Most seeds consist of three parts: embryo, a miniature plant inside the seed; endosperm, stored food
reserves for the growing embryo; and seed coat(testa), which encompasses and protects the embryo and
endosperm from damage, excess water loss, and other unfavorable conditions.
The embryo has one or more miniature seed leaves (cotyledons), an embryonic stem (plumuleorepi-
cotyl), an embryonic root (radicle), and a hypocotyl, the transition zone between the embryonic stem and
root. Among angiosperms, plants that have two cotyledons are classified as dicotyledonsand those with
a single cotyledon as monocotyledons. Gymnosperms may have as many as 15 cotyledons.
There are about 250,000 different seed-bearing plants in the world. Each species has its own mor-
phologically unique form of seed, which can be identified by its size, shape, color, and other external fea-
58 CHONG ET AL.Figure 1 (A) Longitudinal section through seed of an angiosperm, Albizia julibrissin(silktree), 5. (From
Ref. 3.) (B) Longitudinal section through seed of a gymnosperm, Pinus ponderosa(ponderosa pine), 6. (From
Ref. 4.) (C) Hypogeous germination: development of Lindera benzoin(spicebush) 2, 3, and 10 days after
germination. (From Ref. 5.) (D) Epigeous germination: development of Acer platanoides(Norway maple) 1, 3,
7, and 19 days after germination. (From Ref. 6.)