HYGROSCOPIC MECHANISMS 551 



This principle of differential imbibition finds a great variety of 

 applications in the construction of hygroscopic mechanisms. In all 

 such cases, the cell-walls on the two antagonistic sides of the motor 

 apparatus differ in the manner described above in their molecular 

 structure, and hence in their powers of swelling and contracting. The 

 requisite antagonism may, however, also be produced in other ways, as 

 will be seen later on in the case of anthers. 



As regards their shape and detailed structure, hygroscopic motor- 

 cells exhibit a good deal of variety. The prosenchymatous thick- 

 walled forms are connected by various transitional types with genuine 

 mechanical fibres. As a matter of fact, ordinary bast-fibres may assist 

 in the production of movements, besides performing their principal 

 mechanical functions. For cells of this composite type Eichholz 

 suggests the name of " dynamo-static " elements. In the same way 

 the term dynamic cell (Eichholz's actual phrase is " specijisch-dyna- 

 mischc Zelle ") may be applied to any thick- walled prosenchymatous 

 element which has its cell- wall molecules or micellae arranged in 

 transverse rings, or very gently inclined spiral series, and which under- 

 goes very well-marked longitudinal contraction on losing water. Par- 

 enchymatous hygroscopic cells are also far from uncommon ; they may 

 be isodiametric, or more or less rod-shaped. The thickness of their cell- 

 walls varies greatly ; very often local thickenings are developed, which 

 play an important part in the mechanism of movement. The walls are 

 also generally lignified, but this feature does not seem to be in any 

 way directly correlated with the hygroscopic properties of the mem- 

 branes. 



The preceding generalisations, which are based upon elaborate 

 researches by Zimmermann, Schwendener, Eichholz, and Steinbrinck 

 (especially upon those of the last-mentioned author), may now be 

 illustrated by a few concrete examples. The peristome of Mosses 

 is an excellent instance of a hygroscopic apparatus formed by the 

 persistence of certain definite portions of the walls of cells, which 

 otherwise become disorganised. Where the peristome is double, 

 hygroscopic properties are usually restricted to the teeth of the outer 

 series. In damp weather, the hygroscopic teeth curl inwards, thereby 

 closing the orifice of the capsule and preventing the escape of the 

 spores, which, of course, cannot be effectively distributed during rain. 

 When the air is dry, the teeth curl outwards, and spore-dispersal can 

 proceed unchecked. Each peristome-tooth is made up of an inner and 

 an outer cellulose lamella. Ontogenetically, these lamellae represent 

 local thickenings of the partition-walls between two adjacent cell- 

 layers ; consequently they are separated from one another by a middle 

 lamella. Now Steinbrinck has shown that the axes of maximum 



