32.] 



CHAPTER II. THE TISSUES. 



161 



leaf- blade of terrestrial forms of Isoetes, and of Cyperacece, Typha, 

 Sparganium, Dasylirion, Phormiura, many Palms, Welwitschia). 

 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 develop- 

 ment of sclerenchymatous hypoderma, the cells of which are 

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

 Aloe verrucosa and the Rose. 



A peculiar form of hypoderma, termed aqueous tissue, is present in the 

 leaves of certain plants (?.g. some Ferns, Polypodium Lingua, Aspidium 

 coriaceum ; species of 

 Tradescantia ; also in 

 the S&itaminese, var- 

 ious Palms, Orchids, 

 Bromeliaceee, etc.) ; 

 it consists of one or 

 more layers of thin- 

 walled parenchyma- 

 tous cells, destitute 

 of chloroplastids, 

 containing much 

 watery sap, and 

 arranged closely to- 

 gether without inter- 

 spaces ; the tissue 

 serves as a reservoir 

 for water. 



FIG. 121. Diagram (after Schwendener) illustrating the dis- 

 tribution of the supporting-tissue or stereom, as seen in trans- 

 verse section of stems : A of Arum, maculaium having isolated 

 cortical stereom-strands; B of Allium, vineale, with continuous 

 pericyclic stereom-ring ; C of Juncus glaucus (hollow), with 

 hypodennal stereom-strands and conjunctive stereom-strands : 

 / vascular bundles ; s stereom-strands ; I air-cavities. 



The hypoderma 

 of the root, the 

 exodermis, com- 

 monly consists of 

 a single layer of cells, but in some 'plants the primitive layer 

 undergoes periclinal divisions, so that the exodermis comes to con- 

 sist 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. 158). 

 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 



V. S. E. M 



