ii8 ENDOGENS. [chap. 



to call such stems endoge?ious, from a notion that the younger 

 bundles were those in the centre of the stem, and that 



they pushed and compressed 



#the older bundles towards the 

 outside. 

 14. But the structure of 

 Woody Dicotyledons requires 

 further examination. 

 Take a cross-section of the 

 stem of an Oak, or of any 

 ^ ^ branching tree from the cooler 



''r^S.mt^^Ti:^''^:^^^ mountain ranges, already several 

 ':^^i':i^::.:^''fl years old. You find in the centre 



r.Dro-vascular bundles ; r /cellular ^\^q rCmainS of thc Original CCllu- 



lar system of the stem reduced 

 to a very narrow cord, and distinguished as the jkith. 

 When young the pith served to contain and to convey 

 nourishing fluids to the growing point; now it is dry 

 and useless. Surrounding the pith is the wood, forming 

 the great mass of the stem A number of concentric 

 rings are distinguishable in the wood, there being as 

 many rings as years that the stem has existed, one ring to 

 a year ; so that by counting the rings you may ascertain 

 the age of the stem. The appearance of rings or annual 

 zo?ies, in the wood, arises simply from the wood formed in 

 summer and autumn being denser, closer grained, and with 

 fewer vessels than that formed in spring. 



With a little care you may notice that there are, as it 

 were, narrow rays proceeding from the pith to the bark. 

 These are actual plates of cellular tissue left between the 

 fibro-vascular bundles, which look like narrow rays when 

 cut through transversely. They are called the medullary 

 rays. They are usually very narrow, much narrower in most 



