Tracheid cell walls

As a tree grows, the cells are produced in concentric lamella in the cambium layer, which is between the bark and the wood. In the spring, when moisture is plentiful and the tree is growing rapidly, the tracheid cell wall is thin (3—4 -lm) and the hoUow center or lumen is relatively large (26—43 p.m). 

Other distinct classes of wood in a tree include the portion formed in the first 10—12 years of a tree s growth, ie, juvenile wood, and the reaction wood formed when a tree s growth is distorted by external forces. Juvenile fibers from softwoods are slightly shorter and the cell walls thinner than mature wood fibers. Reaction wood is of two types because the two classes of trees react differentiy to externally applied stresses. Tension wood forms in hardwoods and compression wood forms in softwoods. Compression wood forms on the side of the tree subjected to compression, eg, the underside of a leaning tmnk or branch. Tension wood forms on the upper or tension side. Whereas in compression wood, the tracheid cell wall is thickened until the lumen essentially disappears, in tension wood, tme fiber lumens are filled with a gel layer of hemiceUulose. 

COMPOSITION OF SPRUCE TRACHEID CELL WALLS (Weight % of Individual Cell Walls)... 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

In softwoods, the main cell type is the tracheid, which is often mistakenly referred to as a fibre. Tracheids constitute over 90% of the volume of most softwoods, and are the principal paper-making cells of softwoods. Their average length is usually between 2 and 4 mm, with a lengthrwidth ratio (aspect ratio) often in excess of 100 to 1, but there is a wide distribution of tracheid lengths, and it is possible for some to be as short as 1 mm and for others to be as long as 5 mm (Table 2.1). The lumen, or central cavity, is several times wider than the cell wall thickness. There is also a difference between spring wood (i.e. cells synthesised in the early part of the annual... 

Figure 2.3 A schematic representation of the structure of the primary (P) and secondary (SI, S2 and S3) cell walls of a softwood tracheid (ML = middle lamella).

The outer secondary cell wall (SI) is comparable in thickness to the primary wall and consists of four to six lamellae which spiral in opposite directions around the longitudinal axis of the tracheid. The main bulk of the secondary wall is contained in the middle secondary cell wall (S2), and may be as little as 1 fim thick in early woods and up to 5 fim in summer wood. The microfibrils of this part of the wall spiral steeply about the axial direction at an angle of around 10 to 20°. The inner secondary wall (S3), sometimes also known as the tertiary wall, is not always well developed, and is of no great technological importance. 

Cell Wall Mechanics of Tracheids , R.E. Mark, Yale University Press, 1967. 

Larson (4,5) fed 14C02 photosynthetically to Pinus resinosa, divided the differentiating xylem into several fractions, and counted the radioactivity of each cell wall component. From these studies, it was concluded that as tracheid maturation occurred, xylose deposition increased, whereas mannose remained relatively constant, and both arabinose and galactose decreased considerably. 

In recent years, Hardell and Westermark (6) scratched Picea abies tracheids with tweezers, collected individual cell wall layers, and then analyzed the average monosaccharide composition. Surprisingly, among the individual cell wall layers no significant difference in the mannose xylose glucose ratio among individual cell wall layers was observed. 

During primary wall formation the plastids contain starch and other materials which stain heavily with uranyl acetate and lead citrate. When the tracheid starts to form the Si layer, the plastid becomes surrounded by an endoplasmic reticulum. While the fate of these compounds is unknown, it can be envisaged that they are used for generation of energy and/or a source of cell wall materials. 

---