MOTILE MECHANISMS IN HIGHER PLANTS 461 



can enter. The slit-like pits are parallel to the long axes of the 

 " bricks " and it is evident therefore that the greatest swelling 

 will be in the direction at right angles to the greatest length of 

 the pit. In the Rose of Jericho, Steinbrinck was able to verify 

 the anticipated difference between the extensibility of the fibres 

 on the two sides. He found that when dried those of the upper 

 surface shortened by about 8 to 9 per cent, of their length as 

 measured when wet, whilst in the case of those on the lower 



Upper 





Lower. 



Fibre on lower side. Fibre on upper side. 



Fig. 1. — Anastatica hierachuntica. 

 Transverse section (a) and fibre from (b) under side, (c) upper side of the vascular ring. 



side the stem hardly showed any change in length as between 

 the wet and dry condition. 



It thus at once becomes intelligible why the branches curl 

 inwards on drying and straighten again when they are wetted. 



The peristome of the moss capsule furnishes another beautiful 

 example of a hygroscopic mechanism of a rather different 

 character. The peristome consists of a ring of teeth, brightly 

 coloured, fringing the orifice of the spore capsule or " fruit." 

 There are commonly sixteen of these teeth (sometimes there are 



