228 BOTANICAL GAZETTE [MARCH 
organization that persists in the roots of all species of the genus. 
The aggregate ray is also found in the reproductive axes of certain 
species, notably B. alba. It appears in the seedling in the mature 
stem of southern species, and as a result of injury when not nor- 
mally present in the stem. 8B. nigra, which has diffuse rays in the 
stem, recalls the aggregate type upon injury to the vegetative axis, 
and shows an aggregation normally in connection with the root 
trace. B. papyrifera, a northern species, recalls aggregate rays in 
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FA CERIN Ex 
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Fic. 3 Fic. 4 
Fics. 3, 4.—Fig. 3, diagram of wounded birch stem, showing relations and 
positions of traumatic features; fig. 4, diagram of oak stem, showing seedling condition 
and traumatic behavior of ray structures. . 
the seedling as a consequence of injury. Traumatic reactions 
should be interpreted in the light of data gained from the organi- 
zation of conservative regions, etc. 
Reactions occurring in the wound cap are the result of hyper- 
trophy and are not to be considered as reversions, since they are 
not correlated with structures that are known from their distribu- 
tion in the seedling and conservative regions to be primitive. 
True reversions in the birch, as exemplified in this investigation by 
B. alba and B. papyrifera, occur opposite the wound and not in 
the wound cap. B. populifolia, which has aggregate rays in the 
