EPIDERMIS. 49 



mechanical adhesion of originally independent filaments, 

 and not produced by cell-division, it is called pseudo- 

 parenchyma. A similar structure is formed by many fungi. 

 True Epidermis is first shadowed in amongst the 

 Muscinece, including the Liverworts, Bog-mosses, and true 

 Mosses, and curiously enough in the Liverworts or March- 

 antiece, a primitive group, we suddenly come upon an 

 epidermis perfectly developed and furnished with complex 

 stomata ; whereas in other divisions of the Muscinece much 

 more highly organized, the epidermis is very rudimentary, 

 or confined to certain portions only. In vascular plants 

 the epidermis consists at first of a single superficial layer 

 of cells differentiated from the meristem. Eventually this 

 layer sometimes S]Dlits into two or more by cell-division 

 taking place parallel to the surface ; however, the outermost 

 layer always forms the epidermis proper. The cells of the 

 epidermis are at first in contact on all sides, consequently 

 without intercellular spaces. The contour of the epidermal 

 cells varies, being elongated in the direction of growth in 

 long leaves and stems, and more or less quadrate in 

 structures with a broad surface; very frequently the side 

 walls are more or less waved. The free or outside walls 

 of epidermal cells become very much thickened and cuti- 

 cularized. As a rule, epidermal cells do not contain 

 chlorophyll or starch ; these are, however, present in the 

 epidermal cells of Ferns and some aquatic plants. At an 

 early period of development stomata or openings through 

 the epidermis are formed. The preliminary for this is the 

 formation of a mother-cell, formed by the division of an 

 ordinary epidermal cell, one-half of which constitutes the 

 mother-cell. This mother-cell is then divided by a wall into 

 two equal parts or cells, termed the guard-cells of the stoma. 



E 



