584 THE AMERICAN NATURALIST. [Voı. 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 (799, 
: 00), Correns ('99b), and Webber (:00). 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 Monocotyledonz and Dicotyledonz 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 


