316 The Journal 
The possibility of producing a vigorous 
hybrid between these trees is immedi- 
ately suggested. In fact one may 
already exist. The London plane, Pla- 
tanus acertfolia, has never been found 
growing wild and exhibits character- 
istics intermediate between the Amer- 
ican and the European planes. But 
without doubt, it would be highly 
profitable to experiment with hybrids 
of known parentage. 
A large field for profitable research 
is thus opened up by the possibility of 
artificially producing trees having excep- 
tional vigor. The oaks, chestnuts, lin- 
dens, and many other important genera 
offer a fertile field for experimentation. 
And at the present time the importance 
of such work can scarcely be over- 
estimated. 
How little has been done is made 
strikingly clear by Prof. Henry’s his- 
torical review. He ascribes to Klotzsch 
the credit for the first hybridization, 
with the production of pine, oak, alder 
and elm hybrids at Berlin in 1845. 
The results were good, but the work 
attracted little attention. 
VIGOROUS WALNUT HYBRIDS 
The frequent production of hybrid 
walnuts in California led Luther Bur- 
bank to take up this genus, and he 
called attention to the valuable qualities 
of the first-generation hybrids, which 
grow so rapidly that experienced for- 
esters will scarcely credit the figures. 
Trees of the so-called Paradox walnut 
(Juglans regia, the Persian or “English” 
x J. californica) at Santa Rosa measure 
80 feet in height and 6 feet in girth 
after fifteen years of growth. The 
hybrid known as Royal (J. californica x 
J. nigra, the black walnut of the 
eastern United States), appears to be an 
even more rapid grower, one specimen 
being credited with a height of 100 feet 
and a girth of 9 feet after only sixteen 
years of growth. Another magnificent 
walnut hybrid is that on the James 
River, Virginia, which was described 
by Peter Bisset recently.’ 
As to the quality of the wood of these 
hybrid trees—a point of prime impor- 
of Heredity 
tance to foresters—Prof. Henry remarks: 
“Tt is a popular belief that fast-grown 
timber is necessarily soft and com- 
paratively worthless. This is a fact in 
most conifers; but in one class of broad- 
leaf trees, the wood of which is char- 
acterized by large pores in the inner part 
of the annual ring, the contrary is true, 
as the faster the timber of these trees 
is grown the stronger and denser it 
becomes. This class includes oak, ash, 
chestnut, hickory, and walnut, the 
species in fact that par excellence 
produce the most valuable timber. 
‘All the more reason, then, for efforts 
to produce fast-growing crosses in the 
case of these precious trees. To quote 
from the conclusion of my paper of 
1910:.‘In countries like our own the 
only hope of salvation for forestry is in 
growing timber rapidly; and we have 
been helped in that by the introduction 
of fast-growing conifers like the larch, 
the Corsican pine, and the Douglas fir. 
But it is essential to grow the more 
valuable classes of non-coniferous tim- 
ber.’ The difficulty of growing the 
ordinary species of oak, ash, and walnut 
is the long period required for their 
maturity, which renders hopeless, except 
on the best soils, all chance of an ade- 
quate financial return. Without vigo- © 
rous first-crosses, the most valuable 
classes of timbers can only be grown in 
limited quantity.” 
THE CAUSE OF VIGOR 
Although many geneticists have 
speculated on the problem, no one has 
yet been able to offer a satisfactory 
explanation of the extraordinary vigor 
displayed by hybrids. Some of the 
Mendelian hypotheses put forward are 
plausible, but have so far remained 
unproven. The observed vigor, as Prof. 
Henry points out, “is distributed over 
the whole plant, and is as conspicuous 
in the roots as in the stem and leaves. 
What we actually observe is not only 
an acceleration of, but also an increase 
of cell-division in all parts of the plant. 
The cells divide very quickly, continue 
to divide, and thus build up a taller 
stem, a more extensive root-system, etc. 
5 Bisset, Peter, “The James River Walnut.’’ JouRNAL oF HereEpity, V, 3, pp. 98-102, 
March, 1914. 
