[92] THE NEW EVIDENCE OF ISOSPORY IN PALAEOZOIC SEED PLANTS

Transcription

1 [92] THE NEW EVIDENCE OF ISOSPORY IN PALAEOZOIC SEED PLANTS BY M. BENSON (With 4 figures in the text) AN intensive study of some sections of the Palaeozoic fructifica- J\ tion Schnetzia Bennieana Kidston, kindly lent me by Dr Crookall after their return to the Geological Survey Museum, London, by Dr Halle, yielded results of twofold interest. A Note relating to them was published last July (3). In the first place they considerably strengthened the evidence of the ovular nature of Schnetzia, as is shown in that Note and in a more recent paper where a revised diagnosis of Calatliiops Goeppert is given (4). In the second place, these female spores and embryo-sacs proved to be of a type unknown in the ovules of recent plants, and showed that the earliest ovules of which we have record of structure, i.e. those of the Lower Carboniferous horizon, bore female spores of the same size as their pollen grains. Not only were these spores of the same size and form but the female spores were as fully cuticularised as the male. Thus there had been no loss of the equipment for aerial transport on the retention of the spore in the seed. As a result the young embryo-sacs had been regarded as pouen grains because the triradiate scar was still visible. It was not until the young tetrads of spores were found that these Schnetzia embryo-sacs (Fig. 2 a) were demonstrated not to be mere spores. Since this first discovery was made several other species of Pteridosperm ovular apparatus of both Lower and Upper Carboniferous strata have been tested in the same manner, and the microtome sections of all those available at the Geological Survey Museum, London, have yielded in varying degrees of preservation minute spores or tetrads. The accompanying figures exhibit transverse and longitudinal sections of ovules in which separation of the carbonaceous matter of the non-cuticularised parts of the ovule has taken place. This carbonaceous matter is exhibited as black masses, almost entirely amorphous, on either side of the sectioned cuticularised

2 New Evidence of Isospory in Palaeozoic Seed Plants 93 parfs. The latter consist simply of the spores or embryo-sacs, and the epidermal layers of the cupules and ovules. The cuticles are exceedingly tenuous, refringent layers determining the position of the wrappings of the spore their epidermal subjacent cells explain the width which gives them a strap-like appearance in - - or I sections. Occasionally, when the section is oblique the wrapping layer looks wider. In some cases the epidermal cells are themselves cuticularised and, on separating partially, resemble bladders. The genuine spores demonstrate that Prof. Boyd Thomson's conclusions (7) as to the Isospory of Recent Seed Plants are confirmed by a study of Palaeozoic Seed Plants. As space is limited I propose to confine this account to the description of such preparations as best illustrate the subject and have been selected for that purpose. Fig. I shows parts of four sections on Halle's shde They are transverse of the younger group of ovules occurring on the same rachis as that examined by Kidston and reproduced from his Memoirs (6) in our Fig. 2 c. Such ovular, bud-like bodies are now included in the form genus Calatliiops Goeppert, so that we may call this body C. Sehuetzia Bennieana. Fig. 1 a, b, c are successive sections (Row I, 2, 3, 4) on this slide In two drawings the carbonaceous matter has been omitted. Fig. I a shows what I regard as a degenerating spore. Fig. i b shows the spore which would have given rise to the embryo-sac with possibly a residuum above it. Fig. i c shows the reduced size of the ovule presumably above or below the sporogenous region. Fig. x dis a complete tetrad of female spores. This is assumed, as there is no trace of a spore in the sections before or after Row I, 7 in which this tetrad occurs. All the drawings were made with a camera lucida and the magnification is shown by the line representing 19-20/x. Fig. 2 a and c are reproduced from Kidston's Memoirs, PI. CVII, figs. 9-13(6) and represent the Calatliiops {Sehuetzia) Bennieana fructification "c" from which he procured the six bodies "a" by maceration. They are magnified a Uttle less than the pollen grain beside them. Two of the larger bodies show a triradiate scar no larger than that of the female spore in Fig. i b. The true $ spore is precisely similar to the pollen grain in external appearance which is given at b. It is interesting to note that the body c was procured by Kidston from the same specimen as that which provided Halle with the material for his shde 3464 which yielded the minute spores in Fig. i. Fig. 2 6 is one of many which are available from the Burntisland PHYT..XXXIV. 2 7

3 94 M. BENSON structural material of Heterangium Grievii. The one drawn by camera lucida here occurs in R.H.C. Coll., CN. 270, i, and is among those of Fig. 3 in the Heterotheea paper(2). Fig. 3 a, b are transverse sections of an embedded piece of Whtttleseya elegans Newb., and are figured from Halle's slide n \c Fig. I. They exhibit two tetrads ensheathed in the epidermal layers of the cupules and seeds. Though in two successive sections (Row I, 2 and 3) they represent the spores of distinct seeds, as they occur in different parts of the sections. The tetrads have had the walls separating their constituent spores dissolved by the drastic treatment to which they were subjected when the fossil was being softened for embedding purposes (vide (5), pp. 5-7). The tetrad in Fig. 3 b has begim to

4 New Evidence of Isospory in Palaeozoic Seed Plants 95 germinate but that of 3 a is similar in size and form to the tetrad shown in Fig. i d, except that it shows none of the inner walls. II/- Fig. 2. IVa Fig- 4- IV6 Fig. 4 a, 6 are also sections from an embedded piece of Whittleseya elegans Newb., but they are cut in the longitudinal plane. In these 7-2

5 96 M. BENSON con.sccntive sections (Slide 360, Row II, i, 2) the same tetrad is represented in two pieces, showing that germination had probably started as in Fig. 3 b. The average linear dimensions are computed easily from the ig-20/^ line which serves equally for all the sections and for the pollen grain in Fig. 2 b. Both Figs. 3 and 4 are examples of Upper Carboniferous structures, as Whilileseya elegans occurs in Westphalian rocks. N.B. All the microtome sections containing the female spores described in this paper were made by Dr T. G. Halle, who employed his new method described on pp. 5-7 of his treatise (5). It was at his suggestion that I procured the loan of these valuable slides. SUMMARY Examples are described from both Lower and Upper Carboniferous rocks of specimens of Calathiops (Goeppert) fructifications. They are shown to be immature ovule apparatus which exhibits minute, triradiate? spores of ig-20/x in linear dimensions. This is shown to be the linear dimension of the pollen grain of Hclerangium Grievii which occurs not only in the pollen sac Heteroiheca Grievii Ci) but in the pollen chamber of Sphcsrostonia ovale which is accepted as the ovule of Heterangium Grievii (l). Pollen grains similar to the above are found in Westphalian rocks, vide PI. CLIII, fig. 4 of Kidston's Memoirs(&). Four figures are given: Fig. 1 a, b, c, d; Fig. 2 a, b, c; Fig. 3 a, 6; Fig. 4 a, b. These are fully described in the text. REFERENCES (1) BENSON, M. (1914). Splicfrostoma ovale. Trans, roy. Soc. Ednib. 50, Part i (No. I). (2) (1922). Heterotheca Grievii, the microsporange of Heterangium Grievii. Bot. Gaz. 74, No. 2. (3) (1934)- '!^ote on. the spores oi Schuetsia Bennieaiia {Kidston). Ann. Bot. 48, No (4) (1935)- The fructification of Catathiops Bernhardti n.sp., including additional notes by Prof. Gothan and a revised diagnosis of the formgenus Calathiops Goeppert. (5) HALLE, T. G. (1933). The structure of certain spore-bearing organs believed to belong to Pteridosperms. A', sveiiska VetenskAkad. Handt., Tredje Serien, 12, No. 6. (6) KIDSTON, R. (1923-5). Memoirs of Fossil Plants. Geol. Survey of Great Britain. (7) THOMSON, R. B. (1934). Heterothally and the seed habit versus heterospory. New Phytol. 33, 41 (where references to his earlier work are given).

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