32.] 



CHAPTER 11. THE TISSUES. 



165 



large air-cavities, either lysigenous or schizogenous (see p. 130) ; 

 generally speaking, they are of schizogenous origin in aquatic 

 plants, of lysigenous origin in land-plants (see Fig. 116 6). These 

 cavities frequently extend throughout the whole length of the 

 root or the leaf, and through an entire internode of the stem ; 

 but they may be interrupted at intervals by diaphragms {e.g. leaf 

 of some Monocotyledons ; root of Hydrocharis ; stem of Alisma, 

 Pontederia, Marsilia). When these cavities are largely developed 

 the member becomes a float {e.g. root of Jussiaea, see p, 64). 



Assimilatory tissue is present in some aerial and some aquatic 

 roots {e.g. some 

 Orchids, Podo- 

 stemaceae, etc.). 



4. The Endo- 

 dermis is, in the 

 great majority 

 of cases, a single 

 layer of cells ; it 

 is but rarely al- 

 together want- 

 ing {e.g. roots of 

 Podostemaceae) ; 

 it sometimes 

 consists of two 

 layers, formed 

 by the tangen- 

 tial division of 

 the cells of the 

 primitively sin- 

 gle layer {e.g. 

 root of Equise- 

 tum ; stem of 

 some Pterido- 



phyta, such as the rhizome of Nephrolepis, and the stem of Salvinia 

 and Azolla). 



Most commonly the cells of the endodermis are thin-walled, 

 with a suberised thickening-zone extending round the lateral and 

 upper and lower surfaces of the wall (see exodermis, p. 161), and 

 showing in transverse section (Fig. 127) as a black dot on the 

 radial wall. This peculiar marking is by no means always pre- 

 sent : it is frequently wanting in the endodermis of the stem in 



ed 



FIG. 127. Transverse section of central portion of the root of 

 Ranunculus repens (x 300): ed the endodermis, enclosing the 

 single central stele ; its radial walls show the sections of the 

 cuticularised thickening-bands ; x the four protoxylem-bundles ; 

 t the solid xylem; s the four phloem-bundles ; pc the pericyclej 

 Y the cortical ground-tissue. 



