464 SCIENCE PROGRESS 



When a cell which is full of water is exposed to the air, 

 evaporation will take place from the surface. The walls are 

 saturated with water and this is of course continuous with the 

 watery contents of the cell. If the wet walls are impervious to 

 air, the thin cell walls will be brought closer to each other as 

 water vapour passes off from the surface of the membrane. If 

 the walls are rigid or thick, of course this approximation may 

 not be possible ; then very considerable stress is developed in 

 the water and if any air happen to be dissolved in it, a bubble 

 will be formed which will at once destroy the cohesion of the 

 water. Other causes may also bring about the same result and 

 the space will then not be filled with air but with water vapour, 

 provided that no air can pass inwards through the wet walls. 



It is clear then that with conditions of this kind actually or 

 potentially existent in cells, movements may readily be produced 

 in tissues, if the cell walls are rendered unequally rigid in a 

 suitable way; investigation has proved that a large number of 

 motile structures depend on a specific adaptedness of their con- 

 stituent cells' which are thus enabled to utilise the cohesive 

 quality of water in such a way as to produce a quite definite 

 series of movements. 



In all the examples at present investigated the movement is 

 primarily secured by the pull inwards of a thin part of the cell 

 wall exerted upon other thicker and therefore less extensible 

 portions. The thickened parts of the wall exhibit a great diversity 

 of pattern and their arrangement mainly determines the particular 

 character of the movement in each case. 



A fairly simple but very effective example of a mechanism of 

 this kind is to be seen in the sporangium of an ordinary fern. 

 The sporangium (of which several are borne together in clusters, 

 on the under side of fern leaves), as is well known, is shaped like 

 a biconvex lens and it is situated on a stalk ; around the greater 

 part of the rim the cells are more protuberant than elsewhere. 

 The inner and party-walls are thickened in a remarkable way, 

 whilst the outer walls are unthickened. This cell band forms 

 the so-called annulus. As the water in the cells of the annulus 

 gradually evaporates and — owing to the maturation of the 

 sporangium — the loss ceases to be made good, a considerable 

 tensile stress is developed within the cells and the unthickened 

 elastic walls are pulled inwards ; but this inwardly curving part 

 of the membrane exerts a proportionally increasing pull on the 



