Earlywood

Figure 3.4 Distribution of lignin in earlywood tracheids of black spruce.

Figure 13. Avicelase 1 + mannan-ase treatment. Degradation of S2 in an earlywood tracheid snowing lamellar aggregation pattern. Scale = 1 fim.

Figure 2. End view of a softwood showing growth rings. Each growth ring consists of a light and dark area. The light portion is called springwood or earlywood, and the dark area is termed summerwood or latewood. 15y ...

Alcohols also promote wettability and penetration of the wood surface. This may easily be shown by the following simple experiment. When equal sized drops of distilled water were placed on the surface of a freshly planed piece of southern yellow pine, the times for the drops to completely soak into the wood were observed. On the early wood it took 65 seconds and on the latewood 179 seconds. When similar drops of 50% ethanol solution were used instead of pure water, it required only six seconds to disappear into the earlywood and 26 seconds into the latewood. However, if a small drop of adhesive syrup, with no hardener added, was placed on the wood surface, no adsorption took place at all. It was surmised that the viscosity prevented its permeation. When the adhesive was diluted with 50% alcohol it was readily absorbed and produced a red stained spot on either earlywood or latewood areas. This showed that the low molecular weight adhesive molecules could readily permeate the wood structure before condensation with the curing agent. 

It is quite probable that the resin is insufficiently cured at the junction between latewood surfaces because the water has not been efficiently removed from the film. One observation which substantiates this is that when the specimens are heated, the sheer strength and wood failure values both increase, and the distinction between earlywood and latewood disappears. This definitely indicates that the problem is due to under cure. 

Fig. 1-2. Transverse section of xylem and phloem of red spruce (P/cea rubens). CZ, cambial zone DP, differentiating phloem MP, mature phloem with sieve cells (sc) and tannin cells (tc) DX, differentiating xylem with ray cells and tracheids (tr) MX, mature xylem, earlywood (EW) with resin canals (rc), lined with epithelial cells (ec) LW, latewood. Note that each ray continuous from the xylem, through the cambial zone, and into the phloem. Light micrograph by L. W. Rees. Courtesy of Dr. T. E. Timell.

At the beginning of the growth the tree requires an effective water transportation system. In softwoods thin-walled cells with large cavities are formed in hardwoods special vessels take care of the liquid transportation. Comparatively light-colored and porous earlywood is thus formed. Later, the rate of growth decreases and latewood is produced. It consists of thick-... 

Fig. 1-6. Cells of coniferous woods. An earlywood (a) and a latewood (b) pine tracheid, an earlywood spruce tracheid (c), ray tracheid of spruce (d) and of pine (e), ray parenchyma cell of spruce (f) and pine (g) (llvessalo-Pfaffli, 1967).

The average length of Scandinavian softwood tracheids (Norway spruce and Scots pine) is 2-4 mm and the width in the tangential direction is 0.02 -0.04 mm (Fig 1 -8). The thickness of earlywood and latewood tracheids is 2-4 /u,m and 4-8 /xm, respectively. 

Fig. 1-13. Transverse section of earlywood tracheids in tamarack (Larix laricina), showing the middle lamella (M), the primary wall (P), and the outer (S,), middle (S.), and inner (S i) layers of the secondary wall. Transmission electron micrograph. Courtesy of Dr. T. E. Timell.

The middle layer (S2) forms the main portion of the cell wall. Its thickness in softwood tracheids varies between 1 (earlywood) and 5 (latewood) jiim and it may thus contain 30-40 lamellae or more than 150 lamellae. The thickness naturally varies with the cell types. The microfibrillar angle (Fig. 1 -16) varies between 10° (earlywood) and 20-30° (latewood). It decreases in a regular fashion with increasing fiber length. The characteristics of the S2 layer (thickness, microfibrillar angle, etc.) have a decisive influence on the fiber stiffness as well as on other papermaking properties. 

Compression wood is heavier, harder, and denser than the normal wood. Its tracheids are short and thick-walled (even in earlywood) and in cross section rounded so that empty spaces remain between the cells. The S, layer is thicker than in a normal wood while the S J layer is absent. The layer contains helical cavities that parallel the microfibrils and reach from the... 

To calculate lignin concentration from refractive index of the specimen, calibration values are required. For measurements in the secondary wall, value Na in Eq. (3) is the refractive index of the S2 region in a matched specimen of holocellulose. This value varies from specimen to specimen and must be measured each time a new specimen is examined (Donaldson 1985a). The only exception is for earlywood and latewood from the same growth ring. For measurements in the middle lamella, Na has a constant value of 1.516 which... 

The accuracy of interference microscopy depends on the calibration standards used. For measurements in the secondary wall, these values are 1.604 for Nb and a range of values for Na depending on the specimen. The variation in Na arises presumably from variation in microfibril angle (Hermans 1946). The need to measure Na for each sample is one disadvantage of this technique that increases the amount of work necessary to perform the measurement. If Na is not measured for each sample, an error of 11% is introduced compared to the usual error of 2% (Donaldson 1985a). Na does not vary between earlywood... 

Table 4.4.1 gives the distribution of lignin in Douglas-fir (Pseudotsuga menziesii) earlywood tracheids (Saka and Thomas 1982). The overall lignin content (W) of the earlywood portion is 0.285 gig). The Klason and acid-soluble lignin determinations should be made on wood taken near the location from which the sections are collected for EDXA measurements. 

Table 4.4.1. The distribution of lignin in Douglas-fir earlywood tracheids as determined by bromination with SEM-EDXA. (Saka and Thomas 1982)...

Fig. 4.3.2A-C. A series of interference micrographs showing the maximum extinction positions of (A) the background, (B) the cell corner middle lamella of Pinus radiata earlywood, and (C) the S2 region of the secondary wall (interference photomicrograph)...

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