258 SUMMARY OF CURRENT RESEARCHES RELATING TO 



A reaction worthy of special attention is that of tannin with osmic 

 acid. If to a solution of tannin a little hydrochloric acid and then a few 

 drops of osmic acid (1 per cent, solution) are added, an intense blue is 

 obtained. With osmic acid alone a black colour is usually obtained ; an 

 exception to this is Sedum TelepMum, which gave blue. 



Usually the tannin is spread imiformly through the cells, but sometimes 

 some of the cells become as it were reservoirs of tannin, while others con- 

 tain watery cell-sap. In Sedum TelepMum numbers of these reservoirs or 

 idioblasts are found in both the upper and lower epidermides. They are 

 easily recognized by being much larger than the adjacent cells. Those on 

 the lower face of the leaf contain in addition a rose-coloured pigment. 



With a salt of iron the ordinary cells give a greenish-black colour, and 

 the idioblasts a bluish-black ; we have here the two forms of tannin side 

 by side. In the genus Primula a number of the species contain very 

 characteristic idioblasts. 



(2) Soluble starch. — This substance occurs in some plants, almost ex- 

 clusively in the epidermal tissue. 



(3) SphserO'Crystals of Linaria striata. — On treating fragments of the 

 epidermis of this plant with alcohol, the sphaero-crystals are seen adhering 

 to the walls of the cells. They are of a yellowish colour, and their organic 

 nature can be demonstrated by the action of heat. 



(4) Crystals of calcium oxalate sometimes occur in the epidermal cells. 

 They are either enclosed in sjDCcial cells (Euonymus latifolius), or are 

 scattered sparsely in the ordinary cells (^Cynosurus cristatus), or occasionally 

 a mass of small crystals in various forms is found (Commelyna communis^. 



(5) Crystalloids and analogous bodies. — Bodies which belong to this class 

 are found in the epidermal cells of Campanula tliyrsoidea. They are 

 coloured yellow by iodine, and swell up under the action of caustic 

 potash. 



(6) Oil. — Occasionally in epidermal cells and stomata, e. g. Weigelia 

 rosea, Hoy a carnosa, &c. Idioblasts containing oil are found in Asarum 

 europseum, AristolocMa rotunda, and Asperula taurina. Those in the upper 

 epidermis of Asarum europseum are 20-35 /a in diameter, those of the 

 lower epidermis 40-70 /x, long, 30-40 /a broad. The oil is coloured brown 

 by osmic acid, and can easily be extracted by ether or alcohol. 



(7) Chlorophyll is often present in epidermal cells, as has been already 

 pointed out by M. Stohr. In Sioertia perennis, Cucurbita Pepo, &c., 

 chlorophyll granules were found inclosing starch- grains. 



(8) Pigments. — Kose-coloured pigments exist in the epidermal cells of 

 certain plants. In AnagalUs arvensis the pigment is confined to special 

 cells. 



The above substances can be classified in two categories from a 

 physiological point of view. 



(1) llie assimilating substance or chlorophyll. — Where this substance 

 occurs in abundance, as, for example, in ferns, the epidermal tissue produces 

 starch largely. 



(2) Substances eliminated by the plant in the course of its) chemical 

 transformations. Among these tannin, soluble starch, calcium oxalate, and 

 oil may be mentioned. The epidermal tissue here acts as a reservoir for 

 the substances which are no longer of service to the plant. 



(3) And finally, ivater, in those plants where the epidermis plays the 

 part of a reservoir, and contains the water required by the leaf for the 

 function of transpiration.* 



• Cf. infra, p. 2G1. 



