402 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



As far as present evidence goes, there is no essential difference 

 between the two except that in parthenocarpic fruits the ovules 

 become shriveled and shrunken and no embryos are formed (Fig. 

 213). To explain why some plants are naturally parthenocarpic 

 (e.g., varieties of Musa, Ananas, Vitis, etc.) while others are not, 

 Gustafson (1939a, o) studied the auxin content of the ovary in a 



Fig. 213. Fruit production in tomato; left, from fertilized ovary; right, induced 

 by hormone treatment. (Photograph supplied by Dr. A. W. Hitchcock.) 



few forms of each kind. In every instance he found it to be higher 

 in the ovaries of the parthenocarpic varieties than in the ovaries 

 of the nonparthenocarpic ones. Obviously, then, the reason why 

 some plants produce parthenocarpic fruits is that their ovaries 

 contain enough auxin to promote growth without fertilization, while 

 those of other plants do not possess it in sufficient quantity and it 

 must therefore be augmented either by pollination and fertiliza- 

 tion, or by pollination alone, or by an external application of the 

 requisite growth-promoting substance. In a detailed review of the 



