LEIPNER, ON SILICA IN PLANTS. 135 



long been known, its position defined, and its quantity 

 determined. It might at first sight appear unnecessary to 

 lay much stress iipon the existence of silica in a new order of 

 plants, Avhen we already have the two orders above named 

 afibrding such abundant examples of its presence, but there 

 are two points of interest in this manifestation of it. 



1st, that as yet, so far as I am aware, the presence of 

 silica has not been noticed in any indigenous exogens, and 



2dly (which is of more importance), that the silica exists 

 in such small and varied proportion, as to offer vastly 

 superior opportunities for investigating its mode of deposition. 



In the Graminese and Equisetacese the silica occurs chiefly 

 in the epidermis, where it exists in the form of a siliceous 

 tissue, exhibiting all the details of the epidermis, the outlines 

 and exact form of the epidermic cells and stomata; and enters, 

 moreover, into the composition of all the various structures 

 which are developed from it, such as spines and hairs. After 

 having destroyed all organic matter, either by burning, or, 

 what is still better, by boiling in nitric acid and subsequent 

 charring, we therefore still retain the epidermic structures in 

 the form of what might be called a siliceous cast. 



With regard to the Rubiacese, I have to make the same 

 remark, viz., that it is chiefly, if not solely, in the epidermis 

 that silica exists. But in order to define its exact position, 

 it will be desirable to recall attention to the structure of the 

 epidermis, and the first formation and after-development of 

 the vegetable cell. 



The epidermis consists of two organs, the cuticle and the 

 true epidermis. 



The cuticle, which, according to the present opinion of 

 most botanists, is present in all plants, is a thin, colourless, 

 transparent membrane, which overlies the true epidermis, 

 and is in general considered to be a secretion from the 

 outside of the epidermic cells below it. Only the stomata 

 are not covered by this membrane. 



Underlying the cuticle is the true epidermis, consisting of 

 one or more layers of flattened and compressed cells. In the 

 order before our notice, the outlines of these cells are all 

 flexuose, the stomata occurring at intervals between them, 

 but only on the mider side of the leaves and near the angles of 

 the stem. 



It is in the underlying or true epidermis, that the silica 

 occurs not only in the Rubiacese, but also in the Gramineae 

 and Equisetacese. But on observing an individual cell, 

 whether epidermic or not, and examining its first formation, 

 its growth and completion, we notice at first a simple vesicle, 



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