MUCILAGE-LAYERS 405 



mucilaginous cell-layers 18S which form a characteristic component of many 

 pericarps and seed-coats. These mucilaginous layers are in the first 

 instance designed to prevent the germinating seed from drying up, 

 and are accordingly on the whole characteristic of xerophytes ; they 

 occur, for example, in a great many Cruciferae and Labiatae. As a 

 rule, it is the outermost layer of the pericarp or testa that consists of 

 mucilage-cells; this is the case, for instance, in Linum, Salvia, 

 Plantago, etc., etc. This superficial location, with its attendant 

 advantages as regards ready access of water and unlimited space for 

 expansion, is, of course, highly appropriate in view of the functions 

 which these cells perform. Occasionally, however, mucilaginous 

 layers are also found in the interior of the testa, or on its inner 

 surface. 



The remainder of this section will be devoted to a more detailed 

 consideration of the mucilage-tissues of pericarps and seed-coats in a 

 few selected cases. 



The mucilage-layer which forms the outer surface of the seed-coat 

 of the Linseed {Linum usitatissimum), is often cited as a typical 

 example of this form of mucilage-tissue. Each of the prismatic 

 cells composing the mucilage-layer is separated from its neighbours 

 by a thin, sharply-defined middle lamella, which terminates externally 

 in the cutinised layers of the outer wall. The secondary thickening 

 layers, both in the outer and the inner walls, possess the pow T er 

 of swelling to an enormous extent when wetted. If they are allowed 

 to absorb water gradually, they at first exhibit a very evident 

 stratification, which soon disappears again as the swelling proceeds. 

 The thin inelastic middle lamellae cannot long resist the powerful 

 tension to which they are subjected, owing to the pressure exerted by 

 the swelling layers against the comparatively tough cutinised layers of 

 the outer wall ; sooner or later they give way, and the outer walls, 

 which are intersected by numerous cracks, are thereupon lifted up by 

 the prismatic masses of swelling mucilage. The outer walls thus form 

 a sort of covering, which prevents the deliquescence of the mucilage 

 from proceeding too rapidly. 



The nutlets of Salvia are likewise provided with a superficial 

 mucilage-layer, composed of prismatic cells, which are about twice as 

 high as their width. In this instance the secondary thickening layers 

 are deposited principally upon the lateral walls ; hence, when swelling 

 takes place, the results are somewhat different from those just 

 described in the case of Linum. The middle lamellae separating 

 adjacent cells do not break down, but form a delicate persistent 

 framework, from the meshes of which the swelling secondary layers 

 protrude in the form of long, tubular structures ; these tubes of 



