160 BREEDING CROP PLANTS 



longer than the stamen and therefore which must be artificially 

 pollinated by hand or crossed by the aid of insects; (b) an inter- 

 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 pro- 

 duced 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 FI 

 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 self- 

 sterile. The few seeds obtained under reduced cultural conditions 



