STEMS OF DICOTYLEDONS 155 



eels. As the tree grows they elongate either vertically, 

 by the lengthening of the twig, or horizontally, by its 

 increase in diameter, until they often appear as long slits. 



Scrape off a little of the brownish, or sometimes almost 

 colorless outer covering. This is the epidermis, and is 

 replaced by the outer corky layer of the bark in older 

 stems. As the stem increases in diameter from year to 

 year this outer covering is broken up and pushed aside to 

 make way for the new growth, so that the bark is con- 

 stantly dying and sloughing off from the outside and as 

 constantly renewed from within. Under the epidermis, 

 notice a greenish layer of young bark ; beneath this a layer 

 of rather tough, stringy fibers called bast, and finally a 

 harder woody substance that constitutes the bulk of the 

 interior of the stem. Cut through this to the very center 

 of the axis and we find a cylinder of lighter, pithy texture; 

 this is the same as the parenchyma or parent tissue that 

 we found pervading the interior of the cornstalk (Sec. 213). 

 It is usually called the pith or medulla, and is the only part 

 present in very young stems. 



Between the woody axis and the bark is a more or less 

 soft and juicy ring called 



220. The Cambium Layer. This is not always easily 

 distinguishable with a hand lens, but is conspicuous in the 

 stems of sassafras, slippery elm, aristolochia, etc. If some 

 of these can not be obtained, the presence of the cambium 

 can be recognized by observing the tendency of most stems 

 to " bleed " when cut, between the wood and bark. This 

 is because the cambium is the active part of the stem in 

 which growth is taking place, and consequently it is most 

 abundantly supplied with sap. This is especially the case 

 in spring, when it becomes so gorged with nourishment 

 that if a rod of hickory or elder is pounded, the pulpy 

 cambium is broken up and the bark may be slipped off 

 whole from the wood. It is the nourishment contained in 

 the cambium of certain plants that tempts goats and calves 

 to bark them in spring, and that enables savages, in time 



