THE DEVELOPMENT OF THE FRUIT 525 



pollination. In other words, the growth of the ovarian and other 

 tissues of the fruit can occur without any stimulus from the accom- 

 panying development of the ovules into seeds, Parthenocarpic fruits are 

 usually, but not always, seedless. In some species fruits will develop 

 and viable seeds will be formed even if no pollination takes place. Such 

 plants are parthenocarpic and parthenogenetic at the same time. (Par- 

 thenogenesis is common in certain strawberry varieties.) Furthermore, 

 many parthenocarpic fruits contain aborted or partly developed seeds, 

 or seeds that, though normal in appearance, are incapable of germination. 

 On the other hand, not all seedless fruits are parthenocarpic. In some 

 cases seedlessness is due to embryo abortion some time after fertilization; 

 unless pollen had been available to furnish the stimulus for fruit setting 

 no later development of the fruit would have been possible. 



It is evident therefore that seedlessness and parthenocarpy are 

 rather distinct phenomena though it frequently happens that the two 

 are associated. 



Seedlessness of N on-'parthenocarpic Fruits. — The immediate cause 

 of seedlessness in fruits that have not developed parthenocarpically is 

 embryo abortion. This in turn may be due either to internal or to exter- 

 nal factors. Frost or freezing temperature after the fruit has set is 

 perhaps one of the most common of the environmental factors leading to 

 this condition; it has been observed repeatedly in pears, apples and 

 peaches. The developing embryo of the seed seems for some reason 

 more tender to low temperatures than the ovarian and other tissues 

 surrounding it. Consequently embryo development is arrested; how- 

 ever, if the growth of the fruit has proceeded far enough it will continue 

 through to maturity, though such fruits are often materially smaller 

 than those containing seeds. In many pear varieties, particularly 

 those that normally are either elongated or pyriform, the seedless speci- 

 mens are generally quite distinct in shape. ^^ Each has a shorter trans- 

 verse diameter through the core, but is much thickened at the basal end. 

 Sandsten^i^ has produced seedless tomatoes by excessive feeding. 

 Though no statement is made as to whether or not these fruits developed 

 parthenocarpically, it is presumable that pollination at least and prob- 

 ably fertilization took place and that seedlessness was due to embryo 

 abortion. 



In a preceding paragraph it was shown that full matm'ity of the fruits 

 on a caprifig tree is usually attained only when some of its pistillate 

 flowers are inhabited by the developing Blastophaga wasp. Ordinarily 

 these fruits matm-e no seeds because few or none of the pistillate flowers 

 are pollinated. In this fruit, then, embryo abortion and seedlessness are 

 associated with a stimulus resulting from the attack of a certain insect. 



Embryo abortion, resulting in seedlessness, is not, however, always 

 due to external factors. For instance, according to one investigator only 



