34 TEE OAK. 



the xylem consists of vessels and cells, the former de- 

 veloped centripetally, while the phloem consists of 

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

 in the center 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 

 behavior in the soil as the young seedling develops fur- 

 ther, and in the light of the above anatomical facts. 



Although the root-system of the young plant is reg- 

 ularly constituted of a series of lateral rootlets spring- 

 ing from the primary root, the orderly arrangement is 

 soon disturbed when the tertiary and other rootlets 

 begin to develop from the secondary rootlets ; more- 

 over, 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 bor- 

 ing into the soil, and, provided the latter is sufficiently 

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

 a foot or more in the first year. As the roots thick- 

 en they exhibit considerable plasticity, as is especially 

 evinced on rocky ground, where the older roots may 

 often be found in cracks in the rocks, so compressed that 

 they form mere flattened sheets many times broader 

 than they are thick (Fig. 8). 



