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belt of cells, capable of growth, situated just outside the 

 wood, and called the cambium layer. These plates of thin- 

 walled cells, known as the medullary rays, are here especially 

 mentioned, because they play an important part in the stor- 

 age of the reserve food-substance formed by the plant during 

 the growing season, and stored away to be employed during 

 the initial growth in early spring. Besides the vessels, wood 

 cells and medullary rays in the wood ring, there are also 

 long, rectangular starch-bearing cells, in many respects not 

 unlike the pith, extending lengthwise of the wood, and at right 

 angles to the general direction of the medullary rays. These 

 may be styled the wood-pith cells. (3) The cambium layer, 

 above mentioned, is made up of small, thin-walled cells, and 

 constitutes the soft layer at which the wood and bark may be 

 separated, especially in spring. The radiating plates of pith 

 cells which reach from the pith to the cambium layer, and 

 above designated as the medullary rays, project beyond the 



cambium, and broaden out in the loose, cellular part of the 

 bark that lies between the somewhat interrupted ring of bast 

 and the cambium. (4) This bast is the tough fibrous part of the 

 bark, and consists of small bundles of thick-walled cells which 

 run lengthwise of the twig, and are usually midway betw^een 

 the rind and the cambium. (5) Upon the exterior of all is 

 a double layer of thick-walled cells. The outer may be styled 

 the cuticle, and beneath this is a much thicker layer of firm 

 tissue, with the contents of many of the cells colored green. 

 The outer and thiiiner layer bears much of the coloring mat- 

 ter which gives mature twigs their characteristic reddish, 

 brown or other color. Between the rind and the ring of bast 

 is a belt of loose, cellular tissue abounding in cavities between 

 the cells known as intercellular spaces. This is the loosest 

 tissue in the twig, and may be called the pith of the bark. 

 It, however, differs from the true pith in the center of the 

 stem in having the cell contents colored green. It is often 

 and appropriately called the green bark, and is the part ex- 

 posed when the thumb-nail removes the rind of a tw^ig but 

 does not pass deep enough to reach the wood layer. 



Therefore, to recapitulate, the twig in cross-section consists 

 of the following parts, beginning at the outside: A double 



