Instant Notes: Plant Biology

The endosperm After the double fertilization of the ovule the first part to grow is the
endosperm, normally consisting of triploid cells. Initially in many plants the
endosperm nucleus divides freely giving an acellulartissue only later devel-
oping cell membranes and cell walls, although others are cellular from the start.
Some remain part cellular part acellular, such as the coconut in which the milk
is acellular. The endosperm grows rapidly and may absorb part of the nucellus
or inner integument, stimulating their cells to break down. It incorporates
within it a large supply of nutrients, a mixture of carbohydrate, lipid and
protein and remains in the ripe seed of some plants which may be mainly
starchy as in cereals such as wheat or oil-rich as in castor-oil, linseed or oilseed
rape (canola). It also contains plant hormones which may stimulate the growth
of the embryo (Topic H4). In some acellular endosperms, nuclei with high
ploidylevels have been found where, it seems, the chromosomes have divided
but never separated. In some plants the endosperm has been all or mostly
absorbed by the embryo by the time the seed is mature, such as in beans and
brazil nuts, both of which have protein-rich seeds.

The embryo The embryo starts to grow either shortly after fertilization or after some delay,
up to a few months (Fig. 1). The first cell division is transverse, with just a few
exceptions, giving a terminal and a basal cell. After this the details vary some-
what, but the terminal cell always gives rise to the stem apexand the cotyle-
dons, which are the embryo’s first leaves and sometimes food store, and in some
to the root apexand root cap as well. The basal cell provides the suspensor,
along with the root tip and cap in some. The suspensor becomes a short string of
cells and the lowest cell swells into a vesicle, but the function of the suspensor is
not clear and it disintegrates with the enlargement of the embryo.
The earliest flowering plants almost certainly had two cotyledons, and in
most living plants the shoot tip develops two lobes that will become the two
cotyledons with the shoot apex between them. Beneath this, in the hypocotyl,
vascular tissue develops. The developing root acquires a distinct cap. In some
plants one of the cotyledons appears to be suppressed. This is a feature of one of
the major divisions of the flowering plants, called the monocotyledonsbecause
of this (Topic R4), although some dicotyledonssuppress one cotyledon in a
similar way. Grasses, which are monocotyledons, have their own unique
embryo formation with a swelling at the tip of the embryo forming two different
outgrowths, the scutellum which may represent the cotyledon, and the

50 Section D – Reproductive anatomy

Suspensor

Vesicle

Cotyledons Stem apex

Fig. 1. Early growth of the embryo in a dicotyledon, Capsella bursa-pastoris.