334 PLANT GROWTH SUBSTANCES 



spectively? Wittwer (95) found that the growth activity in extracts 

 from corn pollen and immature grains is not destroyed by prolonged 

 heating. According to McLane, syngamin seems to be insoluble in ether 

 at pH 5.4, passes through a collodion membrane, and can be removed 

 in part by treating corn extracts with activated charcoal. 



Fruit Setting 



The present evidence seems to point to the following sequences in 

 the function of hormones in relation to fruit setting. Pollen germinating 

 on the stigma either produces a hormone (32) or through secretion of an 

 enzyme liberates a hormone from inactive combinations in the style and 

 ovary (98,50). The male gametophyte, therefore, has at least two func- 

 tions, fertilization of the egg and causing the ovary to grow. The latter 

 action seems to be closely associated with prevention of abscission of the 

 young fruit. 



Van Overbeek (68) believes that not only the carpels but also the 

 ovules are controlled in their initial growth by a hormone originating 

 in the microgametophyte. By injecting NAA or indolebutyric acid 

 (IB) into the ovaries oi Melandrium and Datura not only parthenocarpic 

 fruit were secured but they contained enlarged ovules with seed coats 

 although no true embryos {66). He suggests that, with some plants at 

 least, even division of the polar nucleus and the egg cell, before fertiliza- 

 tion and triple fusion, may be brought about by the presence of pollen 

 tubes in the style. Gustafson (32) has shown that pollen extracts when 

 applied to pistils may cause them to develop into parthenocarpic fruit, 

 while Laibach (45) and Thimann (84) found that pollen contained 

 hormones. However, the greatest hormone effect on the fruit absciss 

 layer probably comes after fertilization from the ovules (1,50), since 

 the hormone content of ovules and developing seeds is exceedingly 

 high (31). 



The disclosures by these studies, but probably even more the discovery 

 by Gustafson (30) that several synthetic growth regulators will induce 

 fruit development without pollination, and by Gardner and Cooper (28) 

 that premature dropping of apples can be retarded by spraying with a 

 dilute solution of NAA, have been of considerable value in horticulture. 

 Moreover, it has created much interest leading to further experimental 

 work on the use of growth regulators in horticultural practice. 



Through the tests by Zimmerman and Hitchcock (99) about 30 



