398 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERM.S 



culty lies in bringing about endosperm formation. Unfortunately 

 the nature of the stimulus which may lead to a division of the un- 

 fertilized secondary nucleus remains unknown. Until this is dis- 

 covered, the only way of overcoming the difficulty would probably 

 be to perfect a technique for excising the adventive embryos and 

 growing them in artificial culture. 



Finally, it may be added that although a method of inducing 

 adventive embryony has undoubted possibilities, there is also a 

 need sometimes for the elimination of adventive embryos. In Cit- 

 rus, for example, where a number of nucellar embryos may mature 

 simultaneously with the zygotic embryo, it is quite difficult to 

 distinguish the two kinds of seedlings in early stages. Further, 

 rue zygotic embryo sometimes becomes crowded out by the nucel- 

 lar embryos so that all the seedlings are asexual. It would be a 

 distinct advantage to the breeder if he could exercise some control 

 over the two processes, eliminating either zygotic or nucellar em- 

 bryos according to his requirements at the moment. 



Induced Parthenocarpy. 13 Some of the world's most important 

 fruits are seedless or have only abortive seeds. As examples may 

 be mentioned varieties of banana, cucumber, orange, pineapple, 

 grape, grapefruit, persimmon, and breadfruit. A good many of 

 these varieties are believed to have arisen by gene mutation, and 

 some have been obtained by hybridization. 



Recently attempts have been made to produce seedless fruits 

 on seeded varieties by withholding pollination and applying certain 

 chemical substances to the pistil. As early as 1849 Gaertner ob- 

 tained seedless fruits in certain cucurbits whose stigmas had been 

 "pollinated" with the spores of Lycopodium. Millardet (1901) in- 

 duced fruit formation in certain varieties of the European grape by 

 pollinating the stigmas with pollen of Ampelopsis hederacea, and a 

 partial development of the ovary in certain cucurbits by treating 

 the stigmas with powdered pollen. A year later Massart placed 

 dead pollen upon the stigmas of an orchid and observed a slight 

 increase in the size of the ovary. Subsequently, Fitting (1909) 

 painted the stigmas with an extract of pollen and ascribed a hor- 

 monal action to the latter. Additional experiments of a similar 

 nature made by later workers (see especially Laibach, 1932, 1933) 



13 For more detailed information on this topic, see Maheshwari (1940), Gustaf- 

 son (1942), and Swarbrick (1947). 



