34 THE OAK 
down as separate, alternate cords of xylem and phloém ; 
the xylem consists of vessels and cells, the former 
developed centripetally, while the phloém 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- 
