226 MINUTE ANATOMY OF FLOWERING PLANTS 



growth, is stretched, and finally rent by the continual 

 enlargement of the wood within; it is weathered and 

 worn, and thrown off in fragments, in some trees raj)idly, 

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

 may acquire great thickness. Similarly in Honeysuckles 

 and Grapevines, the layers 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 close- 

 fitting, tabular cells, with outer walls much thickened and 

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

 sists only for the first year or two. It is found, therefore, 

 in the case of stems, only on herbaceous plants, 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 the usual elements of vascular tissue are repre- 

 sented. The midrib, for instance, possesses a typical 

 fibro-vascular bundle, with phloem and xylem portions, 

 derived from the branching 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 the leaves, but also to bring in sap and to 

 distribute it throughout every part. 



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

 pulp, are of various forms, usually loosely arranged, so as 

 to give copious intercellular spaces or air passages commu- 

 nicating throughout the whole interior (Figs. 381, 382). 

 The green color is given by the chlorophyll grains, seen 

 through the transparent w^alls of the cells and through the 

 translucent epidermis of the leaf. 



In ordinary leaves, having an upper and under surface, 

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

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

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



