132 PLANT LIFE. 



loosen from the i)Iant they may produce a new plant, as in 

 the tiger-lily (see *\ 361-364). Both base and blade may be 

 used for storage, as in the century-plant; or the entire leaf 

 may serve the same jjurpose, as in the cultivated cabbage. 



163. Structure. — Three regions in each jjart may be dis- 

 tinguished, as in the root and stem : (i) tlie epidermis ; (2) 

 the cortex ; both continuous with that of the stem ; (3) the 

 Steles, continuous with those of the stem -when the latter 

 contains several steles, or branches of it when the stem con- 

 tains a single stele. 



164. (a) The petiole. — The structure of the petiole agrees 

 iir all essentials "with that of tlie stem (see •, 124 ff. '). The 

 epidermis forms the outer surface, frequoitly with hairs or 

 emergences (see ^[128, 129). The cortex consists of 

 rounded or cylindrical thin-walled cells, the outer layers 

 containing chlorophyll, and frequently with angles much 

 thickened for strength. Mechanical tissues forming strands 

 or bands are also frequently present in the cortex. In water 

 plants, e.g., in water-lilies, large intercellular chambers, 

 often forming extensive canals, arc present. There may be 

 a single stele, surrounded 1)}' an cndodermis and containing 

 seyeral or many yas( ular bundles ( B, fig. 15S ); or there may 



.-I r. 



Fig. 1 5S.— Diagrams of trans\'erse sections of petif>les showing two most common 

 structures. A, petiole witli several steles. /', petiole with one stele, containing a 

 number of bundle pairs. <-, cortex; en, endodermis ; //v, phloem; j-, xylem • 

 111, |nlh ; ;-, pith rays 'I he letters i . li stand on the upper or ventral side of petiole' 

 — .'\tter Van Tie^hem. 



be se\'eral steles, each surrounded b\- a S]iccial endodermis 

 and consisting oflittle more than a ]iair of vascular bundltjg 



