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.

Leaf & 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 ends;
• 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 canal.

• Gymnosperm 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".
• Resin 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 + wax (rosin).

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..

Needles

• 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.