138 Our Field and Forest Trees 



The wood vessels through which crude sap 

 moves toward the leaves, and the bast tubes 

 through which elaborated sap travels away from 

 the leaves, are clustered together, and with them 

 are tough fibers to give support and strength. 

 Botanists call the whole sheaf of vessels, tubes 

 and fibers a " fibro-vascular bundle." 



The stem of a very young seedling, cut cross- 

 wise, is just a honeycomb tissue of thin-walled 

 cells, all alike. It is all pith, and nothing else. 



The stem of a tree one year old will show a 

 few bundles, making a dotted circle around the 

 outside of the young pith. 



Next summer more bundles will form, in the 

 spaces between those of last year, but a very little 

 nearer to the outside of the stem. By the time the 

 tree is several years old there is an almost un- 

 broken ring of fibro-vascular bundles all around 

 the pith. Wood-vessels and fibers are on the 

 inside of this ring — bast-tubes mingled with 

 fibers are on the outside. 



In some shrubs and young trees, the ring of 

 fibro-vascular bundles is so close and compact that 

 the stem is like a strong tube filled with soft pith. 

 This is the case of the elder branches and willow 

 twigs. Both can easily be emptied of their pith 

 and whittled into whistles — as all country boys 

 know. 



As the elder stems grow old, their pith dis- 

 appears, leaving them hollow. The elder — so 



