380 



Vegetable Physiology. 



organ as a continuous layer, forming a kind of skin com- 

 pletely investing and perfectly enclosing all the succulent struc- 

 tures, &c. The cells of this layer do not participate in the vital 

 activity of the succulent tissue, and, when complete, are found 

 filled Avith simple watery fluid, seldom containing starch, colour- 

 ing matter, or the like, which exist in the subjacent paren- 

 chyma. In the great majority of plants that side of the walls- 

 of the epidermal cells which is turned towards the external air 

 becomes thicker than the other sides, this being the case to a 

 greater or less extent in different cases, in excessive instances 

 giving the hard horny character to the surface of the leaves 

 observed in many evergreen plants. This thickening results 

 from the deposition of new lamellae of cellulose on the inside of 

 the exposed wall, which deposition may go on until the cavity is 

 nearly filled up and the cell converted into a solid body (fig. 3). 

 Upon leaves and young stems the thickening layers are generally 

 formed of hard cellulose, obstinately resisting decomposing agents; 

 but the epidermis of many seeds, although of great solidity, has the 

 cells thickened or filled up by lamelLne of a less resisting modifica- 

 tion of cellulose, as we see in the horny skin of the seeds of beans, 

 and still more remarkably in the skin of the seed of the quince 



Fiff. 



Terpendicular section of the epidermis or skin of the leaf of the garden hyacinth, soaked in solution 

 of potash: a, the pellicle (never coloured blue by iodine), apparently formed by the chemical 

 alteration of the outer layer of the wall of the cells; b, ihc lamellai thickening the side of the 

 cell next the surface ; the laminated structure is invisible when Iresh, and they then [are 

 Coloured yellow by iodine, but after the action of potash the lamella; become distinct, and axe 

 coloured blue by iodine ; c, the subjacent cells of the leaf. Magniticd 400 diameters. 



and linseed (fig. 4), where the cells are thickened by lamellae of 

 a gummy consistence, which softens and swells up in hot water 

 like the tissue of the seed-lobes of the bean and other leguminous 

 plants. Still further alterations of the epidermal cells, dependent 

 chiefly on chemical operations, as the formation of resinous or 



