Variation in the morphological features of tracheid tips was observed within and among sterns of larch (Larix leptolepis Gord.) with several different angular displacements from the vertical. The formation of compression wood increased with an increasing angle of stern deviation. Decrease in tracheid length occurred with thc development of compression wood within a growth ring. Compression wood also was accompanied by an increase in the occurrence of abnormal tips. Flattened and Lshaped tips of tracheids increased in number with the development of compression wood and the number of smoothly tapered tips decreased, indicating that the intrusive growth between adjacent cells does not proceed smoothIy in compression wood. It is considered that the spatial readjustment of developing cells is restricted in compression wood, and results from the great increase in the rate of cell division. A negative correlation between tracheid length and morphological abnormalities of tracheid tips was found within growth rings, apparently associated with the severity of compression wood development.
Compression wood in some softwoods having helical thickenings on the inner surface of normal wood tracheids were examined using a scanning electron microscope. Helical thickenings of Taxus, Torreya and Cephalotaxus have narrow bases, and are loosely attached to the innermost layer of the secondary wall, while those of Pseudotsuga, Picea and Larix have broad bases blended tightly with the microfibrils of the S3 layer in normal wood. The transition from normal to compression wood entails a preservation of the thickenings in Taxus, Torreya and Cephalotaxus, while they are replaced by helical ridges and cavities in Pseudotsuga, Picea and Larix. The direction of helical thickenings gradually changes from an S- to a Z-helix, or a Z- to an S-helix in the course of the transition from normal to compression wood, or vice versa in Taxus, Torreya and Cephalotaxus. Helical checks never occur in these species. In Pseudotsuga, however, helical thickenings can be deposited as an additional layer on the helical ridges. The results obtained in the present investigation revealed that the orientation of the thickenings did not always coincide with that of the innermost microfibrils of the secondary wall layers, indicating that helical thickenings may be considered as a layer independent of the secondary wall.
In the course of the righting movement in young spruce trees (Picea jezoensis Carr. var. hondoensis Rehd.) inclined at 45°, the occurrence of compression wood associated with the development of vascular cambium in the shoot was observed. In shoots, the recovery first took place at the mid point, a few days after inclination. The observations of serial cross sections taken from the apex downward revealed no appreciable difference in the development of the procambium-cambium continuum between the upper- and underside of the shoot. The formation and structure of primary tracheary elements were similar, irrespective of the site of the procambium in the shoot. No compression wood cells occurred before the vascular cambium cylinder was complete. The stimulus of compression wood formation is received only by the differentiating secondary xylem tissues derived from the cambium cylinder.
Septate wood fibers were abundant in the following parts of growth rings of Kalopanax pictus Nakai: 1) around the vessels, 2) in the vicinity of ray cells, 3) in terminal regions of the growth rings. Septum formation in wood fibers progressed from the initial region (pore zone) towards the terminal region within a current growth ring with progressing 1ignification of the wood fiber walls. Many septate wood fibers at the end of the growth ring had radially continuous septa. Karyokinesis was observed in severa1 wood fibers before the initiation of septum formation, while lignification was in progress after the completion of the S3 layer deposition. This suggests that the septation starts in parallel with the progress of lignification after the deposition of the S3 layer.
Differences in response among differentiating tracheids to the stimulus of stem inclination were examined at three heights in stems of young trees of Taxus cuspidata Sieb. et Zucc. A change in the orientation of the helical cell wall thickenings from an S- to a Z-helix with a simultaneous absence of an S3 layer were the first anatomical responses to appear. These changes first occurred in differentiating xylem on the underside of the upper segment of the stem after 4 days inclination. The gravistimulus for compression wood formation was transmitted basipetally within the stem as the tilting period was increased. After 10 days, the xylem on the underside of the upper segment of the stem contained five mature cells with a Z-helix, indicating that deposition of the thickenings required 2 days. The present results suggest that the rate of cambial division and xylem differentiation differs longitudinally in an inclined stem. For Taxus cuspidata, a change in the orientation of the helical thickenings should be a useful marker for deciding whether or not the differentiating cells have perceived the stimulus for compression wood formation.
The distribution of guaiacyl and syringyl lignins in the secondary xylem tissues of normal and compression wood of Buxus microphylla var. insularis Nakai was examined by visible light (VL) microspectrophotometry coupled with the Mäule and Wiesner colour reactions and by UV -microspectrophotometry, and compared with normal wood of Betula ermani Cham. Buxus formed compression wood on the lower side of the leaning sterns, and the secondary walls of the vessels and fibre-tracheids showed excessive lignification, resembling the S2 (L) layer of compression wood tracheids in gymnosperms.In normal wood of both species, the Mäule colour reaction indicated that in Betula the secondary walls of fibres contain larger amounts of syringyl units in the lignins than other tissues, and that in Buxus the secondary walls of fibre-tracheids contain both syringyl and guaiacyl units. The vessel walls of both speeies contained higher amounts of guaiacyl units. Heterogeneity of the syringyl-Jignin distribution was found in the secondary walls of Buxus fibre-tracheids.In compression wood of Buxus, on the other hand, the spectra of the secondary walls of the vessels and fibretracheids after the Mäule reaction showed low absorbances compared with the normal wood, whereas, after the Wiesner reaction, their secondary walls gave high absorbances. In addition, the UV -absorption maximum of the secondary fibre walls shifted from 274 nm to 279 nrn, and the UV -absorbances of the vessei and fibre-tracheid walls greatly increased in compression wood. The results obtained in the present study demonstrated that in normal Buxus wood the secondary walls of the vessels and fibre-tracheids contain both guaiacyl and syringyl units, though the syringyl unit is a rninor constituent in the vessel walls, and that both cell types increase their contents of guaiacyl units, especially in the outer parts of the secondary walls during their changes from normal wood to compression wood. The present study also suggested that the Wiesner reaction may be used for examining the content of lignin and the proportion of guaiacyl to syringyl units in lignins.
Anatomical characteristics and lignin distribution of ‘compression-wood-like reaction wood’ in Gardenia jasminoides Ellis were investigated. Two coppiced stems of a tree were artificially inclined to form reaction wood (RW). One stem of the same tree was fixed straight as a control, and referred to as normal wood (NW). Excessive positive values of surface-released strain were measured on the underside of RW stems. Anatomical characteristics of xylem formed on the underside of RW and in NW stems were also observed. The xylem formed on the underside exhibited a lack of S3 layer in the secondary fibre walls, an increase of pit aperture angle in the S2 layer, and an increase in lignin content. Some of the anatomical characteristics observed in the underside xylem resembled compression wood in gymnosperms. These results suggest that the increase of microfibril angle in the secondary wall and an increase in lignin content in angiosperms might be common phenomena resembling compression wood of gymnosperms.
To assess the characteristics of tension wood (TW) in Trochodendron aralioides Sieb. et Zucc., seedling stems were artificially inclined at angles of 30° (TW- 30), 50° (TW-50), and 70° (TW-70) from the vertical. At all angles, the growth promotion was pronounced on the upper side of the inclined stems, where excessive tensile growth stress was observed. A gelatinous layer (G-layer) formed in the tracheids of TW. The cell wall structure of the tracheids in TW was S1 + G. The G-layer had a small pit aperture angle <10°. TW-50 showed larger tensile growth stress, a thicker G-layer area, and a smaller pit aperture angle of the Glayer than TW-30 and TW-70. Lower levels of Klason lignin and hemicellulose and higher levels of α-cellulose content were observed in TW-50. In addition, an increase in glucose content and a decrease in xylose content in holocellulose were observed in TW-50. Therefore, it can be concluded that the degree of TW varied with different inclination angles.