Agriculture Reference
In-Depth Information
internal structure of the onion seed is illustrated in Fig. 2.19. The embryo is
curled within the seed and consists of a short root below the shoot apex, which
is located along with the primordium of the first leaf at the base of a slit at the
lower end of the cotyledon. Following germination, the first leaf will emerge
from this slit. The cotyledon forms the bulk of the embryo and consists mostly of
small cells densely packed with reserves of globular fat, protein and sugar
phosphate (phytin) (de Mason, 1990). The beginning of conducting tissue is
seen as a central procambial strand. At the tip of the cotyledon, embedding into
the surrounding endosperm, there is a swelling termed the haustorium. During
germination this absorbs nutrients from the endosperm reserves and transfers
them to the growing cotyledon. The cells of the thick-walled endosperm are also
packed with reserve globules of protein and lipids.
During germination the lower portion of the cotyledon elongates first and
the root emerges. Cell divisions occur throughout the cotyledon as it initially
elongates. The primary root grows downwards and the slit containing the
Fig. 2.19.
Longitudinal section through a mature onion seed with embryo. The
embryo consists of a root, a hypocotyl with shoot apex and a long cotyledon. The
procambium extends the entire length of the embryo. The cotyledon ends in a
haustorium, which absorbs nutrients from a thick-walled endosperm during
germination. A small amount of nucellus lies under the seedcoat at one end (from
Esau, 1977. Courtesy of John Wiley and Sons Inc., New York).
