REPRODUCTION AND DISPERSAL 917 



considerably with the species. In many cases the act of pollination forms a stimu- 

 lus of sufficient intensity to inaugurate continued development; this condition is 

 well illustrated in certain orchids in which fruit development has been started by 

 dead pollen or pollen extract placed upon the stigma. In other cases it is the grow- 

 ing pollen tube which initiates fruit development, as in Geranium and in various 

 orchids. In the Cucurbitaceae the fusion of gametes is necessary for complete fruit 

 development, although pollination alone stimulates considerable growth. In still 

 other cases the growing ovules are an important stimulus, as in the grape, where 

 the size of the fruit increases with the number of seeds. 



Parthenocarpy. In striking contrast to ordinary fruit production 

 is parthenocarpy, or the development of fruit without the fusion of 

 gametes. Familiar illustrations of parthenocarpy are afforded by a 

 number of seedless varieties of cultivated fruits, as in oranges, grapes, 

 and bananas; while only certain varieties of grapes and oranges are 

 seedless, the cultivated banana never produces seeds. 1 In some cases 

 of parthenocarpy, pollination seems to be necessary for fruit development, 

 but it is quite unnecessary in certain figs, where fruit development occurs 

 without the aid of any known external stimulus. The most striking 

 case of all is in Balanophora, a plant which is quite without functional 

 pistillate flowers, but which produces fruits containing viable seeds. In 

 this genus the pistillate flower is reduced to a protuberance with rudi- 

 ments of a style and an embryo sac. One species (B. globosa) lacks 

 staminate flowers, and even in those species which produce pollen, it is, 

 of course, entirely useless, affording one of the best illustrations of 

 the retention by a plant of a useless organ. Balanophora is a holopara- 

 site, and it may be that there is some connection between its parasitism 

 and its loss of sexuality. 



In recent years the number of plants which are known to be able to develop 

 parthenocarpic fruits has been considerably increased; among such plants are the 

 persimmon, gooseberry, hop, and certain varieties of the apple and the pear. It is 

 also becoming clear that in most cases neither pollination nor any other known 

 external stimulus is necessary to secure fruit development. In several cases, as in 

 the gooseberry and the persimmon, the seedless fruits mature earlier than do the 

 seed-bearing fruits. Obviously fruit production without seeds is wholly useless so 

 far as the perpetuation of the plant is concerned. 



Variations in the size and the structure of fruits and seeds. Probably 

 no other plant organs are as invariable as are fruits and seeds, and for 

 this reason the few variations which are known have an unusual interest. 

 Seeds which develop singly or which are not crowded during develop- 



1 Plants with parthenocarpic fruits are, of course, propagated vegetatively ; it is sup- 

 posed commonly that they originated suddenly as mutants. 



