Seeds and Seedlings - Seed Testing - page 27

dicotyledons
Figure 2.13 shows the essential structures of dicotyledonous seedlings. A primary root irst develops root
hairs which aid in absorbing water and dissolved minerals. Within the irst several days after emergence,
secondary roots develop. These serve to more fully anchor the seedling and greatly increase its absorptive
capability. Adventitious roots are secondary roots which arise from seedling structures other than the pri-
mary root (i.e., hypocotyl, mesocotyl, or epicotyl).
The hypocotyl is that portion of the seedling axis immediately above the primary root and below the
cotyledons. In species with epigeal germination, the hypocotyl elongates to pull the cotyledons above the
soil. In this process, a hypocotyl arch (or hook) is formed which serves to pull the cotyledons through the
soil, thereby protecting the food stores and growing point. Upon exposure of the hypocotyl arch to light
(i.e., upon emerging through the soil), the arch straightens out and stops growing. The hypocotyl contains
conducting tissues that connect the aerial and subsoil portions of the seedling.
Species with hypogeal germination do not undergo hypocotyl elongation; therefore, the hypocotyl is,
for all practical purposes, not discernible.
Upon completion of hypocotyl elongation, the cotyledons shed their seed coat and unfold. Cotyledons
are embryonic leaves which serve as the irst photosynthetic organs of the seedling. Therefore, they turn
green upon exposure to light. In addition, they serve as a nutrient source. As the seedling grows, it is
nourished by the cotyledons, which begin to show shrink lines as they become depleted. Gradually, the
cotyledons shrink and fall off as the primary leaves are developed and become photosynthetically active.
The epicotyl is that portion of the seedling between the cotyledons and the primary leaves. Species
with hypogeal germination undergo elongation of the epicotyl. In this process, an epicotyl arch or hook is
developed. The epicotyl arch is also light sensitive, causing it to straighten and stop growing shortly after
emergence through the soil. As stated above, the function of the arch is to pull the primary leaves and
shoot apex through the soil without damage to the growing point. Conducting tissue of the epicotyl links
the above and below ground parts of the seedling. Scale leaves are often present on the epicotyl below the
primary leaves.
The shoot apex is the terminal part of the seedling axis, containing the growing point (meristematic
region) which will continue to develop additional leaves and the aerial portions of the plant.
Monocotyledons
Figure 2.14 shows the essential structures of monocotyledonous seedlings. The primary root of many
monocots is often short lived, and is replaced by secondary roots which may be either adventitious or lat-
eral . Adventitious roots are clearly visible in corn, where they arise from the mesocotyl. Lateral roots are
those which arise from another root. A seminal root system includes the primary root and adventitious roots
which all appear approximately simultaneously during germination. In wheat, the primary root is often not
distinguishable from the adventitious roots; therefore, all the roots collectively make up the seminal root
system. A specialized tube-like structure, called the coleorhiza , is present in grasses. It serves as a protec-
tive sheath for the radicle upon emergence from the seed. Soon afterwards, its growth stops and the radicle
grows through it.
The hypocotyl is not present in most monocot seedlings, however, a mesocotyl is present in certain
species. The mesocotyl is thought to be a result of a junction of part of the cotyledon to the hypocotyl. In
corn, the mesocotyl is clearly visible as a portion of the seedling axis above the scutellum and below an
enlarged swelling located a short distance above the seed.
In monocotyledonous grasses, the single cotyledon is modiied into an absorptive organ surrounding
the embryonic axis. This structure is called the scutellum and its function is to absorb food from the endo-
sperm for the developing embryo.