132 PLANT LIFE. 



loosen from the plant they may produce a ne\v 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 purpose, as in the cultivated cabbage. 



163. Structure.- -Three regions in each part may be dis- 

 tinguished, as in the root and stem : (i) the 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 

 in all essentials with that of the stem (see *\ 124 ff. ). The 

 epidermis forms the outer surface, frequently with hairs or 

 emergences (see ^[128, I2 9)- ^ ne 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, are present. There may be 

 a single stele, surrounded by an endodermis and containing 

 several or many vascular bundles (, fig. 158); or there may 



A B 



FIG. 158. Diagrams of transverse sections of petioles showing two most common 

 structures. A , petiole with several steles. B, petiole with one stele, containing a 

 number of bundle pairs, ec, cortex; ed, endodermis; /, pericycle ; f, bundle pair; 

 m, pith ; r, pith rays. The letters A, B stand on the upper or ventral side of petiole. 

 After Van Tieghem. 



be several steles, each surrounded by a special endodermis 

 and consisting of little more than a pair of vascular bundles 



