The most widespread of the gymnosperms
are the conifers: 50 genera and over 550 species. They can found
world-wide under varying conditions. In the Americas they dominate
the forests of the US West and Canada, then drop down the eastern
coast dominating in the early successional stages; they continue
and dominate the coastal and dry sandy forests of the SE.
Typically they are found under
what we would consider the less productive conditions of either
long cold seasons, dry areas or nutrient deficient soils ( sandy,
less mature soils of the west or acidic swampy mucks).
Their history extends back to the
late Carboniferous ( 300 MYA) where their drought resistant features
might have been critical during their evolution during the Permian
when the earth turned cold and dry.
& wood structure:
Although pine trees come to mind for
most, when referring to gymnosperms, not all gymnosperms have needles
as we'll see later with the ginkgo and gnetums.
Those that do have needles hold onto
them year round, and can do so because the trees have tracheids,
(as opposed to the vessels of angiosperms),
- long, slender cells with tapered, overlapping
- water moves upward from tracheid to
tracheid through pit pairs, thus preventing large gas bubbles from forming
and thus no caviatation during freezing/defrosting periods. Water can
move through the plant with little seasonl disrtuption.
Note in the images below: on right..the
wood cut is parallel to the tracheids.. the "dots" are the contigous
pores. On the left is a cross-section cutting throught the tracheids -
to the left of the image are the thiner walled cells that form in the
spring and to the right, the heavier walled cells layed in the summer
when less water is available. The big 'hole' at about 2 o'clock is a resin
wood is generally softer than angiosperms because it has
less lignification . In the summer, angiosperms wood has more
cell walls that also contribute to its ability to withstand more
of "a beating".
is the antimicrobial material that seals wounds from insects and protects
the tree from being eaten due to its foul taste. It is also flammable
and thus it turns over the nutrients faster and clears the underbrush
so that it makes sure to have enough water available for survival by
eliminating the competition. The gymnosperms do not burn due to their
often 1 ft thick cork that is fire resistant and helps to insulate the
phloem against freezing in the winter. Resin is made of turpentine +
Majority of conifers retain their needles
for anywhere from 2-5 years, with a total dropping of needles every 5
years. The bristlecone variety of gymnosperms however is an exception
to this rule as it can hold its needles for up to 40 years ( ?) ( anwer
they live under such lousy conditions.. dry and low nutrient they don't
have the reserves to build up a new stock every few years..
- thick cuticle covers the epidermis
beneath which 1+ layers of hypodermis - thick walled cells.
- The stomata are sunken
- The mesophyll cells have projections
to increase surface area and generally contain 2+ resin canals
- 1-2 vascular bundles are found in
the center surrounded by transfusion tissue to conduct materials between
vascular and mesophyll cells
- there are dead tracheids in the middle
to support the structure
- the endodermis surrounds the vascular
tissue preventing water loss.