3o6 THE STRUCTURE OF ECONOMIC PLANTS 



The Leaf. — The petiolate basal leaves which form a rosette in 

 the first phase of vegetative development are variously incised and 

 deeply lobed or lyrately pinnatifid. The petiole encircles the 

 stem for about a third of its circumference, and its basal margins, 

 which are winged, continue as a ridge as far as the lobes of the 

 lamina. (Fig. 154, /4.) The vascular supply consists of five collat- 

 eral bundles which occupy positions corresponding to the lobing of 

 the petiole. Each bundle is made up of three to five units which 

 are separated from one another by parenchymatous rays. The 

 bundles are crescent-shaped with the open arc directed toward 

 the adaxial surface of the petiole, and transverse veinlets cross- 

 connect the main ones. There is a well-defined cambium in the 

 principal veins which gives rise to some secondary vascular tissue. 

 The phloem is bounded by a zone of mechanical tissue, comparable 

 to the pericyclic strands in the stem; and the parenchymatous cells 

 adaxial to the bundle and between the rows of the secondary xylem 

 may also be thick-walled. 



The subepidermal layers are chlorophyllose and there are chloro- 

 plasts in the cells immediately outside the mechanical tissue of the 

 bundles, in the phloem parenchyma, and in the parenchymatous 

 rays separating the radial rows of xylem in each bundle. Antho- 

 cyanin may occur in the cells of the epidermal layers, but this con- 

 dition is a variable one. 



The surfaces of the blade are much roughened, and the veins 

 form prominent abaxial ridges. The degree of pubescence is not 

 constant, but commonly there are epidermal hairs sparsely distrib- 

 uted over both surfaces, especially along the veins on the abaxial 

 one. The large, stiff, unicellular hairs extend from a basal group 

 of slightly raised epidermal cells. (Fig. 154, D.) The latter are 

 roughly pentagonal or hexagonal in surface view except over the 

 veins, where they are rectangular and much elongated, with their 

 long axes parallel to the vein. The mesophyll consists of a single- 

 layered palisade region and the spongy parenchyma which is several 

 cells in thickness. The air passages are large and some of the 

 palisade cells may be transversely divided so as to form a partial 

 double layer. (Fig. 154, C.) Chloroplasts occur in all the cells in 

 the mesophyll and the guard cells, but are absent in the tissue 

 surrounding the principal veins. At these points, the parenchyma- 

 tous cells are compact and there is a sheath of mechanical tissue 

 as in the petiole. The venation is pinnate-reticulate with fre- 



