1921] WHITAKER—BIRCH AND OAK 223 
metamorphosis. The homogeneous character of the ray is indicated 
by solid black. The depressions of the cylinder, corresponding in 
position to the large rays in the vicinity of the annual ring, are to 
be noted. 
The type of ray characteristic of the majority of our forest 
trees is the diffuse ray. The organization of this is shown at the 
right (B, B’). Just outside the leaf trace (black) the ray is char- 
acteristically aggregate, a situation parallel to that of the early 
organization of the compound ray. Farther out, however, the 
rays spread out and subdivide, instead of being more closely 
united, and vessels and fibers again appear in the foliar segment. 
This permits the passage of more water and food substance, and 
is of advantage to trees subject to the rigors of a northern winter. 
In tangential aspect the rays are several cells wide, which differen- 
tiates them from the simple or uniseriate rays of the Conifers. 
This type of ray is found in the mature stem of C. stricta and C. 
equisetifolia. 
Ray organization in birch and oak 
Brrcu.—Fig. 1 represents a transverse section through the stem 
wood of Betula nigra, and shows the type of ray characteristic of 
most of our forest trees. Fig. 2 shows some of these diffuse rays 
in tangential aspect. They are about three or four cells in width, 
but without any interspersed fibers. It is an interesting fact 
that most birches of southern origin‘or affinity retain the aggregate 
type of ray organization in their vegetative axes, while those of 
northern range are characterized by the diffuse type of ray. ° B. 
populifolia, for instance, which is essentially a southern birch, has 
persistent aggregate rays in the normal stem wood. B. pumila 
and B. lutea may be taken as examples of the northern species. 
This difference in ray organization would seem to indicate the 
evolutionary relationships occurring in response to weather 
exigencies. 
B. alba, an introduced and also indigenous species, may be 
taken as an example of a birch belonging to the temperate region, 
and will be used in this investigation because of its intermediate 
position geographically and structurally, where no special advantage 
is to be gained by recourse to other species. 
