USES OF IRRITABILITY. 655 



the outside. It appears more important, however, that the irritable parenchyma 

 contains no intercellular spaces; the cells, which are moreover thin- walled, have 

 abundance of so-called intercellular substance between them, which has the property 

 of swelling up. A touch on the inside of the filament causes it to curve along its 

 whole length, and Pfeffer succeeded in showing here also that if the filament is cut 

 across, stimulation causes an expulsion of water to take place at the cut surface. 



The point around which physiological interest centres in the phenomena of 

 irritability so far described, exists of course in the recognition of the internal processes 

 causing them, and which we have been able to trace back to the protoplasm, so that, 

 in fact, we perceived that it is the protoplasm which is properly irritable, whereas the 

 external effect, the irritable movement itself, depends chiefly upon the extensibility and 

 elasticity of the cellulose walls. But the question as to the uses which the plants 

 referred to derive from these remarkable adaptations, leads us in an entirely different 

 direction of physiological investigation. With respect to Dioncsa muscipula, and the 

 obviously very different adaptations in Drosera, the statements in Lecture XXIV 

 may be referred to ; it was there pointed out that the very complicated effects 

 of stimulation in these plants seem nevertheless to exercise only a somewhat in- 

 significant effect on their total nutrition. It is probably otherwise in the case of 

 irritable stamens, the movements of which are evidently calculated to be started 

 by insects visiting the flowers for the sake of the honey, whence, in accordance with 

 the other mechanisms of the flower in each case, the pollen from the anthers has 

 a chance of sticking to the body of the insect, to be rubbed off subsequently on 

 the stigma of another flower. Irritability here therefore occurs in the service of 

 reproduction, while in other flowers other adaptations serve the same purpose. 



So far as I am aware, no one has as yet attempted an explanation of the use of 

 the irritability of the leaves of Mimosa ; but I believe that I am able to afford one. 

 For I have often had opportunities of observing that after a severe hail-storm, when 

 plants of the most various kinds, and even robust plants, close to my Mimosas 

 before the window or in the open, have been dashed and broken by the 

 hail-stones, the Mimosas, in spite of their delicate structure, have come out quite 

 uninjured ; a few minutes after the rough weather they expanded their leaves again 

 entirely unhurt. The matter is easily explained. The blows of the first drops 

 of rain or of a single hail-stone cause aU the leaves of the Mimosa to pass into the 

 irritated condition, the primary stalks to hang down limp, and the double rows of 

 leaflets to close together like clasp-knives ; the now limp and pendent leaves may be 

 struck even by large hail-stones without taking harm, because they yield to the blows 

 like pendent threads, whereas stiff leaves are pierced or their turgescent stalks bruised 

 simply because they resist. This action is, so far as the laminae are concerned, 

 promoted to the utmost by the fact that the folded secondary petioles and leaflets 

 can only be struck by a hail-stone in a particularly unfortunate case. Just as against 

 hail, this behaviour (reminding us of the contraction of many animals when danger 

 threatens) may also probably be of use on other occasions, as, for instance, when 

 large animals invade the place where Mimosas grow, where at the same time the 

 very strong prickles to the right and left of each leaf-base are of service; both 

 arrangements must also render it difficult and distasteful for phytophagous animals 

 to eat the foliage of the Mimosae. A more exact insight into the matter is of course 



