224 BOTANICAL GAZETTE [MARCH 
Oax.—The oak is the outstanding arboreal form with compound 
rays. It is this that makes oak wood in great demand for building 
purposes. The northern oaks show the compound type of ray 
structure, while on the other hand the southern oaks retain the 
aggregate type of organization in the mature stem wood. Dis- 
coveries of fossil oaks (1, 2) from the gold gravels of California 
(Miocene) indicate that the aggregate condition was the general 
one in previous geological epochs. Hence on the basis of com- 
parative anatomy and of geological record the aggregate ray 
seems to be the primitive one for the oak. 
‘The compound ray was apparently an evolutionary response to 
the demands of a rigorous winter and to the need of storing up food 
in abundance. The fact, observable on any hillside in winter, 
that the oaks of northern latitudes, particularly the seedlings or 
saplings, retain their leaves late into the winter is an interesting 
evidence of their southern derivation. The older trees gradually 
become early deciduous. 
Fig. 3 shows a transverse section of a stem of Quercus rubra. 
In the center is a compound ray, composed undoubtedly of homo- 
geneous cells from which vessels and fibers are conspicuously 
absent. Fig. 4 shows some of these compound rays of the same 
species in tangential aspect. These may advantageously be com- 
pared with the tangential view of the aggregate rays of the birch 
as shown in fig. 2. 
Birch 
Wounpep STEMS.—Fig. 5 represents the polished disk of a 
wounded birch log, in which the wound has become so nearly 
healed that it appears far on the inside of the stem, and would not 
be discernible from the outside. The return to apparent normal 
conditions of growth is often very complete in the birch, and bark 
forms again on the outside. It also occasionally happens that 
bark grows over wounds of such large extent that the two edges 
of the injury have not grown together. This region of overgrowth 
or hypertrophy is known as the wound cap, and is represented at 
the top of the figure, above the wound. It may be noted in 
this connection that growth on the side of the stem which has 
