TISSUE SYSTEMS 



327 



In a growing point of the sporophytes mentioned above, whose 

 structure has been described, indications of these three systems 

 can be seen. In fig. 701 we have a representation of such a 

 growing point. The outer layer of cells is seen to be continuous 

 over the surface ; it forms 

 the dermatogen, which when 

 adult becomes known as the 

 cjiidermis. Centrally there is 

 a mass of cells which show a 

 general tendency to become 

 longer than broad. This is 

 Itnown as the plerome ; it de- 

 velops generally into a bulky 

 strand or cylinder of tissue 

 known as the stele, in which 

 well-definedcoUections of cells 

 knoivn as vascular bundles 

 originate. Between the two 

 there is a cylinder of tissue 

 known as the^ieriblein, which 

 consists of ground or funda- 

 mental tissue. In some grow- 

 ing points with small-celled 

 meristems these three sys- 

 tems can be seen to originate 

 indifferent layers of the meri- 

 stem. In others the differentiation cannot be traced so far in the 

 direction of the apex. In further development the stele is 

 usually found to contain a certain amount of ground tissue 

 besides the vascular bundles, yfe may therefore distinguish 

 between stelar and extra-stelar ground tissue, the latter alone 

 arising from the periblem. 



Fig. 701. Growing point of the stem of a 

 Dieotyledon. After Douliot. d. Dermato- 

 gen. pe. Periblem. jj/. Plerome. ?. Young 

 leaf. The dark lines are exaggerated to 

 indicate the limits of the pleromCj peri- 

 blem, and dermatogen. 



The Epidermal Tissue System. 



The body of nearly all plants which consist of a mass of 

 cells is covered by a thin membrane composed of a sheet of 

 cells. In the shoots of the higher plants this is derived from 

 the entire dermatogen, and is known as the epidermis. It is 

 usually only one cell thick. In the roots the dermatogen gives 

 rise at the apex to many layers of cells, forming the root-cap. 

 In most Dicotyledons the innermost of these layers can be 

 traced backwards over the surface, and bears the root-hairs. 



