:2-26 JiiycTF anatomv of FLoWERiyo plants 



growth, is stretelied, and finally rent by the continual 

 enlargement of the wood within; it is weathered and 

 worn, and thrown off in fragments, in some trees rapidly, 

 in others more slowly, so that the bark of old trunks 

 mav acquire great thickness. Similarly in Honej'suckles 

 and Grapevines, the la3-ers of the inner bark or liber 

 loosen and die, and come off in strips when only a year 

 or two old. 



523. The epidermis, consisting of a single layer of clo-se- 

 titting, tabular cells, with outer walls much thickened and 

 coated with a layer of matter impermeable by water, per- 

 sists onl}- for the first year or two. It is found, therefore, 

 in the case of stems, only on herljaceous fjlants, and on the 

 twigs and young parts of perennials, as a rule. 



ANATOMY OF LEAVES 



524. In the framework of leaves — ribs, veins, and vein- 

 lets — all tlie usual elements of vascular tissue are repre- 

 sented. Tlie midril), for instance, possesses a typical 

 fibro-vascular bundle, witli pldoem and xylem portions, 

 derived from the l.irancldng of the fibro-vascular system 

 of the stem. In the veinlets, however, the conducting 

 elements become reduced to simple series of hollow cells 

 and fibers. The woody framework serves not only to 

 strengthen tlie leaves, but also to bring in sap and to 

 distriljute it througliout every part. 



525. The living cells of the leaf, making up the green 

 pul[i. are of various forms, usually loosely arranged, so as 

 to give copious intercellular spaces or air passages commu- 

 nicating tlu-oughout the wliole interior (Figs. 381, 382). 

 The green color is given by the chlorophyll grains, seen 

 through the transparent walls of the cells and through tlie 

 translucent epidermis of the leaf. 



In ordinarv leaves, having an upper and under surface, 

 the crreen cells form two distinct strata, of different arrange- 

 ment. Tliose of the upper stratum are oblong or cylindri- 

 cal, and stand endwise to the surface of the leaf, usually 



