The Formation of Interlocked Grain in African Mahogany (Khaya spp.) Analyzed by X-Ray Computed Microtomography

Interlocked grain occurs when the orientation of xylem fibers oscillates, alternating between left- and right-handed spirals in successive wood layers. The cellular mechanisms giving rise to interlocked grain, thought to involve the slow rotation of fusiform initials within the vascular cambium, remain unclear. We suggest that observations of wood structure at the cellular level, but over large areas, might reveal these mechanisms. We assayed timber from several commercially-important tropical angiosperms from the genus Khaya (African mahogany) that exhibit interlocked grain using X-ray computed microtomography followed by orthogonal slicing and image processing in ImageJ. Reconstructed tangential longitudinal sections were processed with the ImageJ DIRECTIONALITY plug-in to directly measure fiber orientation, and showed grain deviations of more than 10o from vertical in both left- and right-handed directions. Grain changed at locally constant rates, separated by locations where the direction of grain change sharply reversed. Image thresholding and segmentation conducted on reconstructed cross sections allowed the identification of vessels and measurement of their location, with vessel orientations then calculated in Matlab and, independently, in recalculated tangential longitudinal sections with the DIRECTIONALITY plug-in. Vessel orientations varied more than fiber orientations, and on average deviated further from vertical than fibers at the locations where the direction of grain change reversed. Moreover, the reversal location for vessels was shifted ~ 400 μm towards the pith compared to the fibers, despite both cell types arising from the same fusiform initials within the vascular cambium. We propose a simple model to explain these distinct grain patterns. Were an auxin signal to control both the reorientation of cambial initials, as well as coordinating the end-on-end differentiation and linkage of xylem vessel elements, then it would be possible for fibers and vessels to run at subtly different angles, and to show different grain reversal locations.

The Impact of Changing Climate on the Cambial Activity during Radial Growth in Some Citrus Species

This study on radial growth in the stem of Citrus was carried out with an aim to notice the behavior of vascular cambium with respect to climatic and age effects. The fusiform initials vary in length from 137 to 363 μm in C. limon, 100 to 463 μm in C. paradisi, 137 to 413 μm in C. reticulata var. kinnow, and 137 to 375 μm in C. sinensis. The length rises with age, followed by decline and then again increase in C. limon. In C. paradisi, there is increase up to maximum and after decline is soon followed by constancy. In C. reticulata var. kinnow, increase in length from top to base in C. sinensis, increase up to maximum followed by a decline. Swelling of cambial cells occurs in the third week of March in C. limon, last week of March in C. paradisi, third week of April in C. reticulata var. kinnow, and second week of April in C. sinensis. The cambium turns dormant in early October in C. limon, late December in C. paradisi, early December in C. reticulata var. kinnow, and early November in C. sinensis. Thus, the cambium remains active for about 6 months in C. limon and C. sinensis, 9 months in C. paradisi, and 7 months in C. reticulata var. kinnow.

Axial variation in the cambium anatomy of Schizolobium parahyba var. amazonicum

Schizolobium parahyba var. amazonicum (paricá) is a promising forest species that has been planted in some states of the Amazon region in Brazil, to meet the demand of the plywood panel industry. The present work involves a study of the variations of the cambium and their impact on derivative tissues at different heights in the stem of S. parahyba var. amazonicum. Except for the tangential diameter of the fusiform initials (DFI) and the width of the xylem cell layer in differentiation (WXD), there was significant statistical variation between the evaluated axial positions for all anatomical parameters of the cambium. A strong positive correlation was noticed between the length of the fusiform initials (LFI) with ray height (RH) [r = 0.79, degree of freedom (DF) = 7, P < 0.05], vessel element length (VL) (r = 0.78, DF = 7, P < 0.05) and fiber length (FL) (r = 0.74, DF = 7, P < 0.05). The results of this study give quantitative support that the LFI is an important prognosis, not only for the VL and FL, but also for the rays, in hardwood species.

Unidirectional splitting and uniting of rays in the cambium of Platanus accompanying the formation of interlocked grain in wood

Developmental changes in the cambium producing interlocked grain wood of <i>Platanus</i>, were determined by an analysis of successive late xylem layers (TLX) at the borders of the annual rings. Intensive splitting and uniting of rays were observed. These occured by intrusion of fusiform cells into the rays and by loss of the fusiform initials which formed the strands separating the rays, respectively. Uniting of rays was unidirectional within the large areas of cambium (domain), so was the splitting, too. There were domains of Z and S-type. Orientation of splitting and uniting of rays at a particular locality of the cambium was reversed at irregular time intervals. It appears that the reason for these reversals was slow movement of domain pattern. The occurrence of alternating Z and S domains brought about the alternation of the grain inclination from right to left and back again in the interlocked-grained wood.

Growth activity of fusiform initials in storeyed cambium

The intrusive growth of fusiform initials was studied in <i>Entandrophragma</i>, This growth makes possible a change in the position of the cell ends of one storey in respect to those in the neighbouring storey, and this is 'turn leads to changes in the orientation of cambial initials. The growth activities of the oppositely directed ends of the same initial are not the same. Not all cell ends are simultaneously actiye. There are groups of ends distributed alternately in a storey which are active or inactive, so that a pattern of growth activity appears in the cambium. The activity of a particular cell end changes in successive time periods. Owing to this, the rate of creeping of one end past. those of the adjoining storey varies. This phenomenon of changes in the activity of cell ends in time may be referred to the phenomenon of the appearance of the growth activity pattern on the cambium surface under the hypothesis of transverse shifting of the elements of this pattern ("active" and. "inactive" groups) in relation to the cambial initials.

Seasonal activity of cambium in some tropical trees. 111. Salvadora persica L.

Seasonal activity of cambium has been investigated in <i>Salvadora persica</i> L., one of the commonest evergrenn tree of northern India, from March, 1974 to February, 1975 in the main stem and the branch. Secondary vascular cambium behaves abnormally on the xylem side, by producing xylem and phloem in succession, resulting in the formation of interxylary phloem. It has a storied (sitratified) cambium organization, comprising short fusiform and isodiametric ray initials. The fusiform initials show characteristically beaded cell walls due to the presence of primary pit fields and are uninucleate. These are highly vacuolate during the active period. The cambial activity starts in the month of March, reaches the peak in August and finally slows down in the main stem. In an young branch, however, it is initiated in March-April and reaches the peak in July and then declines till November. It once again showed an upward trend gradually. It appears that the initiation of the combial activity in this tree takes place by a hormone produced in the newly formed leaves, coupled with the high rainfall, enhanced relative humidity, optimum temperature, and short-day conditions. Size measurements for the fusiform initials showed considerable variations during the span of a year. A comparision of the organization of three types of phloem 'normal', 'included' and 'internal' occurring in this plant has also bean presented.

Size correlations among cambial initials and their derivatives in Polyalthia longifolia Thw.

The length and breadth of the cambial initials and their derivatives have been examined in <i>Polyalthia longifolia</i>, a tropical tree possessing non-storied cambium. Taking the average size of the initials and the elements originating from them, most of the sieve-tube elements have been found to be slightly shorter in length than the fusiform initials. On the other hand, a few of these are still shorter - almost half of the fusiform initials, due to transverse or somewhat oblique divisions in the sieve element mother cells. The vessel elements are slightly shorter but 5-6 times wider than the fusiform initials. The parenchyma strands, in phloem comprising cells storing starch or tannin (pps), in xylem accumulating starch only (ssps), are more or less equal to fusiform initials indicating that the xylem and phloem mother cells forming parenchyma cells have not undergone any major change except for transverse divisions. The individual vessel-associated parenchyma cells (v.a.p. cells) are wider but much shorter in length as compared to the starch-storing parenchyma cells (s.s.p. cells) indicating that more transverse divisions have occured in the strands of the former than those of the latter. Among all the cambial derivatives, the fibers exhibit maximum increase in length, due to intrusive growth. The ray parenchyma cells are slightly longer than the ray initials possibly due to the elongation of these initials during their transformation into vascular ray cells.