79^ PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



is applied to tlie under side of an organ from the upper side of which the parenchyma 

 has been removed, water may sometimes be seen to escape also from the horizontal 

 cut surface of the parenchyma ; it is certain that during irritation water escapes from 

 the lower parenchyma ; it gives off a small portion of it to the upper parenchyma (as 

 is shown by the measurements that have been quoted), a larger portion flows off at the 

 sides through the intercellular spaces, and a smaller portion apparently enters the central 

 fibro-vascular bundle. The whole amount of water that escapes to the lower paren- 

 chyma is so small that it is no doubt at once consumed at these spots at the moment of 

 irritation. 



Since water escapes from the parenchymatous cells of the under side when irritated, 

 and passes into the intercellular spaces, the air must be at least partially expelled from 

 the latter ; and this is evidently the cause of the darker colour of the irritated parts 

 already observed by Lindsay. PfefFer fixed a petiole in the normal condition so that the 

 contractile organ could not bend when irritated ; when he touched a point of the irrit- 

 able side he saw the darker colour spread instantaneously from the point of contact. No 

 other explanation of this phenomenon is possible than that the air is expelled from the 

 intercellular spaces and replaced by water, which would cause a smaller amount of light 

 to be reflected from the interior. The expelled air will collect, in consequence of the 

 laws of capillarity, in the larger intercellular spaces round the central fibro-vascular 

 bundle, from which it will easily reach the petiole. 



We do not however at present know how a light touch or concussion causes the 

 strongly turgid cells of the under side to lose a portion of their water through their walls 

 and then to take it up again with great energy. 



In the diurnal position of the organ slight transverse folds are seen to run along 

 both sides which after irritation become more shallow on the upper but deeper on the 

 under side, showing that the consequent curvature causes a slight passive compression on 

 the under side. This side first of all contracts in consequence of its loss of water and of 

 the elasticity of its cell-walls, and then becomes still further compressed by the down- 

 ward curvature of the upper side\ 



In the contractile organs of the leaflets of Oxalis Acetosella^ , where the anatomical and 

 mechanical contrivances are similar to those of INIimosa, this compression is much 

 stronger, and the folds make their appearance on the under side w^hen the organ is irri- 

 tated. Pfeflfer states that a decrease in mass also takes place, and since a very con- 

 siderable elongation of the upper parenchyma is required for the movements, there must 

 be a more considerable transference of water from the under side. The organs of Oxalis 

 diflfer from those of Mimosa in remaining irritable after the intercellular spaces have 

 become filled with water ; but when in this state they become flaccid on irritation ; it is 

 probable therefore that a portion of the water passes from the contractile organ into 

 the tissue of the petiole and lamina. The depression of the leaves of O. Acetosella and 

 stricta when sunlight falls suddenly upon them ^, is, like the irritable movements, attended 

 with flaccidity, and has been determined by Pfeffer to be of the same nature. 



(b) The anatomical and mechanical contrivances in the Stamens of Berberideae'*, the 

 Gynostemium of Stylidium, and the Leaues of Dioncea muscipula and Drosera^ have at 





^ [The most recent publication on the irritability of the leaves of Mimosa pudica is by Pfeffer in 

 Pringsheim's Jahrb. fiir wiss. Bot. 1874. — Ed.] 



2 See Sachs, Bot. Zeit. 1857, pi. XIII. 



3 See Batalin, Flora, 1871, No. 16. 



* The irritability of the stamens of Berberidese differs from that of the same organs in Cynaraceoe, 

 in being displayed on the inner surface only of the filament. The parenchyma of the irritable portion 

 possesses also no intercellular spaces, these being replaced by a copious ' intercellular substance.' 



^ [The peculiar motion of the hairs on the surface and margin of the leaves of Drosera, by means 

 of which, with the assistance of the viscid substance excreted by them, insects are captured and 

 apparently devoured, was first observed by Roth in the eighteenth century (Von der Reizbarkeit der 



