tures [7]. The endosperm contains stored food reserves composed of carbohydrates, proteins, oils, and
other biochemical substances. All seeds contain stored food reserves. In some, the amount can be quite
small. Generally, the larger the food reserve, the greater the vigor of the seedling. Plump seeds usually
have more food reserves than small, shriveled seeds. Also, food reserves are found in the cotyledons of
some species.
The seed coat may appear dull, highly glossy, smooth, wrinkled or pitted, hard or soft, thick or thin,
or any combination of these characteristics. Many seeds also have attached wings or other appendages. In
seeds having two seed coats, the inner membranous one is usually thin, transparent and physiologically
active; that is, it restricts gaseous exchanges and movement of biochemical substances [8]. The outer seed
coat is hard and thick. A non viable seed may contain an empty seed coat without an embryo or one that
is reduced and shrunken. Seed coverings play an important role in protecting the seed and in influencing
germination.
III. SEED GERMINATION
Most seeds begin to germinate (resume activity) soon after being exposed to or planted in a moist, warm
soil or germinating medium. The germination process begins with a swelling of the seed as it takes up or
imbibes moisture. Usually, the radicle emerges first from the softened or ruptured seed coat, grows down-
ward, and develops into the primary root system. The plumule grows upward to form the stem. During
early growth, the young seedling derives its nourishment from the seed’s cotyledons and/or endosperm.
Cytokinins—members of the group of plant hormones, including kinetin, that act synergistically with
auxins to promote cell division but, unlike auxins, promote lateral growth—promote the mobilization of
the food reserves toward the developing shoot and to the root, which begins to function and absorb nutri-
ents from the soil or medium.
In some instances, the cotyledon or cotyledons remain beneath the surface of the ground, hypogeous
germination (Figure 1C), although in most species they push above the surface, epigeousgermination
(Figure 1D), turn green, and perform the functions of leaves, but are not true leaves. The food reserves
continue to nourish the seedling until photosynthesis occurs at a rate capable of supporting the plant, usu-
ally when the first true leaves are formed. At this stage, germination is completed and, in most cases,
seedlings are capable of independent existence and germination is completed.
IV. PHYSIOLOGICAL AND ENVIRONMENTAL FACTORS
Each species of plant has its own unique requirements for moisture, temperature, oxygen, light, and other
factors.
A. Moisture
The need for moisture is the most important prerequisite for triggering germination. Whereas some seeds
require little moisture for germination, others, such as those from Nymphaeaspp. (water lilies) and other
aquatic plants, must be completely submerged in water. Seeds with hard or impermeable seed coats re-
quire special treatment (softening or scarification) to allow efficient uptake of water. Generally, water
reaches the seed through contact with the soil or germinating medium. Once the germination process be-
gins, an adequate moisture level must be maintained as temporary drying can result in death of the seed
or seedling. Too much moisture can cause the soil or germinating medium to become saturated and de-
prive the seed of oxygen, leading to death. Water uptake by seeds during germination has been described
using a three-stage model [9]:
- Stage I (Imbibition)
Dry seeds have a high negative water potential (100 to 200 mPa) because of the colloidal properties
of the seed coat. The surfaces of proteins, cellulose, starch, and other substances must first become hy-
drated. The uptake or imbibition of water in stage I is physical, resulting in softening or rupturing of the
seed coat and an increase in the volume of the seed. As the seed imbibes water, its internal water poten-
tial rises.
GERMINATION AND EMERGENCE 59