pinnules folded upwards on one another, all along the 

 axis, like butterfly-wings ; the four leaflets moved up and 

 laterally towards the apex of the leaf, closing over one another 

 like the flaps of a fan ; then the whole leaf-stalk bent down- 

 wards, getting closer to the stem, and a general collapse took 

 place. These movements, as had been explained by Darwin, 

 were effected either by alternately increased growth on the 

 opposite sides of the leaf, leaflet, or leaf-stalk, or by means of an 

 aggregated mass of cells called a j)ulvi7uts, which formed a little 

 swelling at the articulation of the leaf- stalk with the parent 

 stem, and consisted of a vascular bundle, wrapped round with soft 

 parenchymatous tissue ; they existed also at the articulations of 

 the leaflets with the common leaf- stalk, and there was a separate 

 one for each pinnule, the swellings here being called stninue. 

 The parenchymatous cells, which were the irritable ones, filled 

 with water drawn from the plant ; their cell-walls were not irri- 

 table but elastic. If the cells were irritated by a touch, or by 

 concussion, or by light or heat, their contained water passed out 

 from them, their turgescence was lessened, and their elastic 

 cell-walls contracted, the contraction affecting the side of the 

 whole mass on the touched side of the jmhinus : the result was 

 that the contraction of the touched side was communicated to 

 the stalk, which moved up or down. In Mimosa the under sur- 

 face of the piUviiiHs was irritable, but the atrtiiiuc were irritable 



