[279] A NOTE ON THE ORIGIN OF LATERAL ROOTS AND THE STRUCTURE OF THE ROOT-APEX OF LYGINOPTERIS OLDHAMIA

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1 [279] A NOTE ON THE ORIGIN OF LATERAL ROOTS AND THE STRUCTURE OF THE ROOT-APEX OF LYGINOPTERIS OLDHAMIA BY A. C. HALKET (With Plate XI and i figure in the text) E 'GlNOPTERis oi.dh.imi.i, a plant of the Carboniferous period extremely common in the coal-balls of Lancashire and Yorkshire, was stated by Dr D. H. Scott in 1924 to be "now almost the best known of fossil plants" ((5), p. 121). The general habit of the plant, the anatomy of its vegetative parts and the structure of its reproductive organs are all well known. In spite of the comparative completeness of our knowledge, however, there still exist many lacunae in the story of the life-history of this plant, some of which may be filled as new material becomes available for study. For example very little, if anything, is known at present about the development and method of growth of the various plant organs. It is advisable therefore to record some information which has been obtained recently concerning the development of the lateral roots of this plant. The roots of Lvginoptcris oldhamia,firstdescribed by Williamson (9) under the name of Kaloxvlon Hookeri, are readily identified by the structure of the cortex, with its two sharply contrasted zones and secretory sacs. Recently, while examining a number of preparations made from calcareous nodules from the British Coal Measures, some sections of well-preserved roots of Lvginoptcris were found which show features not previously described. Two sections are of special interest. These are both of young tetrarch roots preserved before the commencement of secondary thickening, and are both cut transversely and in the plane of origin of a very young lateral root. For convenience of reference these sections will be called Root i and Root 2. Root I occurs in slide 17, Root 2 in slide 24 in the Bedford College Collection; both preparations were cut from nodules from the Upper Foot Mine, Shore, Littleborough, and supplied by the Lomax Palaeobotanical Laboratories of the Lancashire and Cheshire Coal Research Association.

2 28o A. C. HALKET Information has been obtained from the examination of these sections relating to (i) the structure of the inner cortex, (ii) the place of origin of the lateral roots, and (iii) the structure of the root-apex. (i) The strueture of the inner eortex. The tissue of the inner cortex in many of the roots found in nodules from the British Coal Measures is not very well preserved and is described as "consisting of rather large cells, loosely packed, so as to leave considerable intercellular spaces between them," and as "evidently a somewhat lacunar tissue" (Scott (4), pp. 52 and 54). The tissue of the inner cortex in Roots i and 2, however, is unusually well preserved and does not altogether agree with this description. In Root I (Text-fig, i B) the tissue of the inner cortex is seen to consist of delicate thin-walled cells closely packed together so that the intercellular spaces are quite small. In parts of Root 2 the same structure can be seen, while in other parts of the tissue it can be seen that large spaces are formed by the breaking down of these cells. The formation of these large spaces may have taken place naturally as the roots grew older, but it seems more probable, from the appearance of the tissue, that the breaking down was due to decay before fossilisation and that the young living roots did not have a definitely lacunar inner cortex such as is characteristic of marsh plants of the present day. The characteristic secretory cells occur among the thin-walled cells and can be recognised by the darkness of their walls. Fungal hyphae are present in the inner cortical tissue of both these roots (Williamson and Scott (lo), p. 737). (ii) The place of origin of the lateral roots. The place of origin of the lateral roots can be determined, since, in both sections, the developing branch root is so young that it has only penetrated about half-way through the inner cortex. In both roots the xylem and phloem groups can be easuy distinguished and it can be seen that a single layer of cells (the pericycle) separates them from a layer of closely fitting cells (the endodermis). Hence the position of each developing root in relation to the tissues of its parent root can be seen. It is clear that a lateral root was formed by the division of the cells of the one-layered pericycle and ruptured the endodermis before making its way through the cortex. The branch roots of Lyginopteris, therefore, arose in the same tissue as do those of modern gymnosperms (Strasburger (7), Reinke(3)). The roots of Lyginopteris, however, appear from the evidence of these

3 Note on the Origin of Lateral Roots of L. oldhamia 281 sections to arise laterally to a protoxylem group not exactly opposite to it as is the case with the roots of modern gymnosperms. It should 1..C.- A. uc. Text-fig. I. A, diagram of transverse section of Root i. X25. B, part of the cortex of this root, A 145. p.z. "peripheral zone" of cortex; i.e. thinwalled parenchymatous cells of inner cortex; e. endoderinis; p.x. group of tracheides of protoxylem; l.r. very young lateral root, cut tangentially; s.c. "secretory cells";/./;, fungal hypha. Both A and B were drawn with the aid of a projection apparatus. The thickness of the line in B indicates the depth of colour of the cell wall, not its thickness. be noted though that Williamson and Scott ((lo), p. 740) state that in older roots, with secondary thickening, the xylem of a branch root is found opposite to the protoxylem tracheids of the main root.

282 A. C. HALKET (iii) The structure of the root-apex. We have at present little information concerning the structure of the root-apex of Lyginopteris.

4 282 A. C. HALKET (iii) The structure of the root-apex. We have at present little information concerning the structure of the root-apex of Lyginopteris. In 1908 Stopes and Watson (C) figured a longitudinal section of an isolated root-apex, which, with other similar apices, was ascribed to Lyginopteris by Weiss in 1913(8). Weiss could come to no conclusion as to the tnethod of growth of the root, though he was inclined to think that an apical cell may have been present as in ferns. The structure of the apex of developing rootlets of this plant can be seen in Roots i and 2. Details of the structure can be seen better in Root 2 for the plane of the section cuts the emerging root longitudinally in the median plane, or approximately in this plane. In Root I, on the other hand, the developing rootlet is cut tangentially. The root-apices of fossil plants are so rarely found that it is thought advisable to record the structure of a single apex even though the conclusions drawn cannot be confirmed by comparison with other apices. The ck'licate walls of the cells composing the apex of the rootlet in Root 2 are sufficiently well preserved to enable the type of organisation to be determined. The arrangement of the cells is shown in the photographs reproduced in PI. XI. Two regions can be distinguished, a central one (the plerome) composed of some slightly elongated cells, and a larger peripheral one (the periblem) consisting of fairly regularly arranged rows of cells continued over the top of the central region. The apical part of the periblem will obviously form the root-cap. This arrangement of cells is the same as that seen in the young branch roots of gymnosperms at an early stage when the differentiation of the plerome is just beginning, but when the characteristic form of the periblem can be clearly distinguished (Strasburger(7), Tafel XXIV, fig. 28, and Reinke(3), Fig. 5). On the other hand, no sign of the presence of the large apical cell characteristic of the roots of most ferns can be seen or of the segments cut off from such a cell; neither can a group of initial cells be seen such as is characteristic of the Marattiaceous type of root. From the arrangement of the cells in this apex, therefore, it may be deduced that the structure of the root-apex and, thence, the mode of growth of the root of Lvginopteris ohihamia were similar to those of modern gymnosperms, as well as to those of Amyelon radieans, the root of Cordaites, another palaeozoic gymnosperm (Halket (l))^ ' References to other papers on the structure of the root-apices of gymnosperms are given in this paper.

5 THE NEW PHYTOLOGIST VOL. XXXI, PLATE:XI p.x 1. HALKET ROOT.STRUCTURE OF OLDHAMIA LYGINOPTERIS Face p. 283

6 Note on the Origin of Lateral Roots of L. oldhamia 283 Lyginopteris oldhamia has fern-like foliage and is one of the many plants of the carboniferous epoch which were at one time thought to be ferns. Furthermore, Lyginopteris oldhamia was the first of these fern-like plants proved to have borne seeds (Oliver and Scott (2)). Hence a special interest is attached to the fact that, even in comparatively minor characters of organisation, such as the structure of the apex and the vertical orientation of the diarch xylem plate in lateral rootlets (Weiss (8)), there should be agreement with modern gymnosperms rather than with ferns. BEDFORD COLLEGE, LONDON November, 1931 REFERENCES (1) HALKET, A. C. The rootlets of Amyelon radicans Will.; their anatomy, their apices and their endophytic fungus. A nn. Bot. 44, (2) OLIVER, F. W. and SCOTT, D. H. On the structure of the palaeozoic seed Lagenostonia Loma.xi, with a statement of the evidence upon which it is referred to Lyginodendron. Phil. Trans. Roy. Soc. B, 197, (3) REINKE, J. Andeutungen uber den Bau der Wurzel von Pinus Pinea. Botanische Zeitung, 30, (4) SCOTT, D. H. Studies in Fossil Botany, 2, Spermophyta, 3rd ed. London (5) Extinct Plants and Problems of Evolution. London (6) STOPES, M. C. and WATSON, D. M. S. On the present distribution and origin of the calcareous concretions in coal seams, known as "Coal Balls." Phil. Trans. Roy. Soc. B, 200, (7) STRASBURGER, E. Die Coniferen und die Gnetaceen. Jena (8) WEISS, F. E. The root-apex and young root of Lvginodendron. Mem. and Proc. Manchester Lit. and Phil. Soc. 57, No. 16, {9) WILLIAMSON, W. C. On the organisation of the fossil plants of the coalmeasures. Part VII. Myelopteris, Psaroniits and Kaloxylon. Phil. Trans. Roy. Soc. 166, Part i, (10) WILLIAMSON, W. C. and SCOTT, D. H. Further observations on the organisation of the fossil plants of the coal-measures. Part III. Lyginodendron a.nd Heterangium. Phil. Trans. Roy. Soc. B, 186, EXPLANATION OF PLATE XI Figs. I and 2 (photographs) Fig. I. Transverse section of Root 2. p.z. "peripheral zone" of cortex; i.e. thin-walled parenchymatous cells of inner cortex; i.s. large intercellular space formed by the breaking down of the cells; e. endodermis; p.x. group of tracheids of protoxylem; l.r. very young lateral root; s.c. "secretory cells." X42-5 circa. Fig. 2. Part of same section enlarged to show the details of the arrangement of the cells as seen in the longitudinal section of the young lateral root. p. phloem; f.h. fungal hypha other lettering as in fig. i. x212-5 circa.

NOTES ON GINKGO BILOBA'

NOTES ON GINKGO BILOBA' WALTER WV. TUPPER (WITH PLATE xx) Among the gymnosperms, one of the groups most interesting from a morphological standpoint is the Ginkgoales, the only living representative of