Average less than ½" x 1/4" usually in clusters, often lower on the trees. Consist of many microsporophylls arranged spirally on the main axis of the cone. Each microsporophyll bears two microsporangia on the lower surface.
Microspore development resembles that of other plants we have studied. Sporogenous cells are surrounded by a protective tapetum. These sporogenous cells form microspore mother cells, or microsporocytes which meiotically form microspores. The microspore nucleus then undergoes several mitotic divisions eventually resulting in a pollen grain with 2 viable nuclei (generative cell and tube cell) and several vestigial prothallial cells - remnants of the vegetative portion. The pollen grain has two large wings which aid in its dispersal by wind.- Pollination
B. Ovulate cone.
This is the typical pine cone. It has many woody
scales arranged spirally around the cone axis. Young ovulate cones occur
in early spring. At this time, each ovuliferous scale bears two
ovules, each containing a single megasporangium. An integument, or protective
layer, is formed over the ovules, and a pore or micropyle remains
on the end of the ovule nearest the center of the cone. It is here that
pollen grains will enter. Each ovule has a single megaspore mother cell
in the center, surrounded by nourishing nucellus which is in reality
the megasporangium.
C. Female gametophyte
Meiosis of megaspore mother cell gives a row of
4
haploid nuclei. Usually 3
degenerate. The remaining one develops
into the female gametophyte. This germination and development of
the female gametophyte takes up to 13 months, and occurs within
the ovule. 11 mitotic divisions occur before any cell walls separate
the nuclei. This is the free nuclear stage. Cell walls will then
begin to form. At this time, 2 or more archegonia begin to differentiate
at the micropylar end. At this point the ovule consists of integuments,
nucellus and gametophyte. gam. contains several archegonia, each with an
egg cell. A micropylar chamber is beneath the micropyle.
D. Pollination
Consists of transfer of pollen grain from the
staminate cone to the ovulate cone, by means of wind. Cross pollination
aided by the fact that the staminate cones are below the ovulate cones,
thus the pollen has to travel some distance before rising to the height
of the ovulate cones. This occurs in early spring, shortly after emergence
of the ovulate cone.As pollen grains fall toward the axis of the ovulate
cone they come in contact with a sticky secretion from the ovule,
which aids in drawing the pollen into the
micropylar chamber, near
to the developing gametophyte. Now the male gametophyte begins to germinate
from the pollen grain, i.e. pollen tube. Thus both male & female
gametophytes develop within the ovule, and are nourished by nucellus.
E. Male gametophyte
The pollen grain is separated from the archegonia
by a layer of nucellus. It is through this nuclear material that the pollen
tube grows, digesting it as it grows. As it grows, the generative cell
divides, forming stalk cell & body cell, which then divides to form
2 sperm nuclei.
A mature male gametophyte then contains a pollen tube, 2 sperm nuclei, and several vegetative nuclei.
F. Fertilization
After about one year, the male & female gametophytes
are mature, and the pollen tube with sperm nuclei has reached the archegonia.
At this time, the contents of the male gametophyte are emptied directly
into the egg cell. One of the sperm nuclei fuses with the egg nucleus,
the remaining sperm nucleus and vegetative portions quickly disintegrate.
G. Embryo
The first step is the formation of a proembryo.
The zygote nucleus divides twice to give 4 free nuclei, which move to chalazal
end of the egg cell. 2 more mitoses result in four tiers of 4 cells. The
4 cells farthest from the micropyle will form the embryo, while the remaining
cells may form suspensor cells which push the embryo deep into the gametophyte
tissue toward chalazal end. gam. continues to grow and store food for the
developing embryo, and for food reserves in the seed. Several embryos may
form in a single seed, but usually only one develops.
The mature embryo consists of several cotyledons,
a shoot tip, a hypocotyl and a
radicle. The embryo
& gametophyte are surrounded by two layers - 1. dried nucellus or perisperm,
2. seed coat from ovule integument. The entire structure
is the seed. Seed maturity occurs about 12 months after fertilization.
Seeds
may be dormant for many years. Some remain in cones until burned by fire,
others shed seeds readily. Gen. L. H. of Pine.
H. Representatives
1. Pinaceae
a. Pinus - pine. Large trees, - pyramidal or flat-topped. Usually 2 or more needles sheathed at the base in a fascicle. Cones varied & characteristic of species.2. Taxaceae - yews
b. Abies - firs - pyramidal - tall and stately. Leaves flat & linear, X-S with no sharp angles. Appear to come from upper ½ of stem. Cones erect.
c. Picea - spruces - very similar to firs. Leaves with angular X-S. Leaves appear to come from all sides of stem. Cones pendant.
d. Tsuga. Hemlocks. Pyramidal with slender horizontal branches. Leaves 2 ranked ----0---- linear, flat, short. Cones small.
e. Pseudotsuga - Douglas fir. major timber tree. 200' X 12', leaves flat. Cones pendulous with bracts extending well beyond each scale.
f. Juniperus - trees & shrubs. Some with leaves spreading & needle-like. Some with scale-like leaves. Cone a fleshy "berry".
g. Larix - larches. Deciduous, found in bogs. Leaves short, linear; clustered on spurs. Cones small. h. Cupressus - Cypress
i. Thuja - Eastern White Cedar or Arbor Vitae
a. Taxus seed like a drupe or nut - often called fruit.
b. Sequoiadendron gigantea - California Big Tree (Sequoia)
c. Sequoia semperivens - Redwood.
d. Metasequoia - Dawn Redwood
e. Taxodium - Bald Cypress
To Top of Page
To Plant Diversity Page
To Tony Futcher's Home Page