584 THE AMERICAN NATURALIST. [Vol. XXXIX. 



the substance of the sperm nucleus of the male parent. But 

 facts have clearly shown that the pollen may also affect the 

 structure of the endosperm in the seed as well as cause the 

 development of the embryo. Since the endosperm holds no 

 genetic relation to the embryo it has seemed very remarkable 

 that it should take on hybrid qualities. It has also been claimed 

 that other regions of the seed or fruit, such as portions of the 

 pericarp were also affected, but it is doubtful whether this is 

 really so or at least whether such changes are truly a feature of 

 the protoplasmic structure and thus deeply seated in the organ- 

 ism as a feature of hybridization. 



It is only within recent years that a satisfactory theory has 

 been suggested for the influence of pollen outside of the embryo. 

 And this explanation rests on the discovery of the activities of 

 the second sperm nucleus which enters the embryo-sac and which 

 is known in some cases to unite with the polar nuclei constitut- 

 ing a triple nuclear fusion within the sac that is generally known 

 as " double fertilization." We have briefly referred to the phe- 

 nomenon in the latter part of the account of " Asexual Cell 

 Unions and Nuclear Fusions " in Section IV and shall take it 

 up now in greater detail. The best account of xenia is a very 

 clear treatment by Webber, in 1900. 



The explanation of xenia upon the facts of "double fertiliza- 

 tion" was proposed almost simultaneously by De Vries ('99, 

 :oo), Correns ('99b), and Webber (:oo). Double fertilization 

 was first observed by Nawaschin ('98) in Lilium and Fritillaria 

 and shortly after was described in greater detail by Guignard 

 ('99b) in other species of the same genera and in Endymion. 

 Since these discoveries the phenomenon has been reported by a 

 number of investigators in many other forms representing widely 

 divergent groups in the Monocotyledonfe and Dicotyledonse and 

 there is every reason to believe that it is widespread in the angio- 

 sperms. A review of the recent literature is given by Coulter 

 and Chamberlain (^Morphology of the Angiosperms, 1903, p. 156). 

 There is no fixed order in the events of the triple nuclear fusion 

 of " double fertilization." The polar nuclei may have united at 

 the time when the pollen tube enters the embryo-sac, in which 

 case the second sperm nucleus coalesces with an organized fusion 



