DEFINITION. 783 



to assume different conditions which alternate under the influence of external or 

 internal causes. 



In those movements which occur during growth the tension of the tissue is 

 concerned only so far as any change in it reacts on growth and modifies it. Periodic 

 movements and those due to irritation, on the contrary, depend entirely on changes 

 in the tension of the tissues which, in this case, is fully developed only when the 

 organ has attained maturity. These alterations of the tension of the tissues do not 

 however induce new permanent conditions, but can be effaced; every change is 

 again reversed by internal forces, and the previous condition restored so long as 

 there has been no structural injury. 



The movements caused by growth occur in unicellular as well as in multicel- 

 lular plants and organs ; those we have now to consider would appear, on the 

 contrary, to take place only when the organs consist of masses of tissue. The reason 

 of this difference is probably that movements of the first kind are always caused by 

 the growth of the cell-wall, those of the second kind by access or loss of water, i. e. 

 by changes in the turgidity of the cells which form the tissued 



Sect. 27. Review of the phenomena connected with periodically 

 motile and irritable parts of plants. It is remarkable that all organs at pre- 

 sent known as coming under this category are, in a morphological sense, foliar 

 structures, as green foliage-leaves, petals, stamens, or occasionally parts of the carpels 

 (styles or stigmas). It is the more striking that no axial structures or parts of stems 

 are contractile in this sense, because the contractile parts of leaves are usually cylin- 

 drical, or at least are not expanded flat, and therefore possess the ordinary form of an 

 axis. There is this further agreement in the anatomical structure of all parts which 

 exhibit these phenomena; — that a very succulent mass of parenchyma envelopes 

 an axial fibro-vascular bundle or a few bundles running parallel to one another ; 

 the elemental structures of these bundles being only slightly or not at all lignified, 

 and therefore remaining extensible and flexible, a fact of importance in reference to 

 the possibility of the movement. With the exception of the leaves of Dionaea and 

 Drosera, the movement always consists of flexions upwards and downwards, gener- 

 ally in the median plane of the organ, the fibro-vascular bundle thus forming the 

 neutral axis of the curvature. The mass of parenchyma which envelopes the fibro- 

 vascular bundle often has the form of a pulvinus, and does not contain in its outer 

 layers any air-conducting intercellular spaces, or only very small ones, while in the 

 inner layers they are larger, especially in the immediate vicinity of the bundle ; these 

 being, according to IVIorren and linger, wanting only in the irritable stamens of 

 Berberis and Mahonia. The tension of these layers of tissue which is generally 

 very considerable, is caused by the stronger turgidity of the parenchymatous cells 



* The ultimate cause of the movements now under discussion is indeed the same as that of the 

 growth of the tissues, namely, the turgidity of the individual cells. In either case an increase in the 

 turgidity of the cells causes an increase in volume of the tissues ; if the organ is still in a growing 

 state, this increase remains permanent ; if it is already mature, it causes only a temporary enlarge- 

 ment, which is effaced when the turgidity again diminishes ; in other words, the variation in 

 turgidity gives rise to complicated movements of various kinds. 



