Dicotyledonous Stem 



4S 



the Hemlock, the pith, not being able to keep up in growth 

 with the other parts, has left the stem hollow, with a rugged 

 attachment of pith at the sides ; whilst in the Walnut and Jessa. 

 mine it has become broken up into thin discs (discoid pith). 



2. Surrounding the pith there is a layer of spiral vessels 

 (the medullary sheath), which is in reality the commencement 

 of the wood or xylem. The function of these vessels, as indeed 

 of all true vessels, is to serve as air carriers. Like the wood, 

 cambium, and liber, the medullary sheath is pierced by the 

 medullary rays. 



Fig. 6S.— Portion of the transverse section through a shoot of Ivy four years 

 old : R, cortex ; B, bast bundles ; h, wood, in which the four annual rings 

 are distinctly visible ; M. pith ; s', medullary rays ; F, ba&t fibres ; zb, 

 cambium : s ', primary, s , secondary bundles. 



3. Outside the medullary sheath comes the xylem or wood, 

 arranged in the form of concentric rings. These rings are 

 formed as follows. During the first year the cambium cells by 

 their division have caused the wood and liber to increase in 

 thickness. During the winter the cambium remains dormant, 

 but as soon as spring returns the cells once more become vitally 

 active, and form a fresh ring of wood on its interior outside the 

 old xylem, and a fresh ring of liber on its exterior inside the old 



