112 PART II. ANATOMY AND HISTOLOGY. [ 31 



98 C], some Umbelliferae and Papilionacese, many Equisetums ; 

 leaf-blade of Cyperaceae, Typha, Sparganium, many Palms). 

 The spines of leaves (e.g. Holly), also entire spiny leaves or 

 stipules, various emergences, such as the warts of Aloe verrucosa 

 and the prickles of the Rose, and the thorny branches of many 

 plants (e.g. Hawthorn, etc.) owe their hardness mainly to the 

 development of sclerenchymatous hypoderma, the cells of which 

 are generally elongated and fibrous, though they may be short as 

 in Aloe verrucosa and the Rose. 



The hypoderma of the root commonly consists of a single layer 

 of cells, which is then the exodermis ; but in some plants the 

 hypoderma consists of several layers (e.g. the Date, Pandanus, 

 Asparagus, etc.). 



The walls of the exodermal cells generally undergo cuticularisa- 

 tion and frequently become very much thickened, especially on 

 the lateral and external walls, in view of the position which it 

 eventually occupies as the external layer of the root (see p. 109). 

 In some cases it presents a peculiar localised thickening in the 

 form of a band extending round the upper, lower, and lateral 

 walls of the cells, a thickening which is therefore confined just to 

 the surfaces which are in contact with other cells belonging to 

 the same layer, and which appears in a transverse section as a 

 dark dot on the radial walls of the cells. 



In some cases the cells of the exodermis are prosenchymatous 

 and sclerenchymatous (e.g. species of Carex). 



When the exodermis is invested by tegumentary tissue, as in 

 aerial roots of Orchids (Fig. 87), some of its cells retain then-Thin 

 unaltered walls, and are the passage-cells, by means of which 

 water can penetrate into the interior of the root. 



3. The general ground-tissue, of stems, leaves, and roots, lying 

 within the hypoderma, consists mainly of parenchymatous tissue, 

 with, frequently, a considerable differentiation of masses of fibrous 

 sclerenchymatous stereom. 



In aerial stems and foliage-leaves, the more external, at least, 

 of these cells frequently take part in the assimilatory processes of 

 the plant ; the cells contain chloroplastids and constitute assimi- 

 latory tissue. Towards the most highly illuminated surface of the 

 member, the cells are frequently so arranged that their longer 

 axes are perpendicular to the surface, that is, are parallel to the 

 incident rays of light; assimilatory tissue of this structure is 

 termed palisade-tissue : the whole of the internal ground-tissue of 

 a leaf-blade is termed generally mesophyll. 



