Biology of Higher Plants — Anatomy and Physiology 223 



sible to distinguish pith and cortex. The vascular bundles and the stem 

 as a whole do not increase in diameter beyond a certain point. Each 

 vascular bundle is enclosed by mechanical tissue composed of long, thick- 

 walled cells. These mechanical tissues, together with those just beneath 

 the epidermis, give rigidity to the stem. 



The functions of a stem include the support of flowers and leaves, the 

 manufacture of foods (in certain instances), the storage of materials, 

 and the conduction of materials. 



III. THE LEAVES 



Leaves vary in size and shape in diff"erent plants (Figs. 53, 55, 57, 60, 

 and 70) . They vary from the needlelike leaves of the pine, to the large 

 leaves of certain palms. A typical leaf consists of (1) a flat, expanded 

 blade (lamina) ; (2) the stalk (petiole) ; (3) the base for attachment. 



/ . E 



Cross section 

 of jbem 



Carpel (pistil) 



cJf Cortical parenchyma 

 ■^{Lndodermis i 



Q[Hypoderrnis\ ,\ 



Pith ' 



Parenchyma 

 --ce//f 



— £ndodermr5 



yylem 



Cambium 



Phloem 



-Xylem 



Fibers 



^_ Per ('cycle 

 ^_Ph/oom p 



Cortex _ _ 

 Epidermis^ ^ 



Fig. 70. — Buttercup (Ranunculus sp.). A, Leaf; B, flower and its parts 

 (carpels are also known as pistils) ; C, root in cross section with its central stele 

 (composed of xylem, phloem, and pericycle) shown enlarged in D; E, stem in 

 cross section with a portion enlarged in F, showing the detailed structures, par- 

 ticularly the vascular bundle. Note the difference in the locations of xylem and 

 phloem in the root and stem. Observe the nectar-producing nectary at the base 

 of the petal. 



