784 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



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 larger con- 

 tractile 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. — 



(i) Those periodic movemaits 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 spontajieous. 



(2) The greater number of spontaneously motile foliage-leaves are also sensitive 

 to the i7iflue7ice of light, and many petals to that of warmth, in such a way that within 

 certain limits any increase in the intensity of the light or temperature causes such 

 a curvature of the contractile organs as to place the leaves in an expanded and 

 completely unfolded position ; while any decrease in the intensity of the light or 

 temperature produces the opposite curvature, causing the leaves to fold up. The 

 expanded position is called that of growth or the diurnal positio7i, the opposite one 

 that of sleep or the nocturnal position. In consequence of this sensitiveness to 

 fluctuations in the light and temperature, these organs make periodic movements 

 depending on the alternation of day and night, which, being induced by external 

 causes, must be clearly distinguished from the automatic or those brought about 

 by internal causes ; and the more so because both kinds usually occur in the same 

 organ, and are combined in various ways\ 



(3) In a smaller number of instances periodically motile foliage-leaves, as well 

 as some reproductive organs which do not exhibit periodical movements, are irritable 

 to touch or concussion. If a particular spot of the organ is only lightly touched or 

 subjected to a slight rubbing from a solid body, the side which is touched becomes 

 concave or contracts ^ The same effect is produced if a stronger impulse acts on 

 any other part of the irritable organ, assisting to excite the property at the irritable 

 part. If the motile part has curved in consequence of the mechanical irritability, it 

 afterwards resumes its previous position, and is then again irritable. Usually, especi- 

 ally in the irritable filaments of the Cynaracese, the surface is covered with hairs by 

 means of which any hght touch, especially the contact of any solid body, as the foot 

 of an insect, is communicated to the whole organ, and acts therefore as a stronger 

 irritation. 



The physiological function of these various forms of movement in the economy 

 of the plant is known only in a few instances, as in the case of irritable stamens, 

 where the insects that visit the flowers cause the irritation and consequent alteration 

 in the position of the stamens, these movements being serviceable for the conveyance 





* This distinction, partly founded on facts that have long been known, is very necessary for a clear 

 insight into the phenomena, and was first brought forward by me in the treatise on the various 

 immobile conditions of the periodically motile and irritable parts of plants (' Flora,' 1863), 



^ This contraction has been actually proved in only a few cases, but must be assumed in the 

 others. 



