224 MINUTE ANATOMY OF FLOWERING PLANTS 



ceous shoot as in a Maple stem of the first year's growth 

 (Fig. 376), except that the woody layer is commonly thin- 

 ner, or perhaps reduced to a circle of bundles. The wood 



376. Diagram of a cross section of a very j^oung exogenous stem, showing six 

 libro-vascular bundles. 377. Same later, with bundles increased to 

 twelve. 378. Still later, the wood of the bundles in the form of wedges 

 filling the space, separated only by thin lines, or medullary rays, run- 

 ning from pith to bark. 



all forms in a cylinder — in cross section a ring — around 



a central cellular part, dividing the cellular core within, 



the pith, from a cellular bark without. As the wood 

 bundles increase in number and in size, 

 they press upon each other and become 

 wedge-shaped in the cross section; and 

 they continue to grow from the outside, 

 next the bark, so that they become very 

 thin wedges. Between the wedges are 

 still thinner plates (in cross section lines) 

 of much compressed cellular tissue, called 

 medullary rays^ which connect the pith 

 with the bark. The plan of a one-year- 

 old woody stem of this kind is exhibited 

 in the diao-rams. 

 •^— -oor^vv^^ 520. When such a stem grows on from 



379. Cross section of year to year, it adds annually a layer of 

 spring wood; wood outside the preceding one, between 

 /, fall wood.' that and the bark (Fig. 379). This is 



exogenous growth, or outside growing, as the name denotes. 

 521. Some new bark is formed every year, as well as 



new wood, the former inside, as the latter is outside of 



that of the year preceding. 



