Seeds and Seedlings - Seed Testing - page 16

2. Asterad - both the basal and terminal cells contribute to embryo development.
B. Terminal cell of the proembryo divides by a transverse wall.
1. Solanad - basal cell plays only a minor role (or none) in the development of the embryo.
2. Caryophyllad - basal cell undergoes no further division, and the suspensor, if present, is
always derived from the terminal cell.
3. Chenopodiad - basal cell and terminal cell both contribute to embryo development.
II. Piperad - the i rst division of the zygote is longitudinal, or nearly so, with no formation of basal
and terminal cells.
Figure 2.5. Comparison of embryo, seed, and fruit development in dicots and monocots. In many dicots (e.g.,
sunl ower, top), the seed is i lled entirely by the embryo (root-shoot axis between two cotyledons). In mono-
cots (e.g., maize, bottom), the fruit contains both the embryo and the endosperm enclosed by the pericarp
(ovary wall) to form a caryopsis, a one-seeded fruit (drawing by Sabry Elias).
Endosperm development
The endosperm serves as the principal nutritive support for the embryo of many species (especially mono-
cotyledons) during both seed development and germination. Endosperm develops concurrently with the
embryo. Development of the endosperm begins with the union of a male and two polar nuclei to form a trip-
loid (3n) nucleus. Nuclear components other than 3n may occur in some species. Endosperm development
may be cellular, nuclear , or helobial . In the cellular type, nuclear divisions are accompanied by cell wall
formation. The nuclear endosperm is characterized by nuclear divisions unaccompanied by cell wall forma-
tion. The nuclei may remain free or may later be separated by cell walls that form in one of three ways: (1)