524 FUNDAMENTALS OF FRUIT PRODUCTION 



flowers. Occasionally some of the pistillate flowers of these clusters are 

 pollinated and develop seeds, but as a rule if the Blastophaga wasps enter 

 the cluster they oviposit in the pistillate flowers and so-called gall flowers 

 result. While the larvae of the Blastophaga are developing in the gall 

 flowers the staminate blossoms of the cluster mature so that their pollen 

 is shed when the mature wasps are ready to emerge. Such flower 

 clusters on the caprifig are known as insectiferous figs. If, however, the 

 Blastophaga wasps do not enter these clusters at the stage when their 

 pistillate flowers are ready for pollination or oviposition, the cluster 

 may or may not persist until its staminate flowers mature their pollen. 

 (From a practical standpoint their remaining and maturing is of no 

 value, since no wasps are in them to emerge and carry pollen to the 

 flowers of pistillate trees.) Such clusters are known as polliniferous figs. 

 In any case they drop off before the insectiferous figs reach full maturity 

 and the dropping is in a way comparable to the June drop of many other 

 fruits. Since pollination is unnecessary for the setting and persistence 

 of the insectiferous fig it must be concluded that the mechanical or chem- 

 ical stimulus resulting from the insect's presence is the real cause of 

 setting. The growth stimulus changes the twigs and branches^" bearing 

 insectiferous figs so that they may be told readily from those bearing 

 only polliniferous figs by their thickness, length and general vigorous 

 appearance. This response, not unlike that frequently attending the 

 injection of some chemical substance into vegetative tissue, is at least 

 suggestive of the complexities involved in fruit setting. 



Seedlessness and Parthenocarpy. — Seedless fruits are found in 

 practically all fruit-producing species. In some cases they are of rather 

 infrequent occurrence, their production apparently depending on unusual 

 conditions of culture or environment. In others they appear frequently 

 and many seedless strains or varieties have been established and are 

 propagated extensively by vegetative means. In such cases the seed- 

 lessness is due primarily to internal causes that are usually but little 

 influenced by changes in environment. 



Investigations with the grape by Stout^^^ have led to this conclusion: "The 

 most effective course in breeding for the development of seedless sorts is suggested 

 by the conditions of intersexualism. Most individuals and varieties producing 

 seedless or near-seedless fruits are strongly staminate. The former can be used 

 as male parents on the latter, which do produce a few viable seeds. Plants 

 strongly male and seedless can be crossed with plants strongly male but weakly 

 female and near-seedless and, also, the self-fertilized progeny of the latter may 

 be obtained. In this way families weak in femaleness may undoubtedly be 

 obtained in which a considerable number of individuals will produce seedless 

 fruits." 



Parthenocarpy refers to the ability of a plant to develop its fruit (1) 

 without fertilization or even (2) without the stimulus that comes from 



