130 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
In the hybrid two such zones are also found. The inner zone is 
composed of tangentially extended tracts of variable form and 
size centering in vessels corresponding to those of the inner 
zone of C. bignonioides and presenting features which are inter- 
mediate between those of the latter species and C. kempfert. 
The direct influence of C. bignonioides is thus made very appar- 
ent. The outer zone of tracheids consists of a limiting layer of 
rather uniform thickness and generally devoid of vessels. Here 
and there a tendency to radial extension may be noted, but there 
is nowhere anything approaching what is to be met with in either 
C. kempferi or C. bignonioides, and the actual structural condi- 
tion must be regarded as a resultant from the interaction of 
these two species, in which a somewhat equal influence appears 
to have been exerted. 
In the radial aspect of structure, the influence of C. bignoni- 
oides is definitely expressed in the hybrid in (1) the more vari- 
able dimensions of the ray cells; (2) in the less coarsely pitted 
walls ; (3) the 2-seriate character of the pits on the radial walls 
of the tracheids. In the tangential aspect, the influence of the 
same species is manifested in the thin walls of the ray cells, 
while the influence of C. kempferi is expressed in the character 
of the marginal cells. Other features of a minor character also 
contribute to the general results thus indicated, a summary of 
all of which may most conveniently be presented in a tabular 
view, from which I have excluded all characters common to the 
hybrid and to the three species. 
Comparison of structural variations in Catalpa. 





‘Teas C. k - E 
Hybrid. | | pey E Sot n " oed No. 
| 
Transverse. | 
Growth rings very broad . . . . . x | X x E 3 
Growth rings narrow, uniform 2 — — — d I 
Spring "e: summer wood obvious . — pri eae st 2 
Spring and summer wood not distin- 
irc bee or iocadiy í defined x — x — 2 
Cells of summer wood u he dna thick- 
walled, zot at all in radial ro — — — X I 
Cells of summe s ino: viciis. shin. 
walled in radial row x x Am. 3 




