Erica's Original Notes A stem, or trunk, is basically a reinforced column with three main uses: a) to support the leaves enabling them to make food, and to display the flowers, so that they are accessible to pollinators (insects, birds, bats and wind); b) to act as a connecting link between the roots, flowers and leaves; and c) to serve as a storage unit.
Stems have a double plumbing system similar to our bodies; the xylem carries water and minerals from the tips of the roots up to the leaves, flowers and fruit, allowing for diffusion to the cells en route, while the phloem, or digestive system, transports sugar as syrup from the chlorophyll leaf factories to the cells of the tree for new growth and nourishment.
Xylem: This amazing solar-run water supply system is activated by evaporation - or transpiration - from the leaves, starting a chain reaction of molecular suction which baffled man for ages as to just how this continuous column of water worked without pumps; 125 feet per hour in the American Elm, for example. This layer of cells is transformed annually into heartwood, consisting of two bands, the light or spring wood and the darker summer wood making up the annual rings we see in the stump of a tree. These rings become clogged with gums and resin, giving stability to the tree. (See notes 1 and 2 below.)
Phloem: This system is so delicate and surrounded by so much intercellular liquid that it is difficult to get pure samples of it, and researchers had to resort to aphids to 'tap in' to the system! Unlike the xylem, these cells, formed annually, are squashed into oblivion.
Cambium: (In woody plants only) Lies between the xylem and phloem, and is responsible for the annual renewing of these two systems; it is known as 'the Membrane of Life' and is of very fine texture.
Sap: thickens in Winter to protect against the formation of ice crystals. It contains an anti-fungal antibiotic; in Spring the food that has been stockpiled in the roots as starch is reconverted to sugar, in order to start up the leaf factories again.
Bark: (In woody plants only) Consists of two parts; the inner is the phloem, and the outer is called the cork layer, dry and dead, which acts as a protective coat against insects and diseases, and, in some cases, like Douglas Fir, against fire. It may contain useful substances such as quinine in cinchona or tannin as in oak, which help resist disease as well as being used as a dye. The Giant Redwood bark is very thick and so full of air that one can punch it with impunity.
Resin: (In woody plants only) A reserve of carbohydrate and an antifreeze, it runs in special channels, helps plug wounds of tree and is unpalatable to animals.
Lenticels: (In woody plants only) The small horizontal slits, or breathing holes in the bark. Barks may have other marks, such as splits, which are due to the ever-increasing heartwood which puts internal pressure on the outer coat.
NOTES Updated 2013 (RP):
Erica’s notes with regard to the “plumbing system” refer more to the stems of flowering plants.
Primitive plants, sometimes referred to as ‘Lower Plants’ in plant classification are indeed lower! Mosses and liverworts hug the ground, live in moist situations and are in close contact therefore with water. They do not have a specialized system for transporting fluids in the plant. For plants to become taller it is necessary to transport water from the soil to the leaves (and food in the opposite direction), and these plants have developed a vascular system and true stems.
Ferns are among the oldest plants historically with a vascular system and this allowed them to grow a certain height above the ground. Before plants classified as ‘Higher Plants’ existed – the gymnosperms (conifers, cycads, gingkoes) and angiosperms (flowering plants) – ferns and tree ferns were among the tallest plants.
In gymnosperms the water flows through xylem elements called tracheids, where the cross cell walls have not completely broken down and water transport is not as efficient. In angiosperms the cross walls have completely disappeared, and you will see this type of xylem element referred to as vessels. The vascular system of ferns shows various transitions, depending on species, of tracheids and vessels.
A problem with aging trees by annual rings
The alternation of light and dark rings occurs because in springtime water is plentiful, the tree is actively growing and produces large xylem cells. In later summer and into winter (when water may be locked in the form of ice), water is less plentiful, growth slows due to drought stress and other seasonal factors, and the tree produces smaller cells. Because the cells are smaller, there is proportionally more cell wall material, and this causes the ring of cells to appear darker.
However, if before winter, there is a period of renewed rainfall and other conditions are good for growth, the tree may start to produce big cells again, and then small cells a bit later on as drought stress resumes. The effect is to produce a second ring, commonly called a false ring. An example of this is the Caribbean pine (Pinus caribaea) found growing in the Dominican Republic which puts on a ring every time there is a wet spell, and may make as many as four to five rings a year. Counting rings on trees which have false rings can lead to overestimation of their age.