Agriculture Reference
In-Depth Information
Figure. 2.8. Fertilization in Douglas ir (after Allen and Owens, 1972).
Embryo and Seed development
Following fertilization, the zygote develops into a proembryo consisting of several tiers of cells. During
this period, a suspensor tier elongates and pushes the embryo initials through the archegonium and into the
nutritive megagametophyte tissue (Fig. 2.9).
As the embryo continues its growth, the suspensor continues to push the embryo deeper into the mega-
gametophyte, whose cells break down, leaving a cavity that the embryo gradually ills. During this middle
phase of embryo development, rudimentary root and shoot meristems develop (Fig. 2.9).
In the inal stages of development, the embryo differentiates into an axis consisting of cotyledons and
root and shoot meristems (Fig. 2.9). The number of cotyledons varies among and within species, ranging
from one (
Ceratozamia
spp.) to twenty-four (
Pinus maximartinezii
). The embryo grows during this time,
illing the embryo cavity. The mature seed will be shed from the cone as it dries and the scales open, releas-
ing the seed. Wings attached to the seeds are formed from tissues of the ovuliferous scale. These wings aid
in wind and water dispersal as the seeds are shed.
