22 LECTURE II. 



The exact form in which silicon is present in cell-walls is not yet 

 ascertained; that is, whether it is present as particles of silica deposited 

 between those of the organic substance, or whether it enters into the 

 chemical composition of the organic substance, forming possibly a silicon 

 cellulose. The silica may be removed from the cell-wall by treatment 

 with hydrofluoric acid, without disorganising it. 



The salts of calcium which are found in cell-walls are the 

 oxalate and the carbonate. Calcium oxalate occurs either in 

 the form of minute granules or as distinct crystals, in various 

 parts of a great number of plants ; for instance, in the walls 

 of the epidermal cells of many Conifers and of species of^ 

 Semperyivum and Mesembryanthemum, in the bast-fibres of 

 many Taxineae and Cupressineae, in the cortex of many 

 Gymnosperms and species of Acer, Fagus, Salix, and in all 

 parts of Welwitschia and other Gnetaceae (Solms-Laubach). 

 Calcium carbonate is frequently present in the walls of hairs, 

 but it is more especially deposited in the ingrowths of the 

 cell-wall, known as cystoliths, which are found in the epidermal 

 cells of the Urticaceae and in almost all the tissues of the 

 Acanthaceae ; it occurs either as granules or as small crystals. 



It is still an open question whether calcium carbonate in cystoliths 

 exists in an amorphous or a crystalline form. Weiss, and more recently 

 Richter (see Fig. 6), have found that cystoliths are doubly refractive, and 

 they conclude that the calcium carbonate is present in the crystalline form ; 

 but this observation has not been confirmed by other observers, such as 

 Sachs and Melnikoff. 



Calcium carbonate is often found as an incrustation on the 

 surface of plants. In some cases it is evident that the incrus- 

 tation has been formed by the evaporation of water, holding 

 the salt in solution, which had been excreted by the plant : 

 in the cases of submerged plants, it is possible that the calcium 

 carbonate may be deposited on the surface in consequence of 

 the absorption of carbonic acid from the water by the plant. 



We have assumed, so far, that the cell-wall is a closed 

 membrane, which shuts off the contents of one cell from 

 those of adjoining cells ; but this is by no means always the 

 case. The sieve-tubes, for instance, consist of cells placed end 

 to end, the transverse walls of which are perforated so that a 



