126 



A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



tion between the inner and outer layers. They do not run 

 far, as a rule, up and do\vn the stem (Fig. 82), no farther 

 than the distance between the successive forkings of the fibro- 

 vascular bundles in the original bundle cjdinder (page 119). 

 They are more prominent in some woods than others, and are 

 especially striking in Oak, where they form the prominent 



radial lines so plain 

 on cross sections, and 

 the irregular shining 

 plates for which Oak 

 is "quartered", that 

 is, cut longitudinally 

 in a way to display 

 them. The Oak has 

 also ducts so large as 

 to be clearly visible 

 to the naked eye, — 

 whence its conspicu- 

 ous grain. 



Stems exhibiting 

 clear distinction of 

 bark, wood, and 

 pith, having cam- 

 bium, annual rings 

 and medullary raj^s, 

 and increasing in- 

 definitely in thick- 

 ness by secondary 

 growth, represent the most highly developed tj^pe, which in- 

 cludes all of our common trees and shrubs. Since they grow 

 by additions of layers to the wood, they are called exogenous. 

 The other prominent type has none of the above-mentioned 

 features, but remains permanently in a primary growth con- 

 dition with the bundles scattered irregularly throughout the 

 stem (Figs. 72, 74). In the belief, since found erroneous, that 

 such stems grow by addition of new bundles inside of the 



Fig. 82. — A 4-year-old stem of Pinus .sylves- 

 tris, with bark partially remo\'ed at the cam- 

 bium : magnified. It shows clearly the medullary 

 rays, primary and secondary, and the annual 

 rings, containing resin canals. (From Stras- 

 burgcr.) 



