31] CHAPTER II. THE TISSUES. Ill 



Morphology. The limits of this tissue vary with the structure of 

 the part concerned. The external limit of the ground-tissue is the 

 layer of cells lying immediately beneath the primary tegumentary 

 tissue. Again, when the member is monostelic, the internal limit 

 of the ground-tissue is the layer termed the endodermis, which 

 abuts upon the central stele ; in this case the ground-tissue consists 

 of several layers of cells bounded externally by the true epidermis (if 

 present), or reaching to the surface, and bounded internally by the 

 stele, when it is spoken of as the cortex of the member of which 

 it forms part. In a polystelic member, the internal limit of the 

 ground-tissue is still the eudodermis, but each individual stele is 

 invested by a distinct endodermis ; here the primary ground- 

 tissue includes not merely the superficial layers (cortex), but also 

 the tissue between and among the steles. 



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

 tissue which can be distinguished morphologically. 



1. The hypoderjnff, is the external region of the ground-tissue: 

 the external layer of the hypoderma is distinguished as the 

 exodermis. 



2. The general ground-tissue. 



3. The cndodermis, the layer of the ground-tissue which abuts 

 on a stele ; in monostelic members the endodermis is the inner- 

 most layer of the cortex. 



Structure. Speaking generally, the ground-tissue consists 

 mainly of parenchymatous cells which have cellulose walls and 

 retain their protoplasmic cell-contents ; however, supporting- tissue 

 (stereom) is largely differentiated in the ground-tissue, whether as 

 eolUnicliyma or as sclerenchyma. In cylindrical members isteins. 

 roots, etc.) the cells are generally somewhat elongated in the 

 direction of the long axis of the member. 



1. The hypoderma of stems and leaves commonly consists of 

 either collenchymatous or sclerenchymatous stereom (see p. 92) : 



collenchymatous hypoderma is especially characteristic of the 

 stems and leaf-stalks of herbaceous Dicotyledons (see Fig. 69, p. 

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

 Marattiacese : 



sclerenchymatous hypoderma may form a continuous layer of 

 more~^oFn[ess prosencnymatous cells (e.g. stem of some Ferns, 

 Equisetum hiemalc, most Selaginellas, leaf of many Cycads, 

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

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



