EPIDERMIS. 



[ 295 ] 



EPIDEKMIS. 



bous plant, and seize a little piece of the 

 ragged edge with a pair of fine forceps, we 

 may strip off large pieces of what appears, 

 to the naked eye to be a thin homogeneous 

 pellicle. When this is placed under the 

 microscope, it is found to be composed of a 

 layer of cells united firmly together by their 

 sides like stones in a pavement, but loosely 

 connected with the subjacent tissue, which 

 adheres here and there to the detached strip 

 in ragged patches. The firm continuous 

 layer of cells is what botanists call the 

 epidermis of plants. Such a layer of cells 

 clothes the entire surface of the higher 

 plants, from the Flowering plants down to 

 those in which the organs, such as the 

 leaves, are reduced to mere layers of cells 

 like the epidermis itself, as in the Mosses. 

 In a very young and delicate state, such as 

 we find it clothing the surface of organs 

 still concealed in buds, or of young ovules 

 in the ovary, it has been called epiblema. 

 A rather more solid form, but still soft and 

 devoid of thickening layers, such as exists 

 on the surface of the growing parts of root- 

 lets &c., is called epithelium. Both these 

 terms appear useless, and only calculated 

 to confuse the student still more than the 

 use of the words epidermis and cuticle, 

 which already endanger misconception from 

 the very different characters of the struc- 

 tures called by those names in animal 

 organs. 



When a layer of epidermis is macerated 

 in nitric acid, a thin pellicle, destitute of 

 cellular structure, becomes detached in 

 sheets from the outer surface of the plate of 

 epidermal cells; this is the cuticle (fig. 199) 



Fig. 199. 



Cuticle of a cabbage-leaf, removed by the action of nitric acid. 



P, hairs ; F, orinces corresponding to stomata. 



Magnified 250 diameters. 



of botanical anatomists, concerning which 

 much misconception has prevailed. As 

 epidermis advances in age it becomes con- 

 siderably solidified, especially on evergreen 

 leaves, and on shoots of shrubs &c. which 

 remain green for a lengthened period, such 



as Aucuba and Viscum. In most cases, how- 

 ever, the epidermis of structures belonging 

 to the stem disappears about the same time 

 as the leaves fall off, and is replaced by the 

 suberous layer of the bark structure, which 

 change is evident externally by the surface 

 assuming a brown colour, the subjacent 

 tissue containing chlorophyll being hidden. 

 The green colour of parts clothed with epi- 

 dermis depends upon the subjacent tissue 

 showing through the transparent epidermis, 

 the cells of which are usually colourless, and 

 filled with watery contents. 



When sections are made perpendicularly 

 to the surface of any fully developed leaf, 

 but above all of those of leathery texture, 

 the walls of the cells next the external 

 surface are found much thicker than the 

 rest, this thickening extending more or less 

 down over the contiguous side walls. When 

 such sections are treated with sulphuric acid 

 and iodine, the greater part of the thick- 

 ness, from without inward, of this outer 

 wall is stained yellow, while the rest of the 

 walls assume the blue colour ordinarily 

 taken by cellulose with these reagents. 

 Some authors suppose that the whole of 

 this yellow part corresponds to the cuticle 

 above mentioned: but such is not the case; 

 if such a section is boiled or macerated for 

 a long time in solution of caustic potash, 

 then washed well with water and treated 

 with tincture of iodine, the thick upper 

 wall also assumes the blue tint, and, more- 

 over, a laminated structure becomes evident 

 in it, showing that it is produced by the 

 deposition of secondary layers inside the 

 cell. The true layer of cuticle (which is 

 dissolved off by the continued action 

 of potash) is really extremely thin 

 in almost all cases. The true nature 

 of this thickening of the outer walls 

 is well illustrated by the epidermis of 

 Viscum (Mistletoe), which remains 

 upon the shoots for many years ; here 

 several layers of cells subjacent to the 

 original superficial stratum become 

 involved in the process of solidifica- 

 tion, and their cavities completely 

 filled up by the secondary deposits. 

 The true structure of the enormously 

 thick epidermal layer of old shoots, 

 as brought out by the action of potash, is 

 seen in the example of PI. 47. fig. 26. The 

 true cuticle is sometimes of considerable 

 thickness, as in the leaves of Cycas (PI. 47 

 fig. 28). The thickening layers of the epi- 

 dermal cells are true SECONDARY DEPOSITS. 



