also the synergidae. As in Gymnosperms and other groups an interesting
qualitative change is associated with the process of fertilization.
The number of chromosomes (see PLANTS: _Cytology_) in the nucleus of
the two spores, pollen-grain and embryo-sac, is only half the number
found in an ordinary vegetative nucleus; and this reduced number
persists in the cells derived from them. The full number is restored
in the fusion of the male and female nuclei in the process of
fertilization, and remains until the formation of the cells from which
the spores are derived in the new generation.

In several natural orders and genera departures from the course of
development just described have been noted. In the natural order
Rosaceae, the series Querciflorae, and the very anomalous genus
_Casuarina_ and others, instead of a single macrospore a more or less
extensive sporogenous tissue is formed, but only one cell proceeds to
the formation of a functional female cell. In _Casuarina_, _Juglans_
and the order Corylaceae, the pollen-tube does not enter by means
of the micropyle, but passing down the ovary wall and through the
placenta, enters at the chalazal end of the ovule. Such a method
of entrance is styled chalazogamic, in contrast to the porogamic or
ordinary method of approach by means of the micropyle.


_Embryology._

The result of fertilization is the development of the ovule into
the seed. By the segmentation of the fertilized egg, now invested by
cell-membrane, the embryo-plant arises. A varying number of transverse
segment-walls transform it into a pro-embryo--a cellular row of which
the cell nearest the micropyle becomes attached to the apex of the