The skeletal remains of a plant may look quite different than it did as a living entity. For trees, the skeletal remains look, for many years, almost exactly like the living plant. Truly the only telltale sign that the plant is dead is the lack of leaves. Conversely, the skeletal remains of a succulent herba­ceous plant are almost nonexistent. It "melts away," leaving virtually no trace of its previous existence.

The difference between these two examples is the tissue construction of the living plant. Although there are a variety of tissue structures in each plant, the basic cells in succulents are thin walled and filled with a dilute solution of carbohydrates and other organic molecules. The cellulose present in the cell walls outside those cells is bathed in aqueous solution. At the first frost, the water freezes and enlarges the cell past its burst point, which then thaws to allow the water to drain out and leave no remaining structure. The plant "melts" to the ground and disappears rapidly.

The cells of woody plants have thick walls and encase a stronger, thicker solution of carbohydrate and cellulose. The freezing point of this solution is much lower than that of water, and if the cells do freeze, the cell walls are able to withstand the hydrostatic force, and therefore remain intact (6).

To be more specific, the wall of a plant cell consists of three layers: primary wall, middle lamella, and secondary wall. The primary wall is thin and the middle lamella is more an adhesive gel than an actual structure. It is the secondary wall that is thick and gives the rigid structure to the cell. In a succulent, the secondary cell wall does not develop and the cell wall has little mass or strength. In cells of a woody plant, not only is the secondary wall thick but often it contains the organic compound lignin. Lignin is a material that is high in carbon, very hard, and very strong. Succulent plants contain virtually no lignin and therefore have a fragile structure and leave almost no skeletal remains. Nut shells are high in lignin and take on an almost rocklike hardness, and their skeletal remains look identical to those of their live counterpart. Lignin is the second most abundant organic com­pound on earth, second only to cellulose. Woody plant species typically contain 15-25% lignin. Lignin has a higher carbon concentration than cel­lulose and therefore has a high heating value.

In herbaceous plants that have persistent lignified stalks ("sclerified stalks") (stems), the variety of tissues in the stem may have characteristics of succulents and some may have characteristics of wood. The result is that the skeletal remains may have some components that remain intact after death and others that essentially disappear.

To describe what the stem of a skeletal herbaceous stalked plant looks like, we begin with the stem parts of a living plant. The parts of a vascular plant stem, be it herbaceous or woody, are basically the same. The stem consists of epidermis (for outer protection), phloem (for transport of food to the plant), xylem (for transport of water to the upper reaches of the plant), and pith (a center core for the incubation of new cells). Of these four parts, the bulk of the mass of the stem cross section consists of xylem and pith. The pith is usually a very light, spongy, almost styrofoam-like mate­rial. In wood, pith is almost nonexistent except in new bud stems. In stalked herbaceous plants, the pith is a significant part of the stem.

It is in the xylem that the mass of the stem resides. Both in woods and in stalked herbaceous plants, the burnable portion of the stem is primarily xylem. If the overall purpose of an energy crop is to fix carbon from the CO2 of the air into a solid form that can be burned for the release of heat, then an energy crop must be one that accomplishes this fixation the best. The fixing of carbon is done through the photosynthesis process while the stor­ing of that carbon is accomplished by the creation of cells and cell walls, which comprise tissue. The photosynthesis process is powered by sunlight; therefore, biofuel energy sources are really one type of solar energy. A good energy crop must not just "fix" carbon but store it as burnable mass. Con­sidering the other demands for the energy the plant may have, such as making leaves, and making fruit or seeds, an energy crop is a plant that efficiently makes harvestable biomass.

To best describe the ability of a plant to create xylem, there are some technical indicators that can be used. Figure 1 illustrates that an SSP skel­etal stem is basically tubular. The "tube" itself is primarily xylem because the epidermal phloem and pith cell walls are usually very thin. For energy crops, it is best to have this tube as large and substantial as possible. In monocot plants and some dicots, the xylem and phloem are not created in tubes but are "bundled" together as highly efficient string elements that are located in the pith. There is not much mass to these bundles and, therefore, not much mass to their skeletal remains. SSPs typically are the dicots that do not form xylem bundles but, rather, xylem rings.

Critical parameters concerning the skeletal stem include the stem diameter (D); the ratio of the stem diameter to the pith core diameter (D/P); the xylem thickness/pith diameter ratio (X/P); and the xylem-to — pith ratio, defined as XPR. The reason for both X/P and XPR is that X/P

ratios the single wall thickness against the total pith diameter, while XPR ratios twice the xylem wall thickness against the pith diameter (or the wall thickness against the pith radius). Another important physical prop­erty of the stem is the density of the dried xylem tissue (kg/m3).