05 
year. Their rate of growth has exceeded all expectations—in 
several cases, as previously reported, 4 feet per year—while that 
of the American chestnut seedling is about one foot per year. 
Furthermore, the erect habit of the American chestnut is very 
evident in these Fy hybrids. This difference from the low-grow- 
ing, shrubby habit of the Japanese species is perfectly obvious to 
one who visits our plantations. The remarkably rapid growth 
of the F; hybrid is evidently due to the phenomenon known as 
hybrid vigor, or heterosis. In addition to this vegetative vigor, 
in many cases a sexual precocity has been manifested; that ts, 
it is not at all unusual for these hybrids to blossom in the third 
year of their growth. (See Report for 1935, BRooKLyN Bort. 
Garp. Record 25: 63. 1936; also Report for 1937, BROOKLYN 
Bot. GARD. RECORD 27: 49. Fig. 4. 1938.) 
The susceptibility to disease of these Japanese-American hy- 
brids has been already reported (see Reports for 1938, 39, and 
40). It has become more and more evident that in these Jap- 
anese-American crosses the American parent is dominant in the 
habit of growth, and incompletely dominant as regards disease 
susceptibility, the two characters with which we are especially 
concerned. Without going into the genetics of the case in detail, 
it is obvious that, under the circumstances, one way to achieve 
the desired result, namely, a disease-resistant chestnut of timber 
type, is to interbreed these Japanese-Americans with each other. 
Of the next generation, some should apparently have the desired 
characteristics. Of these, one or more may be absolutely pure, 
but others would have to be bred further to produce the desired 
result, i.e., to serve as reliable breeding stock. Another way 
would be to back-cross the Japanese-Americans with resistant 
Japanese, and after selecting the tall individuals, another inter- 
breeding should produce pure, tall, and disease resistant offspring 
in greater numbers.* This situation was foreseen as far back as 
1937, when we began interbreeding the hybrids and back-crossing 
with resistant Japanese and Chinese. 
Thus we already have a considerable number (now 4 years old) 
of 2nd generation or F, hybrids resulting from these crosses. 
* This reasoning is, of course, in the present state of our knowledge, largely 
hypothetical. 
