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The Life
Cycle of Pinus
(Campbell 6th Ed. 605 Fig. 30.9; 7th Ed. 597 Fig. 30.6)
Jack pine (Pinus banksiana) is the most common and widespread pine
found in Manitoba. The sporophyte
generation consists of roots, stems and needle leaves. The needle leaves
are green and photosynthetic and occur in clusters of two on short shoots arising
from a branch. The reproductive structures, seeds (fertilized ovules) and microsporangia, are produced in cones (strobili). The female
or ovulate
(seed) cone consists of a spiral series of overlapping ovuliferous
scales each producing two ovules
on the upper surface. Each scale is subtended by a small fused structure on
its lower surface called a bract.
In jack pine, the female cone remains closed until heat, such as from a forest
fire, causes the ovuliferous scales to separate. The bracts in pine are microscopic
and difficult to see.
Examine a Jack Pine branch with male and female cones
Examine
the slide of a young first year ovulate cone of Pinus cut in longitudinal
section
Note the ovuliferous scales and subtending bracts in a variety of planes of
section. Also note the integument,
micropyle
and megasporangium of the ovule.
Examine
the megasporangium of a young first year ovulate cone of Pinus cut
in longitudinal section
Note the megasporocyte (megaspore mother cell), surrounded
by small dense staining "nurse" cells.
The megasporocyte divides by meiosis to produce the four haploid megaspores, three of which degenerate. The nucleus of the surviving megaspore divides repeatedly by mitosis and a multicellular megagametophyte composed of thousands of haploid cells is produced within the enlarged megaspore. Within the megagametophyte archegonia develop, which contain a large central egg cell.
Examine the slide of a mature pine ovule cut in longitudinal section
Examine
the slide of a mature pine ovule cut in longitudinal section under high power
Note the archegonia and nutritive tissue of the
megagametophyte, the megasporangium and the integument. You
should also notice the cells in the mid-region of the integument that have developed
thick cell walls. This represents an early stage in the development of the protective
seed coat.
The male (staminate) cones are small (1-2 cm) and borne on the same tree as the ovulate cones. The staminate cone consists of a spiral series of microsporophylls (male fertile leaves) that bear two microsporangia on their lower surface. Within a young microsporangium there are thousands of microsporocytes (pollen mother cells). Each divides by meiosis to form four microspores that are arranged in a spherical tetrad. These four will give rise to the microgametophytes (pollen grains).
Examine a branch with staminate cones:
I - Prior to pollen release (mid-spring)
II - Following pollen release (late summer)
Examine a longitudinal section of a mature staminate cone of Pinus.
Note the microsporophylls and microsporangia.
Observe
the winged pollen grains within a microsporangium (10x)
Observe
the winged pollen grains within a microsporangium (40x)
At the end of this period of development the microsporangia split open, releasing the pollen grain. Concurrent with pollen release, the scales of the young ovulate cones separate slightly. Some of the wind dispersed pollen grains drift down between the scales and adhere to a small drop of sticky fluid secreted through the micropyle of the ovule. The pollen grains are drawn down the micropyle and contact the tip of the megasporangium. A pollen tube arises and begins to digest its way through the tissue of the megasporangium. Growth of the pollen tube is slow in Pinus. The time interval between pollen germination and fertilization is one year. During this period of pollen tube growth, the megagametophyte has developed to the mature archegonium stage.
The cell of the microgametophyte divides by mitosis to form a sterile cell and a spermatogenous cell. The latter cell divides again to form two non-flagellated sperm cells. The pollen tube continues to grow through the megasporangium tissue and eventually enters an archegonium of the megagametophyte. The tip of the pollen tube bursts, discharging the two sperm cells into the cytoplasm of the large egg cell. One sperm cell fuses with the egg cell nucleus (syngamy) to form the zygote. The second sperm cell and the other cells of the microgametophyte (tube nucleus and sterile cell) degenerate.
Once fertilization has taken place the mature ovule is henceforth referred to as a seed. The zygote divides by mitosis to form the embryo protected within the seed during its development. The early stages of embryo development in Pinus are complex and will not be discussed here. A mature seed of Pinus consists of an embryo (the new sporophyte plant), a source of stored food (the nutritive tissue of the megagametophyte) and a protective seed coat. The mature embryo consists of two growing points, the shoot and root apical meristems, a root cap and 7-8 cotyledons.
Examine a slide of a mature seed of Pinus and note the above mentioned features
In Pinus some of the surface tissue of the ovuliferous scale remains attached to the seed, forming a membranous wing. These ovuliferous scales separate and the winged seeds fall from the cone. Wind movements disseminate the seeds a considerable distance from the parent plant. The embryo, enclosed within the dessication resistant seed coat, can remain dormant through extended periods of unfavourable environmental conditions. With seed germination, the embryo begins development into the new sporophyte generation.
