88o PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 
turgidity will cause an expansion, a diminution of turgidity will cause a contraction 
of the mass of cells, and these changes in volume will be accompanied by corre- 
sponding curvatures of the organ. The conditions which modify the turgescence of 
growing cells may be identical with those which modify the turgidity of cells which 
have completed their growth ; in the former case every variation of turgescence 
involves an alteration of the volume of the cell which is made permanent by growth, 
in the latter case the alteration of volume is only temporary, and can be effaced by a 
variation of the turgidity in the opposite direction. From these considerations it 
becomes apparent that the study of the phenomena of movement will contribute to 
the development of a mechanical theory of growth, and vice versa. 
Sect. 28. Eeview 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 elements composing 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, which consists of flexions upwards and downwards, 
generally 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 Morren, linger, and Pfefi"er, 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 parenchy- 
matous cells on the one hand and the elasticity of the axial bundle and epidermis on 
the other hand. As far as observations go at present, especially those made on the 
larger contractile organs, the tendency to extension is greatest in the middle layers 
of the parenchyma between the epidermis and the axial bundle, but the elastic 
resistance of the epidermis is less than that of the bundle. 
If we now consider the nature of the movements in reference to the causes which 
directly operate to produce them, we may, in the present state of our knowledge, 
distinguish between three different kinds, viz, 
(1) Those periodic moveinents which are produced entirely by internal causes, 
without the cooperation of any considerable external impulse of any kind. Such 
movements may be termed automatic or spontaneous. 
(2) Spontaneously motile foliage-leaves are also sensitive to the influence of lighi^ 
in such a way that within certain limits any increase in the intensity of the light 
^ This is also true for Dioncea if the motile parts and not the whole leaf be considered. 
