the reserve-food is stored. There are
also intermediate states. The position of the embryo in relation to
the endosperm varies, sometimes it is internal, sometimes external,
but the significance of this has not yet been established.

[v.02 p.0012]

The formation of endosperm starts, as has been stated, from the
endosperm nucleus. Its segmentation always begins before that of the
egg, and thus there is timely preparation for the nursing of the young
embryo. If in its extension to contain the new formations within it
the embryo-sac remains narrow, endosperm formation proceeds upon the
lines of a cell-division, but in wide embryo-sacs the endosperm is
first of all formed as a layer of naked cells around the wall of the
sac, and only gradually acquires a pluricellular character, forming
a tissue filling the sac. The function of the endosperm is primarily
that of nourishing the embryo, and its basal position in the
embryo-sac places it favourably for the absorption of food material
entering the ovule. Its duration varies with the precocity of the
embryo. It may be wholly absorbed by the progressive growth of the
embryo within the embryo-sac, or it may persist as a definite and more
or less conspicuous constituent of the seed. When it persists as
a massive element of the seed its nutritive function is usually
apparent, for there is accumulated within its cells reserve-food, and
according to the dominant substance it is starchy, oily, or rich in
cellulose, mucilage or proteid. In cases where the embryo has stored
reserve food within itself and thus provided for self-nutrition, such
endosperm as remains in the seed may take on other functions, for
instance, that of water-absorption.

Some deviations from the usual course of development may be noted.
Parthenogenesis, or the development of an embryo from an egg-cell
without the latter having been fertilized, has been described in
species of _Thalictrum_, _Antennaria_ and _Alchemilla_. Polyembryony
is generally associated with the development of cells other than the
egg-cell. Thus in _Erythronium_ and _Limnocharis_ the fertilized
egg may form a mass of tissue on which several embryos are produced.
Isolated cases show that any of the cells within the embryo-sac may
exceptionally form an embryo, _e.g._ the synergidae in species of
_Mimosa_, _Iris_ and _Allium_, and in the last-mentioned the
antipodal cells also. In _Coelebogyne_ (Euphorbiaceae) and in _Funkia_
(Liliaceae) polyem