400 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



a size of 4.1 by 7.3 cm. Also, out of 50 cucumber ovaries injected 

 with extracts of cucumber pollen, three continued growth. 

 Of these last, one attained a size of 4.3 by 20.3 cm., comparing 

 favorably with a normal cucumber. 



As a result of these experiments, supported by some microscopic 

 studies, Yasuda arrived at the following conclusions: (1) pollen 

 tubes secrete or carry some chemical substance which diffuses into 

 the tissues of the ovary and thereby induces fruit formation; (2) 

 if the pollen tubes are allowed to grow only up to the base of the 

 style, so as to permit chemical diffusion but not fertilization, the 

 resulting fruits are devoid of seeds; and (3) seedless fruits can also 

 be obtained by pollinating the stigmas with immature, overma- 

 ture, or incompatible pollen, or by using extracts of pollen. 



The fact that extracts of pollen, like the pollen tubes themselves, 

 could also induce parthenocarpic growth left no room for doubt 

 that the stimulation is entirely chemical in nature. In 1934 

 Thimann showed that many pollens contain considerable quantities 

 of an auxin or growth substance, and this led some workers, notably 

 Gustafson (1936, 1938a, 6), to experiment with some synthetic 

 hormones to see if they could bring about fruit formation . 



Several substances were tried, chiefly indoleacetic, indolepropi- 

 onic, indolebutyric, w-naphthaleneacetic, and phenylacetic acids. 

 These were made up into a lanolin paste of about 0.5 to 1 per cent 

 strength and smeared on the stigma. In some cases it was found 

 better to remove the style just above the ovary and apply the paste 

 to the cut surface of the latter; this facilitated the diffusion of the 

 chemical into the ovary. The flowers were subjected to three dif- 

 ferent kinds of treatments. Some were pollinated in the usual 

 way, others were treated with the substances named above, and 

 the rest were left untreated. The first lot produced normal fruits, 

 while the third withered and dropped without forming any fruits. 

 As a result of the second treatment, some parthenocarpic fruits 

 were formed in tomato, pepper (Fig. 212A), tobacco (Fig. 2125), 

 eggplant, crookneck squash, Petunia, and Salpiglossis , although the 

 average weight of these fruits was somewhat lower than that of 

 normal fruits. 



Gardner and Marth (1937) employed a different technique. 

 They sprayed the ovaries with aqueous solutions of several sub- 

 stances, of which naphthaleneacetic acid was found to be the most 



