Download 1: Early development of Capsella

EMBRYO DEVELOPMENT
When a pollen grain fertilizes an ovule inside of the ovary, development of the embryo begins.
Embryonic development, occurring within the growing seed, creates a new individual from a fertilized
egg. The embryo goes through several increasingly complex stages as its mitotic divisions continue.
These stages include the early proembryo, the globular stage, the heart stage (cotyledons begin to
emerge), the torpedo stage (differentiation of the vascular tissues begin), and finally the mature embryo.
The endosperm of most developing seeds is at first non-cellular or liquid. In both corn and coconuts, a
portion of the endosperm remains liquid after the outer part of the endosperm has become cellular. After
the endosperm has become cellular, it begins to convert available nutrients into insoluble reserves such as
starch, proteins, and vegetable oils, storing up these products for the future use of the embryo during seed
germination.
The final important feature of embryo development within the seed is the cessation of growth when the
embryo attains full size. This is commonly called embryo dormancy. This is a temporary, physiologically
imposed dormancy. If it fails, the embryo continues to grow and the seed germinates within the fruit, the
seed becoming useless as a means of reproduction.
1. Obtain slides of various stages of Capsella embryo development. Identify as many of the
following stages as you can: globular, heart-shaped, torpedo, and mature.
2. Compare what you see to slide pictures of embryo development.
3. Examine a prepared slide of a corn grain. Identify the endosperm, cotyledon, coleoptile, root, root
cap, coleorhizae, and shoot apical meristem.
Seedling Development--A typical seed consists of:
1. Embryo - the young plant within the seed consisting of:
a. Epicotyl - will form the leaf of the new plant.
b. Hypocotyl - will form the stem of the new plant.
c. Radicle - will form the root of the new plant.
2. Endosperm - food reserve derived from fertilized polar nuclei.
3. Cotyledons - will serve as food for the sprouting plant.
4. Seed coat - a structure derived from the wall of the ovule to protect the inner parts.
5. Hilum - the funicular scar on the seed coat.
6. Micropyle - a hole through the seed coat.
See if you can identify these structures by looking at the seeds you dyed to test for viability.
There are two types of seedling development: epigean and hypogean. In epigenous development, the
cotyledons emerge from the soil. In hypogenous development, the cotyledons remain in the soil. Look at
the corn and pea seedlings and see if you can determine which kind of development these plants undergo.
1: Early development of Capsella
The asymmetric division of the zygote gives rise to the embryo and the extra-embryonic
suspensor, through which nutrients are transported to the embryo (46). The suspensor also
pushes the embryo into the lumen of the embryo sac (47). In later development the
uppermost cell of the suspensor gives rise to the hypophysis of the embryo (48), which takes
part in developing the root cap and tip.
The embryo is simple and lacks some structures present in the mature plant. However, the
shoot apical meristem, from which all later above-ground meristems are derived, becomes
established at this stage.
The various appearance of the embryo at various stages has been used to provide a context
for discussing developmental events. Their names are given here.
In the following images, the descendants of the apical cell are shown in yellow, and the descendants of the basal cell are shown in
pink.
from Chun-Ming Liu
Embryogenesis
Plant embryogenesis begins with an asymmetric cell division, resulting in a smaller apical (terminal)
cell and a larger basal cell. This first asymmetric division provides polarity to the embryo. Most of the
plant embryo develops from the apical (terminal) cell. The suspensor develops from the basal cell. The
suspensor anchors the embryo to the endosperm and serves as a nutrient conduit for the developing
embryo.
Further cell division leads to the globular stage. The three basic tissue systems (dermal, ground, and
vascular) can be recognized at this point based on characteristic cell division patterns. The globular shape
of the embryo is then lost as the cotyledons (embryonic leaves) begin to form. The formation of two
cotyledons in dicots gives the embryo a heart-shaped appearance. In monocots, only a single cotyledon
forms.
Upright cotyledons can give the embryo a torpedo shape, and by this point the suspensor is degenerating
and the shoot apical meristem and room apical meristem are established. These meristems will give rise to
the adult structures of the plant upon germination. Further growth of the cotyledons results in the torpedo
and walking-stick stages. At this point, embryogenesis is arrested, and the mature seed desiccates and
remains dormant until germination.