74 



MORPHOLOGY OF STEMS. 



JO* 



surround a purely cellular central part (the pith), while sur- 

 rounded by a cellular external rind, the bark, or outer bark. 

 The diagrams (Fig. 128-130) rudely show some stages in the 

 formation of the zone of wood. The fibro- vascular bundles 

 originate in the bases of the leaves, and develop outward into the 

 forming leaves as well as downward into the forming stem. 



142. First Year's Growth. The wood, even in a herbaceous 

 or annual stem, at the completion of the first year's growth, 



forms a zone or tube, 

 enclosing the pith. But 

 it is traversed by plates 

 (in cross-section lines) 

 of parenchyma, or eel 

 lular tissue of the same 

 nature as the pith, 

 which radiate from 

 that to the bark, and 

 thus divide the wood 

 into wedges. These 

 lines, forming what is 

 called the silver-grain 

 in wood, are the MED- 

 ULLARY RAYS. They 

 represent the cellular 

 system of the wood it- 

 self, or untransformed 

 parenchyma. Being 

 pressed by the woody 

 wedges, their cells are 

 laterally flattened. In 

 some stems, the med- 

 ullary rays, or many 

 of them, are comparatively broad and conspicuous; in others, 

 thin and inconspicuous or irregular. The growth of the woody 

 wedges is soon complete, except at the outer portion, next 

 the bark : here they usually continue to grow through the 

 season ; that, is the wood grows externally. The general ana- 



FIG. 131. Longitudinal and transverse section of a stem of the Soft Maple (Acer 

 dasycarpum ), at the close of the first year's growth ; of the natural size. 



FIG. 132. Portion of the same, magnified, showing the cellular pith, surrounded by 

 the wood, and that by the bark. 



FIG. 133. More magnified slice of the same, reaching from the bark to the pith : 

 a. part of the pith ; b. vessels of the medullary sheath ; c. the wood ; d , d. dotted ducts in 

 the wood ; e, e. annular ducts ; /. the liber, or inner fibrous bark ; g. the cellular envelope, 

 or green bark ; h. the corky envelope ; t. the skin or epidermis ; k. one of the medullary 

 raya, seen on the transverse section. 



