AN OCCURRENCE OF PERFORATED TRACHEIDS IN THUJA OCCIDENTALIS L.

Size: px

Start display at page:

Download "AN OCCURRENCE OF PERFORATED TRACHEIDS IN THUJA OCCIDENTALIS L."

James Blake
5 years ago
Views:

1 AN OCCURRENCE OF PERFORATED TRACHEIDS IN THUJA OCCIDENTALIS L. BY M. W. B ANN AN Department of Botany, University of Toronto {Received 28 February 1957) (With Plate and i figure in the text) In a recent communication (June, 1956) to the New Phytologist a report was given on the occurrence of perforated tracheids in the secondary xylem of a conifer, Sequoia sempervirens. While no phylogenetic significance was attributed to this condition, it was recorded because of its uniqueness. The perforated tracheids were observed in a radial section, the condition extending through parts of two successive growth rings. The origin of the wood was unknown and no more material was available. The author has for some years been carrying on investigations of cambial activity in conifers, particularly as related to the multiplicative divisions of fusiform initials. In these studies many thousands of serial sections have been prepared, mostly of the secondary xylem, and amongst these one occurrence of perforated tracheids was discovered. It was found in the peripheral growth in the distal part of a large, lower branch from a mature tree of Thuja occidentalis. The annual increments were very narrow and the perforated condition persisted through six rings with a total width of i.0 mm. Since more is known on the general circumstances under which the perforations developed than in the case reported for Sequoia, the information is here provided. The sequence of events in the formation, enlargement and disappearance of the perforated condition is shown in Fig. i which presents a series of diagrams made from serial tangential sections of the secondary xylem. Fig. la depicts the antecedent condition, namely two contiguous cells of normal aspect and of similar size. In the next tangential section. Fig. ib, a hole appeared in the wall, slightly above the centre of either cell. Vertical extent of the gap on its appearance was 26 ^. The aspect at this time is illustrated in Plate i«. A gradual increase in size of the perforation occurred through the successive tracheids of the growth of the next two years, when in the late wood of^ the second year semitransverse walls appeared in both tracheids (Fig. if). At this point each of the two initiating cambial cells evidently underwent a pseudotransverse division. The products of the anticlinal division of cell i were slightly unequal, the lower daughter initial (ib) being shorter and bereft of ray contacts. As so often happens under such circumstances (Bannan and Bayly, 1956), the smaller and poorly positioned cambial initial failed. The radial file of elements terminated in a narrow, shortened tracheid (Fig. \d), the sequence indicating a drastic size reduction in the final periclinal divisions of the failing initial immediately prior to loss of activity and passage into maturation. Enlargement of the perforation continued through the growth of another four years to a maximum vertical extent of 180 Ji (P'igs. \d-f, Pl. \b, c). Then, in the late wood of the ring laid down five years after the first appearance of the perforation, the gap was closed over (Fig. i_^, Pl. \d). The partition was thinner than the walls on opposite sides of the cells, but the presence of bordered pits demonstrated the existence of secondary layers 132

2 Perforated tracheids in Thuj a 133 (PI. i^). The closing of the perforation was evidently associated with internal changes in one of the two fusiform initials involved, a physiological alteration related to the transformation from fusiform initial to ray initial. In the conifers most of the initials of new rays arise by segmentation and reduction of fusiform initials (Bannan, 1953), and here another example is provided of this metamorphosis. In the tangential section following the closure, the xylem cell on one side is of reduced dimensions (cell 2A, Fig. lh), and two 30 Ai 60 JJ 60U 390^ 60;i 30JL1 SO(J -2A IB- -2B Fig. I. Diagrams from serial tangential sections of the secondary xylem showing stages in appearance, growth and closure of perforation in a succession of tracheids from the same cambial initials. The radial distance between the stages selected for illustration is given in microns. sections later the file of fusiform elements has come to an end (Fig. li). Late in the growth of the following year a new ray one cell high appeared in line with the upper half of the former file of fusiform elements. Clearly the fusiform initial in question had undergone a transverse division, with the upper half becoming reduced to a ray initial while the lower half ceased activity and passed off into maturation.

3 134 ^- ^- BANNAN The closing of the perforation, and the fact that this event was obviously associated with internal changes in one of the two contiguous fusiform initials, may be taken as evidence that the fusiform initials had retained their identity throughout the six-year duration of the perforated condition in the xylem. In other words, the primary walls and lining laters of cytoplasm probably remained intact in the lineal succession of cells which functioned as fusiform initials. The development of the perforation at the same locus in the derived xylem elements, however, would seem to indicate that there was a local deficiency in the wall-synthesizing mechanism in the fusiform initials which was transmitted to their xylem derivatives. The actual dissolution of the wall apparently occurred at some time during the radial expansion or maturation of the derived xylem elements. In this connection the structure of the secondary walls bordering on the perforation is of interest. In some cases the secondary walls were cut off abruptly at the gap margin, even through the middle of a bordered pit. At other times the wall forming the rim was rounded (PI. le), indicating that in at least some cases the dissolution of the primary walls in the perforate area occurred before deposition of the surrounding secondary walls. The occurrence of perforate tracheids in Sequoia and Thuja cannot be regarded as having phylogenetic significance. There is no homology with vessel formation in other vascular plants. Rather it would seem that one is dealing here with a physiological aberration, a local deficiency in the synthesizing mechanism concerned with surface growth of the wall. The converse situation, namely an excess of surface growth, is more common. It is revealed in such anomalies as wall buckling, folding and invagination. REFERENCES B.'VNNAN, M. W. (1953). Further observations on the reduction of fusiforni cambial cells in Thuja occidentalis L. Canad. Jour. Bot., 31, BANNAN, M. W. & BAYLY, ISABEL L. (1956). Cell size and survival in conifer cambium. Canad. Jour. Bot., 34, JANE, F. W. (1956). Perforated vertical tracheids in Sequoia sempervirens Endl. Nezo PhytoL, 55, LEGEND TO PLATE Plate I. Photographs showing appearance, enlargment and closure of perforation in tracheids. a, first appearance; b, intermediate stage in expansion; c, maximum size; d, closure; e, aspect of wall at rim. Magnification a-dy.2$o, e ^700.

4 THE NEW PHYTOLOGIST, 57, i PLATE I N PERFORATED TRACHEIDS IN THUJ.A. {Facing p. 134)

Lecture 19. A Sieve Plate with large Sieve Pores. Secondary Phloem. Secondary phloem (cont d)

Lecture 19. A Sieve Plate with large Sieve Pores. Secondary Phloem. Secondary phloem (cont d) Lecture 19 Secondary phloem (cont d) Secondary Phloem in Tilia americana (American Basswood) Secondary Phloem of Tilia Stained with Toluidine Blue & viewed with Crossed Polarizers. Secondary Phloem A Sieve