IOWA ACADEMY OF SCIENCES. 
87 
At about this time (May 14-21) fertilization occurs. 
There is a great deal of variation in the time, some ovules 
not being fertilized until a week later than some others. 
Thus in some cases fertilization does not occur until two 
w T eeks after pollination. Not until the first of June were 
the embryos large enough to be detected by the naked eye. 
There is thus much ground for the belief of many horticul- 
turists that fertilization does not occur for some time after 
pollination. The embryo increases rapidly in size, absorb- 
ing and replacing the endosperm as well as the nucellar 
tissue. During the time that' the embryo sac is increasing 
in size so rapidly, the cells of the nucellus increase in 
number but little, but enlarge enormously, becoming 
eventually many times as large. The cell walls are very 
thin, and the nucleus almost indiscernable, and are very 
readily displaced and absorbed by the embryo, which 
comes to fill the entire seed at maturity. 
As has been stated, members of the genus Prunus have 
two ovules, only one of which develops. Several inter- 
esting facts were found in a study of this ovule. Until 
about the time of pollination there is no perceptible dif- 
ference between the ovules, the embryo sacs being per- 
fectly normal in each. Very soon, however, it is found that 
one of these ovules has gained the ascendency, and is de- 
veloping more rapidly than is its mate. The latter is 
soon crowded to one side, the tissues of the nucellus be- 
come disorganized, and the entire ovule is gradually 
flattened against the wall of the ovary by the pressure of 
the other. At length it disappears. The persistence of 
life in the embryo sac even after the partial disintegration 
of the nucellar tissue was especially marked. The egg 
cell and the definitive nucleus with the enclosing sac, all 
staining appearing normal, could be distinguished even 
after the surrounding cells could not be recognized, and 
the entire ovule was shriveled. Fertilization does not occur 
in the other ovule until later, so that it can not be that the 
first ovule fertilized is the one that develops. The fact, 
also, that it is very rare for one to find both ovules matured 
