Horticulture Principles and Practices

The two basic modes of seedling emergence are associated with what parts of the
seed emerge first from underground. In certain species, the cotyledons (seed leaves)
emerge above the ground, preceded by a characteristic arching (hook) of the hypocotyl
(a stemlike axis). This mode of shoot emergence is called epigeous (epigeal)
germination (Figure 9–6) and is associated with dicots. However, in certain dicots such
as the pea (Pisum sativum), the cotyledons remain underground. The cotyledons, once
above ground, gradually change from a creamish color to green. In squash (Cucurbita
maxima), the cotyledons are important in photosynthesis. However, in castor bean and
garden bean, these seed leaves do not contribute significantly to photosynthesis. The
cotyledons gradually shrivel in size and eventually drop when the seedling becomes
completely autotrophic. These true leaves are called seed leaves.
In other species such as the grasses, the endosperm remains underground so that
the plumule emerges first. This mode of seedling emergence is associated with monocots
and is called hypogeous (hypogeal) germination (Figure 9–7). The cotyledon
remains underground and eventually decomposes. In this mode of germination, the
epicotyl (rather than the hypocotyl) elongates.
First true
leaves
Epigeous Germination
A type of seed germination
in which the cotyledons
rise above the soil surface.
Hypogeous
Germination
A type of seed germination
in which the cotyledons
remain below the soil
surface.
FIGURE 9–6 Epigeous or
epigeal germination of bean
seed. Cotyledons emerge above
the ground.
Hypocotyl
Cotyledon
Withering
cotyledon
Seed
Primary
root
Seed coat
Primary
root
Coleoptile
First leaf
Prop roots
FIGURE 9–7 Hypogeous or
hypogeal germination of corn
seed. Cotyledons remain underground.
Primary root
Radicle
Coleorhiza
9.13 Seed Germination and Emergence 301