40 BOTANY. 



rudimentary tissues may be distinguished before the per- 

 manent tissues have formed. Thus an outer layer, the 

 dermatogen, whose cells divide only at right angles to the 

 surface, eventually develops into the epidennis. In the 

 centre is a mass of elongated cells, the plerome, from which 

 the fibro-vascular system develops, while between plerome 

 and dermatogen is the perihlem, in which arise the various 

 tissues of the fundamental system. 



66. The Epidermal System of Tissues. — This is the sim- 

 plest tissue-system, as it is the earliest to make its appear- 

 ance, in passing from the lower forms to the higher. It is 

 also (in general) the first to appear in the individual devel- 

 opment of the plant. It is sometimes scarcely to be sepa- 

 rated from the underlying mass, as in most lower plants ; 

 but in most higher plants it frequently attains some degree 

 of complexity, and is sharply separated from the under- 

 lying ground-tissues. 



67. In the simpler epidermal structures of the lower 

 plants the cells are generally darker colored, smaller, and 

 more closely approximated than they are in the subjacent 

 mass; in some of the higher fungi a boundary tissue may 

 be easily separated as a thickish sheet, but probably in such 

 case a portion of the underlying mass is also removed. In 

 many lower plants there is absolutely no differentiation of 

 an epidermal portion. 



68. The epidermal systems of ferns and flowering plants 

 consist usually of three portions : (1) a layer of more or less 

 modified parenchyma — the epidermis proper — ^bearing two 

 other kinds of structures which develop from it, viz., (2) 

 hairs, and (3) breathing-pores. 



69. Epidermis. — The differentiation of parenchyma in 

 the formation of epidermis, when carried to its utmost ex- 



