8io 



PLANT GROWTH 



lO 



1 951). In the light of present knowledge, this is gibberellin A, or a related com- 

 pound. Its role in fruit growth is as yet unknown, but it can induce parthenocarpy. 

 As with the leaf, the abscission of the fruit depends on a cessation of the auxin 

 supply. The fact that the auxin is formed in "waves," which do not necessarily 

 overlap, may mean that there are temporary lacunae in the supply of auxin to the 



Fertilization 



Endosperm 



becomes 



cellular 



1 



Completion of 

 embryo growth 



I Relative strength and 

 duration of hormonal 

 1 stimuli 



^ 



Weeks after fertilization 



Fig. 13. Scheme of "hormone" production (at top) and fruit drop (below) in the apple. 

 The hormone was determined by ether extraction and bioassay, using parthenocarpy of 

 tomatoes as test; for this reason both auxins and gibberellins may be included in the activity. 



From Luckwill, 1953. 



peduncle. On this account some of the fruit may fall at specific times during the 

 growing season. This is especially true of apples; almost all varieties suffer from 

 "June drop" and some have a "first drop" period before that. These dropping 

 periods have been correlated with the auxin production (Luckwill, 1953) into a 

 general picture which has a good deal of verisimilitude (Fig. 13). It is, of course, 

 not certain that the abscission depends only on the auxin coming from the seed, 

 but the data correspond fairly well. It is well known, too, that apples with excep- 

 tionally few seeds are likely to fall during the season. 



Finally, it is interesting that the sex of flowers appears to be under hormonal 

 control, auxin diverting it towards femaleness. In cucumbers the projjortion of 

 female flowers can be clearly increased by early treatment with lAA or NAA 

 (Laibach and Kribben, 1950). In acorn squash plants, which produce male 

 flowers near the base and female flowers at about the 20th node, treatment with 

 NAA caused female flowers to appear at about the 9th node. Most striking example 

 of all is hemp (Cannabis sativa) in which the sexes are borne on separate male and 

 female plants; male plants can be made to bear female flowers by auxin treatment. 

 For this, 0.05% NAA in lanoline was used (Heslop-Harrison, 1956). It is sugges- 

 tive, too, that in the acorn squash which gradually becomes female, the last-formed 

 female flowers yield parthenocarpic fruits; there would seem, therefore, to be a 

 continuous increase in the auxin supplied to the flower-buds, causing continuous 

 change from male through female to parthenocarpic. The possible significance 

 of these findings for horticulture may be very great. 



