THE ANGIOSPERMAE 1451 



tophytes only. Others have effects on the morphology of the progeny, where 

 the characters appear as recessives. 



On the other hand, in cases of weak self-incompatibility, it is the ovary 

 or the ovules which seem to cause the differentiation between successful 

 and unsuccessful pollen tubes. Darwin, and after him Bateson and Gregory, 

 noted that in illegitimate unions in Primula approximately half the ovules 

 are fertilized in all cases and the later writers suggested a differentiation 

 between ovules as the cause. 



Competition between pollen tubes seems to be indicated in experim nts 

 with Nicotiana tahacum, where it has been shown that pollination with 

 abundant pollen yields a relatively uniform Fi generation, while if only a 

 few pollen grains are used the offspring are more variable and aberrant types 

 appear, which are suppressed by more liberal pollination. In the heterozy- 

 gous Oenothera herteriana when self-pollinated, only heterozygous embryos 

 are formed and the homozygotic combinations fail to appear. This has been 

 attributed to preferential fertilization of ovules by pollen tubes which do not 

 carry the same complex of genes. It is more likely to be due to a combination 

 of lethals in the homozygotes. 



The Interpretation of Double Fertilization 



The significance of the fertilization of the endosperm nucleus has never 

 been satisfactorily explained. Navaschin, its discoverer, looked upon it as a 

 sexual act and considered the endosperm to be a misshapen twin of the 

 regular embryo. Strasburger called it a vegetative fertilization and thought 

 that the stimulus to development was the secret of its importance. This 

 accorded with his idea of endosperm formation as a continuation of pro- 

 thallus building. Schiirhoff, on the other hand, denied all sexual character 

 to the fusion, which to his mind is a purely nutritive act which he calls 

 trophomixis. 



The anomalous nature of endosperm as a polyploid tissue which is 

 neither sporophyte nor gametophyte should not be overlooked. Moreover 

 it is by means of this specialized tissue that the young sporophyte is nourished 

 and there is no other provision for its nourishment. This is a unique situa- 

 tion in the plant world and it is not surprising, therefore, that unique cir- 

 cumstances should attend its origin. It is obviously important that the 

 development of the nutritive tissue should be coupled as regards timing 

 with the development of the embryo. Putting aside the deeper question of 

 the nature of the sexual stimulus, the fact remains that union w^ith the second 

 male nucleus is the signal for development to begin and that without it the 

 endosperm nucleus does not usually develop. Other, possibly chemical, 

 stimuli to development must act in parthenogenetic embryo sacs, stimuli 

 which in such cases affect both oosphere and endosperm development. 



If we accept the Porsch theory of the embryo sac, both polar nuclei are 

 the equivalents of ventral canal cells and are the sister cells of gametes, 

 actual or potential. That, united together and combined with a male 



