2 THE PLANT WORLD. 



let. At tliL- base of the twig is usuall}' found a elustcr of scars 

 niarlving the position of the bud-scales which feU off when the 

 bud expanded. The twig consists of central pith, surrounded by 

 a woodv zone. Outside of the woody zone is a zone of softer 

 tissues, a discussion of which would take us too far into anatomy 

 for the purposes of this paper. On the outside is the epidermis, 

 which may become ruptured on the older portions by the growth 

 in diameter. Alost of this outer zone becomes, in branches, the 

 layer commonly known as the bark. At regular intervals lateral 

 structures, leaves, are produced upon the growing stem, which, 

 at the advent of winter, are shed (in deciduous plants) by means 

 of the production of a corky layer near the base of the petiole. 

 The scars thus caused on the sides of the twigs are known as leaf 

 scars. In the axils of the leaves are normally produced the 

 lateral buds or nascent branches. The terminal portion of the 

 twig also usually has a bud which will continue the growth the 

 following season. These parts will now be taken up more in de- 

 tail. A few representative examples are given in each case. 



PitJi. — The pith is usually solid and continuous, white in color, 

 and circular in cross section. The pith is diaphragmed in the 

 walnut and hackberry, that is, though solid in the younger por- 

 tions, it soon separates into numerous thin, transverse plates. In 

 the grape vines the pith is interrupted at the nodes by woody 

 partitions. The color is brown in the smooth sumac, Rhus iila- 

 hra, and in .UhvifJiiis glandulosiis. The cross section is rhom- 

 boidal in the buckthorn, Rhaiinius, and in some other distinctly 

 4-angled twigs, and is 5-angled or star-shaped in the oaks. 

 Smila.v, and other monocotyledons have no distinct pith.. 



Tivig. — The surface may be smooth, pubescent, glaucous (Acer 

 Negiindo) ; roughened with lenticels (Rhus copallina) ; or spiny 

 ( Ribcs, Ruhus, Siuihi.v) ; cylindrical, angled, roughened by 

 corky ridges (bur oak, sweet gum, i'hiius ahita) : various shades 

 of brown, yellow, red, green and gray. Tn some cases the odor 

 of the bruised bark is characteristic as in sassafras and spice 

 bush. 



Leaf-scars. — The arrangement of the leaf-scars may be op- 

 posite and 4-ranked, or alternate. Tn a few cases there are more 



