tg2t1] WHITAKER—BIRCH AND OAK 231 
however, appear in response to a mechanical stimulus, as in the 
birch. 
Fig. 22 represents a transverse section of a wounded stem of 
Q. velutina. The extent of the wound is considerable, and healing 
has taken place to a comparatively small degree. The wound cap 
is restricted as a consequence to the edges of the wound, and it is 
here that reversionary structures make their appearance. The 
immediate region of the wound cap blots out all large rays, but 
directly behind this laterally the rays become aggregate, and 
finally compound again as they approach the back of the stem. 
Fig. 23 shows some of the rays in the wound cap of fig. 22 under 
higher magnification. Here the aggregate nature of the rays is 
quite apparent, especially if it be compared with that showing a 
typical compound ray (fig. 3). In the immediate vicinity of the 
wound the rays are all small, and in tangential section appear 
similar to the diffuse rays of the birch. If one were to interpret 
conditions here as being reversions, with no reference to the other 
parts of the stem or to conservative regions, he would have to 
postulate the diffuse ray as the primitive one for the oak, a situation 
which is in no wise borne out by the facts of the case, as in neither 
fossil forms nor in conservative regions of existing species are 
diffuse rays found. This illustrates the danger of judging trau- 
matic features on their face value without regard to other organs 
or to the past history of the plant. Only those structures occurring 
as a result of wounding which can be shown to be characteristic of 
southern and fossil forms, or of the seedling or conservative regions, 
can be accepted as significant in connection with wounding in the 
northern species. 
Fig. 24 represents under fairly high magnification a traumatic 
wood ray of Q. velutina in transverse section. It is also illustrative 
of the manner in which the aggregate ray of the seedling becomes 
compounded in the older saplings. 
It is of interest to note in connection with wound reactions 
that oak galls produce a return to aggregate condition similar to 
that ensuing when a stem is wounded in any other way. The 
organization of the gall itself is complicated, and need not be 
considered in the present connection. 
