ed
(3).

It would be important to know the centers of origin of the tannins in
the chestnut, their translocation, and whether they are translocated
through or over graft-unions. In other words, will a susceptible scion
when grafted on a resistant rootstock become more resistant because
antibiotic substances formed in the roots of the resistant rootstock are
translocated into the scion?

From a number of older grafts of non-resistant Japanese-American hybrid
scions on Japanese or Chinese rootstocks it appears that this indeed
might be the case. These grafts, some of which are 16 years old, appear
to be more resistant than the original hybrid tree, even if not as
resistant as the rootstock.

This would indicate the possibility that the antibiotic substances are
produced in the roots and translocated into the scion. However, the
possibility still remains that the compounds are formed also in the
leaves and translocated to the base of the tree. To clarify this whole
problem an experiment with Chinese-American grafts in different
combinations is under way. Preliminary results show that antibiotic
substances are formed in upper parts of the plants, but that they are
not translocated downward across the graft union. Thus it was found that
Chinese branches grafted on two year old American seedlings remained
resistant, without the American seedlings showing any increase in
resistance. In future experiments the upward translocation will be
studied in detail on grafts of American scions on Chinese seedlings.

Some Abnormal Conditions

1. _Sterility_

Sterility occurs quite commonly in interspecific hybrids either because
the chromosomes fail to pair in meiosis or because the parent genes when
brought together in the hybrid interact in some way deleterious to the
formation of sex-cells. Furthermore, cytoplasmic sterility is likely to
occur in a wide cross.

Sterility has been encountered in several instances in American x
Chinese and Japanese x American hybrids. In most cases it is a case of
pollen abortion only; either anthers fail to develop completely as shown
in Fig. 5, B, or the anthers develop but are much reduced in size and
contain no functioning germ cells.

Pollen sterility is not sporadic in a given individual: it is uniform
throughout the flowering branches. The individual flowers are
arranged on the catkin axis as in the normal flowers (Fig. 5). But
when the flowers open, a hand lens reveals 3-5 tiny, memb