8o8 PLANT GROWTH lO 



that in each cell the nitrogen increases proportional to the cell surface, and this 

 in turn, it has been suggested, means that the enlarging cell maintains a layer of 

 protoplasm of approximately constant thickness (Pearson and Robertson, 1953). 

 This relationship, which no doubt holds for other fruits, is similar to that of enlarg- 

 ing fragments of tubers (p. 792). 



The sources of auxin for all this growth are by no means understood. Recent 

 work indicates that no one single source is responsible and that several auxin 

 sources succeed one another in making possible the whole process of fruit growth. 

 That auxin is an essential and causative factor is certain, because the whole pro- 

 cess of fertilization and seed development can be set aside, and normal-sized fruit 

 still be obtained, if auxin is applied. This results in seedless, or parthenocarpic 

 fruits. In a few cases, including tomatoes, gibberellins have a similar effect. 

 - That parthenocarpic fruits are capable of more or less normal development 

 has, of course, been known for a long time, because several fruits of commerce, 

 including the banana and navel orange, are parthenocarpic. The work of Yasuda 

 and of Gustafson, in the 30's, was the first to show that petunia and tobacco fruits 

 could be made to develop by extracts of pollen almost as well as by the pollen it- 

 self (see Gustafson 1942 and 1951 for reviews of early work). Later a variety of 

 horticultural fruits have been made parthenocarpic, — including pears, squash, 

 watermelons, peppers, strawberries and tomatoes, — the last even on a semi-com- 

 mercial scale. The procedure is always the same, namely to spray the freshly- 

 opened flowers with an auxin in suitable concentration. If it is desired to ensure 

 complete seedlessness the stamens may be first removed with fine scissors. Auxin 

 application after pollination leads to fruits with a reduced number of seeds (Fisch- 

 nich and Liibbert, 1955). However, the main advantage of commercial auxin 

 treatment is not really seedlessness but an increase in the number of fruit that 

 are set. The method is particularly valuable, therefore, in weather that is unfavora- 

 ble for natural fruitsetting ; e.g. greenhouse tomatoes in the winter months (How- 

 lett, 1 941). The fruits are often rather small, but at least in some experiments nor- 

 mal-sized fruits have been obtained. Apparently the ethyl ester of lAA is the most 

 effective auxin for the purpose, though 2,4-D (XI, p. 763) has been widely used on 

 tomatoes and a-2-naphthoxy-propionic acid seems the most effective on pears. 



The natural supply of auxin to the ovary begins with the pollen, which several 

 workers have shown to contain moderate quantities of auxin {cf. above). However, 

 the number of pollen grains falling on one stigma can be quite small and the 

 quantity of auxin so transferred could hardly be enough for more than a minute 

 amount of growth. This growth, however (or perhaps the act of fertilization) 

 modifies the enzymes of the style and the ovary wall so that, in tobacco at least, 

 they now can liberate an auxin from the ovary tissue (Muir, 1942, 195 1). As a 

 result the styles and ovary tissues soon contain more auxin than the pollen could 

 have supplied. The flower petals behave in an oppositive way, since they fall off" 

 after pollination, presumably because of reduced auxin formation, and application 

 of auxin will make them stay on, at least for a time (Fischnich and Liibbert, 1955). 



The second step is fertilization, which, in the apple, causes growth of the endo- 

 sperm, first as free nuclei in a coenocyte {cf. coconut milk) then as separate cells; 

 thereafter it causes growth of the embryo. The seed appears to contain no free 



