538 



SECRETORY AND EXCRETORY SYSTEMS 



its margins. They either lie in the immediate neighbourhood of the 

 bundle-ends, or are actually intersected by them. Very often the 

 distal ends of the terminal tracheides penetrate into the centre of such 

 a corpuscle (Fig. 218). 



It is comparatively unusual for silica to be deposited in the cavities 

 of cells. In various Orchidaceae, Scitamineae, and Palms, and also 

 in the Hymenophyllaceae, the fibrous strands are accompanied by 

 numerous small tongue-shaped cells, the stegmata of Mettenius. In 



Fig. 218. 



T.S. through a silica-corpuscle in the leaf of Loranthv.s europaeus. The corpuscle is 

 penetrated by a bundle-end, composed of a few spiral tracheides. 



these cells the walls next the fibres are more or less extensively 

 thickened ; the opposite walls bulge outwards, and consist of a thin 

 median strip, which becomes gradually thicker towards either margin. 

 Each stegma contains a mass of silica, which, as a rule, fills the cavity 

 more or less completely, and seems to be devoid of any organic basis. 



Mobius has described the small spherical concretions of silica which 

 occur in the leaves of Callisia repens (Commelynaceae). The bodies in 

 question, which have a rough surface, are located in flattened cells cut 

 off on the outer side of ordinary epidermal elements by periclinal 

 walls. Each cell contains several concretions, which are separated 

 from one another by outgrowths of the cell-wall ; the cell-cavity is 

 thus transformed into a system of passages, in which the silica con- 

 cretions are enclosed. 



Space will not permit of more than a bare reference to the peculiar 

 silica-corpuscles that occur in the Podostemaceae, Chrysobalaneae, 

 and in certain other families. 



Silicified cells and silica-corpuscles have been included in the 

 category of excretory organs, mainly because nothing definite is known 



