NUTS OF THE SHELLBARK HICKORY 
This is one of the commoner hickories of the central United States; its nuts, here shown 
natural size but without their husks, differ greatly from those of the pecan. 
of a cross between the two species is shown in Fig. 15, the following illustration. 
Apparently this alteration in the nature 
of the division of the cells is not asso- 
ciated with any visible change in their 
structure. Miss Marshall, who exam- 
ined for me many sections of the growing 
points of hybrid poplars and their 
parent species, could find, as the result 
of three months’ observations, no tang1- 
ble differences in the size of the cells or 
nuclei, in the number of the chromo- 
somes, etc. 
“Tt is possible that the stimulus which 
causes growth (7. e., cell-division) to 
commence and to continue is some 
soluble chemical compound or enzyme. 
The enzyme in the hybrid may be more 
complex and more effective than the 
enzymes in the species. Whether the 
injection of soluble matter obtained 
from a hybrid into the growing points 
of one of the parent species would 
stimulate the latter to increased cell- 
division, might be worth trying, if the 
experiment could be carried out. 
“Whether the amount of vigor in 
hybrids is directly associated with the 
degree of relationship between the 
individuals which are crossed is a 
disputable point, but one of practical 
The result 
(Fig. 14.) 
interest in the selection of parents for 
crossing experiments. One of my most 
vigorous hybrids (Populus generosa) is 
derived from two parents so little 
related that they are placed in two 
distinct sections of the genus. A cross 
between two races of the common alder 
shows considerable vigor, though the 
parents are so closely allied that they 
can only be distinguished by the most 
trivial characters.” But whatever the 
explanation of this vigor may be, no one 
who has worked with hybrids is likely 
to question its existence; and that fact 
is sufficient to make breeding justifiable. 
PROPAGATION 
“An important question is the propa- 
gation of these vigorous crosses, once 
they are created. The first-cross does 
not come true from seed, and it would 
be a great drawback if we were obliged 
to wait till the newly made trees bore 
flowers and fruit. The first-cross, in 
short, can only be multiplied by vegeta- 
tive reproduction. This is easy when 
the trees are readily propagated by 
cuttings, as in the case of poplars and 
317 
