160 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 32. 



still the endodermis, but each individual stele is invested by a 

 distinct endodermis ; here the fundamental tissue includes not 

 merely the cortex, but also the tissue between and among the 

 steles. The relations of the parts are very much the same in a 

 schizostelic as in a polystelic member (see Fig. 116 5). 



The following are the regions or layers of the primary extra- 

 stelar tissue which can be distinguished morphologically. 



1. The tegumentary tissue, in those parts in which there is no 

 true epidermis. 



2. The hypoderma is the external layer (or layers) of the funda- 

 mental tissue where a true epidermis is present ; when the integu- 

 mentary tissue is not a true epidermis but is derived from the 

 external layer of the cortex, ihe hypoderma is derived from the 

 outermost layer but one of the primitive cortex (periblem) ; in the 

 root, the hypoderma is distinguished as the exodermis. 



3. The general ground-tissue. 



4. The endodermis, the layer of the ground-tissue which abuts 

 on a stele; in the root the endodermis is the innermost layer of the 

 internal cortical tissue. 



Structure. Speaking generally, the extra-stelar fundamental 

 tissue consists mainly of parenchymatous cells which have cellu- 

 lose walls and retain their protoplasmic cell-contents ; however, 

 supporting-tissue (stereom) is largely differentiated in the funda- 

 mental tissue, whether as a collenchyma or as sclerenchyma. In 

 cylindrical members (stems, roots, etc.) the cells are generally 

 somewhat elongated in the direction of the long axis of the 

 member. 



1. The tegumentary tusue derived from the periblem has already 

 been described (p. 154). 



2. The hypoderma of stems and leaves commonly consists of 

 stereom, either collenchymatous or sclerenchymatous : 



collenc/iymatous hypoderma is especially characteristic of the 

 stems and leaf-stalks of herbaceous Dicotyledons (see Fig. 91, p. 

 132), but it occurs also among Pteridophyta in the petioles of the 

 Marattiaceae : 



sclerenchymatous hypoderma may form a continuous layer of 

 more or less prosenchymatous cells (e.g. stem of some Ferns, 

 Equisetum hiemale, most Selaginellas, Casuarina, leaf of many 

 Cycads, Conifers, some Orchids, etc.) ; or it may form numerous 

 isolated strands (e.g. stems of Cyperacese, species of Juncus [Fig. 

 124 0], some Umbelliferae and Papilionaceae, many Equisetums ; 



