Hayes and Garber - Cucurbit Breeding

270 BREEDING CROP PLANTS
very noticeable. Increases in size of fruit are also of much
importance.
For the commercial grower or the fruit breeder, it is essential to
know which varieties are self-sterile. In order to illustrate the
conditions generally found regarding sterility, a compilation of
some results is presented in Table LXXII. Citations to literature
are given so that the reader may go to the original sources when
he desires to know what category any particular variety belongs
to.
The causes of sterility have been determined in some cases.
In the strawbeny it is due to at least two causes (Valleau, 1918) :
1. The dioecious condition.
2. The production of aborted pollen grains or microspores in
otherwise normal anthers.
In the grape, Dorsey (1914) has found sterility to be associated
with both hybridity and the dioecious condition. The
varieties which produce reflexed stamens seldom produce fertile
pollen.
Dorsey states that:
"Sterility has been found to be due to the pollen rather than in the
pistil. Sterile pollen in the grape results from degeneration processes
in the generative nucleus or arrested development previous to mitosis
in the microspore nucleus."
Pollen abortion occurs both in pure and hybrid forms but is not
considered a cause of lack of fertility as abundant pollen is produced
in the grape.
In the plum, pollen abortion is not as a rule the cause of selfsterility.
The outstanding features as given by Dorsey (1919)
are:
"(a)
A constancy of expression of self -sterility even in P. domestica
in which about one-half of the varieties are self-fertile ; (6) the occurrence
of cross-sterility; and (c) the slow growth of pollen tubes under
the condition of self- and cross-sterility."
This type of sterility is comparable with that in the tobacco
crosses previously discussed, where sterility resulted from slow
pollen tube growth. In this case the pollen tube growing
from the pollen grain into the tissues of the style never reaches the
embryo sac. The self-sterile condition is believed by Dorsey to
be a dominant character in the plum and to be inherited, segregation
into sterile and fertile forms occurring at reduction division.