1918] LANGDON—RAY SYSTEM OF QUERCUS 321 
and that of the large aggregate rays. When the rays are strongly 
developed, the dipping in of the annual ring where it crosses a large 
ray is sharper, thus explaining the narrow, depressed segments. 
From observations of transverse sections of twigs from Quercus 
alba, Q. bicolor, and Q. macrocarpa I find that there is evidence 
of retardation in growth of the tissues in the immediate vicinity 
of the wide rays, especially noticeable in the marked dipping in of 
the annual rings where they cross the large rays. However, aside 
from a few extreme cases, this Checking influence of ae wide a 
rays does not explain the 5 
of the wood of Quercus. Their cause may be traced more directly 
to the effect of the leaf traces upon the general growth and form 
of the woody cylinder. Since the principal function of the xylem 
is the conduction of water from the soil to the outer parts of the 
plant, it is obvious that the maximum upward movement of solu- 
tions in the stem would be through the tracheidal tissues and vessels 
in direct line with the leaf traces. This would cause an acceleration 
in growth and the consequent outward projection of those 5 regions 
of the woody cylinder associated with leaf traces, while the neigh- 
boring conducting tissues, namely, the so-called depressions from 
which the main conducting streams had been diverted to the 
petioles of the leaves, would fail to maintain their normal rate of 
growth. 
This condition of the secondary xylem may persist for a number 
of years, but there is a gradual diminution in the size of the depres- 
sions until at length the cambium layer and the xylem assume a 
circular outline. This may be due to the fact that as the wood 
increases in age its capacity for water conduction decreases, owing 
to the choking of the lumina of the vessels of the central regions 
of the wood with tyloses. In typical heart wood trees, such as the 
oak, the sap wood is limited in certain species to the youngest 
annual ring, and in some cases merely to the tracheary tissues of 
this ring. The narrowing of the active conducting zone would 
then be likely to cause a more even development of the woody 
tissues around the entire stem. A fact worthy of note in connec- 
tion with this characteristic formation of the cambium and xylem 
in the stem of the oak is that corresponding to the degree of 
