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 × Chinese and Japanese × 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, membranous perianth-segments for each tiny flower, whitish in color, and more or less connected at their bases. A minute rounded mass appears in the center of the flower, perhaps primordia of abortive stamens, but this does not develop further. The catkin begins to take on a brownish color and at length the whole catkin, in case it is staminate, drops off. If it is androgynous, the staminate part drops off, or withers.
These male sterile trees appear to have a normal, sometimes excessive, development of the females, and are quite prolific nut producers. Information on the occurrence of female sterility in the hybrid trees is incomplete, but the indications are that at least partial sterility is frequent.
[Illustration: Fig. 5. A. Normal androgynous catkin (female flower at
base); B. Androgenous catkin with sterile pollen. From Sleeping Giant
Chestnut Plantation, Hamden, Conn. Photo by Mary Alice Clark, Conn.
Agric. Expt. Sta. July, 1949.]
2. Triploid Hybrid