Hayes and Garber - Cucurbit Breeding

160 BREEDING CROP PLANTS
longer than the stamen and therefore which must be artificially
an inter-
pollinated by hand or crossed by the aid of insects; (b)
mediate type; and (c) forms in which the stamens and pistil
are so arranged that self-fertilization is the usual rule.
The Petunioides section contains numerous varieties and
species. Many of these are grown as ornamental flowering types.
Parthenogenesis. Parthenogenesis, meaning the production
of viable seed without pollination, was shown by Goodspeed
(1915) to occur in N. tabacum, variety Cuba. Under normal
conditions its occurrence is rare. Wellington (1913) did not
find parthenogensis in a considerable series of experiments
and with numerous treatments under greenhouse conditions.
Several species as well as several commercial varieties of N.
tabacum were used in this study. Howard (1913) states that
parthenogenesis in N. tabacum does not occur under normal
but may occur under abnormal field conditions, at Pusa,
India.
Sterility. Studies of crosses between N. tabacum varieties and
N. sylvestris, which belongs to the Petunioides section, have been
made by Goodspeed and Clausen (1917). The FI generation
proved to be nearly sterile, although a few apparently normal
pollen grains were produced. These could not be caused to
germinate in their own stigmatic fluid or in other media. A few
normally maturing ovules capable of fertilization were produced
by theFi plants. If the plants were kept under poor cultural
conditions and the flowers pollinated by their respective parents
approximately 1 per cent, of the number of seeds normally produced
was obtained. If back-crossed with the sylvestris parent,
'practically 10 per cent, of the offspring of the seeds produced are
pure sylvestris. When crossed with tabacum, part of the plants
from the seeds produced seem to be of normal tobacco type and
are fertile; others resemble tabacum but are sterile. The F\
plants closely resemble the particular variety of N. tabacum
which is used as one of the parents.
Studies of self-sterility in tobacco crosses have been made by
East (1919a,6,c). East and Park (1917, 1918) studied crosses
between N. Forgetiana and N. alata which are self -sterile, and
N. Langsdorffii, a self-fertile species. Alata and Forgetiana
varieties sometimes produce seed late in the flowering season,
although during periods of rapid growth they are entirely selfsterile.
The few seeds obtained under reduced cultural conditions