404 STORAGE SYSTEM 



the cell-walls, certain of the secondary thickening layers being 

 mucilaginous in character, and hence specially capable of retaining 

 water. Nevertheless, mucilage-cells of this latter type are not treated 

 separately here on account of the morphological relations of the muci- 

 lage, since these are of no particular importance from the anatomico- 

 physiological point of view. The distinguishing characteristics of these 

 tissues is, on the contrary, of a physiological nature, and consists in the 

 fact that the absorption of water by the cells of typical water-tissues 

 is directly dependent upon the presence of a living protoplast or at 

 any rate a plasmatic membrane whereas this is not so in the case of 

 typical mucilage-tissues (with mucilage derived from cell-walls). 



Where a mass of mucilage consists of greatly thickened cell-walls, 

 it is often very distinctly stratified ; the mucilaginous thickening layers 

 exhibit their characteristic physical and chemical properties as far as 

 our observations extend from the moment that they are laid down 

 [i.e. they do not acquire these properties by secondary modification]. 

 As a rule the primary layers of the wall do not become mucilaginous, 

 unless the mucilage-cells are collected into groups or rows ; in this case 

 the primary layers often break down and disappear, so that extensive 

 cavities (Tiliaceae) or long ducts (Fegatella) are produced. The muci- 

 laginous thickening layers may be laid down uniformly over the whole 

 extent of the cell-wall, but in other instances are confined to a limited 

 area; in the latter event, the cell-cavities often become reduced to mere 

 slits, and are generally pushed very much to one side. 



Attention has already been directed, on a previous occasion (p. 114), 

 to the epidermal cells with thickened mucilaginous inner walls which 

 occur in a variety of plants ; otherwise mucilage cells, in leaves, and in 

 vegetative organs generally, are almost always idioblastic in character, 

 whether they are solitary or collected into groups or rows. A few 

 leaves {e.g. those of Conocephalus ovatus and Rhizophora mucronata) con- 

 tain typical mucilage-cells in addition to a characteristic water-tissue 

 devoid of mucilage. In Bhizophora mucronata elongated mucilage-cells, 

 with the excentric type of thickening, are found at the boundary of the 

 water-tissue and the palisade-layer, projecting for some distance into 

 the latter. 



The bark of a number of desert-plants (Haloxylon, Eurotia, Calli- 

 gonum, Halimodendron) contains groups of mucilage-cells, which, 

 according to their discoverer, Jonsson, arise from a phellogen, and 

 hence receive the name of " mucilage-cork." 1S ' These cells sooner or later 

 burst through the overlying layers of ordinary cork, and thus come to 

 the surface ; thenceforth they act not only as water-storing elements, 

 but also as organs of water-absorption. 



A further instance of water-storage is furnished by the so-called 



