EPIDERMIS. 107 



Statice, Armeria) occur numerous lime-scales scattered over the surface without any- 

 direct relation to the ends of the vascular bundles. Each of these appears on 

 the outer face of a small group of cells of peculiar form, and exactly similar groups 

 of cells occur in species in which no excretion of lime occurs, as Armeria vulgaris, 

 plantaginea, Statice scoparia, latifolia, purpurascens, alata (Mettenius). They consist 

 of eight cells, arising from one epidermal cell which appears in surface view rounded 

 and quadratic. This is divided by two walls, perpendicular to the surface and to 

 one another, into four : each of the latter again divides by a perpendicular wall into 

 two : one very small one forming the inner angle, and one being peripheral. The 

 cells of these groups are thin-walled, and contain dense finely granular protoplasm. 

 Their outer walls lie in many species at the surface : in others, especially thick- 

 skinned species, they form the base of hollow depressions, e. g. Statice alata, 

 purpurea, monopetala. 



(3) In water-plants, especially submerged ones, the whole epidermal surface is' 

 often equally covered with a thick layer of calcium carbonate. Reinsch ^ found 

 the lime-covering on the upper side of the swimming leaves of Potamogeton natans 

 continuous over each stoma during the active vegetation of the leaf. In many land- 

 plants also, which form lime-scales, — Saxifrages, e.g. S. crustata, Statice spec. — the 

 whole epidermis is covered with a thin crust of lime. 



The origin of the lime-coverings remains to be investigated. We may almost 

 imagine in the case quoted under (3) a precipitate having been formed by the re- 

 moval of carbonic acid from the lime-containing water, and explain the lime-scales 

 over the ends of the vascular bundles by the evaporation of drops of lime-con- 

 taining water, especially since the exit of such drops on the young leaf in the Ferns 

 and Saxifrages really takes place : and we may explain the incrustation which occurs 

 near the scales as arising from partial solution of the scales in water containing car- 

 bonic acid, and subsequent repetition of the evaporation. These are plausible 

 explanations, for which however the proof is wanting : for the excretion of Ume in 

 the Plumbagineae it is not admissible. 



Analyses of the lime-coverings showed in Potamogeton, besides calcium car- 

 bonate, traces of Silicic acid and oxide of iron (Reinsch), in Saxifraga crustata 

 (Unger l.c) to 4-146 parts calc. carb. 08 17 of carb. of magnesium. 



The scales of the Ferns, Saxifrages, and Plumbagineae leave behind a colour- 

 less gelatinous residue when the lime is dissolved with nitric acid. 



Incrustations of carbonates of alkalies are described in the case of the foliage of species 

 of Tamarix, Rfiaumaria, shore-plants. Notices on this: de CandoUe, Physiol, p. 237; 

 Treviranus, Physiol. Vol. II. p. 101; Unger, Anat. und Physiol, p. 369. Definite in- 

 vestigations do not exist. 



1 Flora, 1858, p. 723. 



