For example, the Chinese female trees were pollinated with pollen from three other Chinese trees (in the diagram "Pollen source No. 1, 2 and 3), which open their anthers on successively later dates. This pollination schedule was used to avoid prolonged storing of the pollen.
[Illustration: Fig. 2. Showing, above, C. mollissima, Chinese chestnut, left, and C. seguini, seguin chestnut, right, parents of mollissima × seguini hybrid below. Note clustered burs in hybrid—more than twice the number appearing in the mollissima parent. Leaves and habit of tree resemble more the mollissima parent. About 1/6 natural size. Photo by B. W. McFarland, Conn. Agric. Expt. Sta., Sept. 25, 1953.]
To carry on pollination the bags are removed and the pollen-bearing catkins are brushed lightly over the stigmas several times, one or more fresh catkins being used in each bag. These catkins are left in the bag. The bags are then replaced and permanently removed when danger of outcrossing is eliminated, in this case 10-14 days after the last pollination.
The number of nuts collected at the time of harvesting compared with the number of female flowers pollinated was taken as a measure of how successful the pollinations were.
The results showed that five days after the commencement of anthesis a high proportion of the female flowers is receptive. The Japanese and hybrid trees have a definite peak of the period of receptivity between 9 and 17 days after anthesis begins; thereafter, receptivity drops off sharply. The data from the Chinese trees indicated that the period of maximum receptivity is longer than in the Japanese and hybrid chestnuts tested. They maintained full receptivity on the 22nd day after the beginning of anthesis.
It is commonly believed that bagging as well as emasculation may seriously affect the yield from controlled pollination. This is not always the case. One of the Japanese trees and one hybrid tree (S8 × J) yielded fully as many nuts from controlled (under best conditions) as from open pollination. On all other trees the effect of bagging was more or less adverse.
The Effect of Emasculation on Nut Yield
Emasculation involves the removal of the unisexual male catkins and the male part of the bisexual catkins. In the course of the controlled pollination work it has often been found that the female flowers drop off in the bag before the burs start to develop. This has especially been encountered in Japanese × American hybrids and back-crosses. It was thought that this perhaps was due to injuries resulting from emasculation. The following small experiment was carried out in order to determine if this was actually true.
A Japanese × American and a Japanese × (Japanese × American) hybrid were used as the female parents. On these trees some flowering branches were bagged which had been emasculated normally, on other branches only the unisexual catkins were removed, while the bisexual catkins were left intact. Some branches were bagged without any emasculation, and some flowering branches were just tagged. The number of female flowers was counted in all cases. Pollinations were performed 3 times, that is, were repeated on the third and fifth day after the first pollination. This is the procedure ordinarily used for our controlled pollinations. Chinese pollen was used on both trees. Nut set expressed as per cent of the number of pollinated flowers, times three, (because ordinarily there are 3 nuts in every bur) was taken as a measure of how successful the pollinations had been. The results are shown in Table 2.
Table II. The Effect of Emasculation on Nut Yield