34 THE OAK 



down as separate, alternate cords of xylem and phloem ; 

 the xylem consists of vessels and cells, the former 

 developed centripetally, while the phloem consists of 

 sieve-tubes and cells. Any cell-tissue which may lie in 

 the centre of the axial cylinder, and surrounded by the 

 vascular bundles, corresponds, in popular language, to 

 pith ; any that runs between the bundles corresponds 

 to medullary rays. 



We now turn to the root as a whole, and examine 

 its behavour in the soil as the young seedling develops 

 further, and in the light of the above anatomical 

 facts. 



Although the root-system of the young plant is 

 regularly constituted of a series of lateral rootlets 

 springing from the primary root, the orderly arrange- 

 ment is soon disturbed when the tertiary and other 

 rootlets begin to develop from the secondary rootlets ; 

 moreover, as the age of the tree increases, the tendency 

 to irregularity is increased owing to the production of 

 rootlets of the higher orders at different places, thus 

 interfering with the acropetal succession of the younger 

 rootlets. 



At first the root-system is especially engaged in 

 boring into the soil, and provided the latter is suffici- 

 ently deep and otherwise suitable, the tap-root will go 

 down a foot or more in the first year. As the roots 

 thicken they exhibit considerable plasticity, as is espe- 

 cially evinced on rocky ground, where the older roots 

 may often be found in cracks in the rocks, so com- 



