382 PLANT PHYSIOLOGY 



Development of the Embryo. — Before the embryo can develop 

 within the seed, pollination and fertilization are necessary. These 

 two processes must not be confused; the former is the transfer 

 of pollen from the anthers to the stigma, while the latter is the 

 actual union of the male and female gametes. The immediate 

 primary effect of fertilization is the growth of the embryo plant. 

 The effect of the pollen on the embryo is a question for the science 

 of genetics, but there are certain secondary effects which are more 

 within the province of the physiologist. Among horticulturists 

 the idea is prevalent that the nature of the pollen determines 

 the development of some of the fruit characters which lie outside 

 the ovules, viz., color and form. Waite (1894) found that Bartlett 

 pears show constant differences depending upon the kind of pollen 

 used, and Lewis and Vincent (1909) showed that similar effects 

 are to be observed in apples. Spitzenberg apples when pollinated 

 by Arkansas Black are deep red in color as compared with the 

 light red fruits obtained when the pollen is from Jonathan apples. 

 Murneek (1926) found that, in the tomato, plants in which ferti- 

 lization had taken place had a higher metabolic efficiency than 

 those in which fertilization was prohibited, showing that stimula- 

 tive effects may even extend into the vegetative organs. Swin- 

 gle (1926) showed that in the date the size, shape, and color of 

 the seed and the rate of development of the fruit varied with the 

 type of pollen used. He suggested the term metaxenia for this 

 direct effect of the pollen on the parts of the seed and fruit lying 

 outside the embryo and endosperm. But it still remains to deter- 

 mine how many, if any, of the effects described by the previous 

 workers (some of which are mentioned above) were truly metax- 

 enic. In some cases they may have been due simply to an increased 

 setting of seed, in which the difference in fruit size and shape 

 might occur as a purely physical result. In such matters the ex- 

 perimenter must constantly be on his guard, since it is very easy 

 to confuse results which look similar and to conclude that they 

 are produced by similar causes. 



Parthenocarpy. — In general, the failure of either pollination or 

 fertilization results in the fall of the flower without the develop- 

 ment of any fruit, but in certain cases the fruit may develop even 

 though there are no fertile ovules to develop into seeds. The stim- 

 ulus of the fertilized egg is consequently not indispensable for 

 fruit development. Among the plants which are known to pro- 



