EXINE PATTERN IN THE POLLEN OF BRITISH SPECIES OF TILIA

Transcription

1 New Phytol. (i971) 70, 3. EXINE PATTERN IN THE POLLEN OF BRITISH SPECIES OF TILIA BY R. ANDREW The Botany School, (Received January 1971) SUMMARY Reference pollen slides have heen made from tvvcntj'three samples of lnaterial identified as Tiliaplatyphyllos Scop., T. cordata Mill., or T. x vulgaris liaync. Up to grains in each slide have heen microscopically examined and each opposing face has been classified as of 'cordata' or 'platyphyllos' type. The survey showed that grains of T. platyphyllos material were homopolar and consistently of 'plalyphyllos' pattern. Some niatcrial of T. cordata was also honiopolar and consistently of 'cordata' pattern. There was, however, an extreme degree of heteropolarity in most of the hybrid material and also in much of that attributed to T. cordata, a large proportion ofthe grains having 'cordata' pattern on one face and 'platyphyllos' on the other. The results show the extreme importance of adequate authentication of pollen typematerial and suggest that in Britain both T. cordata and T. platyphyllos are native, hut that much liybridizatioa has occurred between them, giving progeny in which the somatic phenotypes show less indication of 'platyphyllos' parentage than the pollen seems to do. For many years Quaternary palynologists, particularly in, have been greatly interested in the vegetational history of the limes, feeling the need to establish unequivocally the native status in Britain of T. cordata Mill., T. platyphyllos Scop, and T. x vulgaris Hayne, the presumed hybrid between the first two. Investigation of the palynologieal evidence has been facilitated on the one hand by the improvement in the technique and standards of resolution in optical microscopy, and on the other by the assistanee given to the interpretations of optical microscopy by transmission eleetron microseopy, and especially of serially sectioned pollen grains (Chambers and Godwin, 191) and later by scanning electron microscopy of whole and fractured grains (Chambers and Godwin, 1971). It is now apparent that the characteristic reticulation seen in optical microscopy of Tilia pollen is due to an exine pattern totally different from that of the commoner case, which is a solidwall mesh or baculabordered mesh. In Tilia each mesh centres upon a massive hollow funnel with perforate walls, a baculoid structure arising fxom the inner layer of the exine. Thus the meshes are all baculacentred, the strands of the apparent reticulum being only the rounded crests of the ridges where the sloping walls of contiguous funnels meet one another. Below these ridges there are only voids. It follows that one cannot apply to grains of this type the terms 'murus', 'brochus' or 'lumen' that are appropriate to the commoner type of reticulate grain. The reticulation can of course still be characterized by measuring the size of meshes and by describing their shapes. The concept of a bacula centric mesh has already been adopted in optical microscopy of Tilia, and the exine pattern is described as based upon a system of perforate funnelshaped bacula opening outwards through the pollen wall (Faegri and Iversen, 194; Beug, 1971). 3

2 4 R. ANDREW It was clear at an early stage that the pollen of T. cordata was characterized by smaller average size, finer reticulation, a rounder outline than that of T. platyphyllos and these criteria were applied to postglacial fossil assemblages in eastern England as early as Further study is now reported. Three criteria employed in early studies of Tilia pollen have not been used here; (a) the basal 'peg' in the pore of T. platyphyllos described by Trcla, though visible in fresh material, disappears in the acetolysis preparation employed for our type slides made as in standard palynological usage; (b) the hexagonal outline of T. platyphyllos (with pores at three of the angles) has not been found to be a consistent character; (c) although the mean size of pollen in T. platyphyllos and T. cordata is substantially different the ranges overlap and the range in pollen from T. x vulgaris is very great, overlapping that of the two other species. On the other hand T. platyphyllos has a mueh Tilia platyphyllos T. X vulgaris T. exp T. cxp T. inter.»cord. T. inter.>eord. 'P. inter. T. exp T. inter.»eord. T. inter. 71lia eordata Sample no. I IS three e/c 0 22 Table i. Analysis of Tilia pollen types 2 II 93 S 9 27 s %of pollen e/p S IS types P/P * 9 * * 74* No. of grains counted iool y 4 iooj Source of plant inaterial Sweden French Jura Poland Wenlock, Shrops. I^eicestershire Staffordshire Matlock, Derbys. Monsal Dale, Derbys. Cainbridge Mildenhall, Suffolk _ Dolgelly, Merioneth Matloek, Derbys. Monsal Dale, Derbvs. Source of pollen sample Stockholm Herbm. U. Herb. U. Herb. Bot. G. U. Herb. Bot. G. Bot. G. U. Bot. G U. Bot. G U. Herb. U. Herb. "1 I CD. Pigott coll. J U. Bot. G U. Herb. Belfast Herb. Cainbridge U. Herb. CD. Pigott coll. CD. Pigott coll. The asterisks in eolumn p/p indicate the presence of exceptionally large 'platyphyllos' mesh (see text). coarser reticulation than T. cordata, and greater heteropolarity of reticulation, characters adopted and characterized quantitatively by Beug (1971). The large mesh of the T. platyphyllos grain often has an irregular polygonal shape (due to the perforate lips of the funnels) and the finer meshes of T. cordata grains are harder to resolve and seem more uniformly round, characters readily visible in Stereoscan eleetron micrographs (Chambers and Godwin, 1971). The difference in pattern of the two faces of the grain is often extremely pronounced, especially so in pollen from the hybrid trees that mostly are regarded as T. x vulgaris. It is apparent that all determinations of specific identity through examination of fossil pollen depend ultimately upon the validity of the identification of reference material from which the pollen referenee samples have been made and this identification in the case of Tilia spp. is by no means simple. Most British taxonomists agree upon a series

3 Exine pattern of Tilia pollen of nonpollen features that characterize T. cordata and T. platyphyllos respectively and that are found in combination in their hybrids. These include flower number, inflorescence position and fruitstructure as well as numerous features of leaf, petiole and shoots such as were employed by Pigott (199) in his studies of the status of the limes in woods on limestone in Derbyshire. Pigott concluded from morphological analysis of the lime tree shoots, from woodland structure and vegetational analyses that the woodlands studied were of considerable antiquity and that T. cordata, T. platyphyllos and their hybrids were present. Eight of the reference slides examined (Nos. 91) have come from trees in these Derbyshire woods identified by Professor Pigott variously as 'c x p', 'inter' or 'inter > cordata' (see Table i). Eight are from other British material in the Botany School herbarium, one is from the Belfast herbarium, six are from trees growing in the University Botanic Garden, one is from Sweden, one from France and one from Poland. They include in total six referred to T. cordata, seven to T. platyphyllos and ten to the presumed hybrid between them. We now record the result of careful microscopic investigation of c. grains in each of these type slides, every grain having been examined by difl:erential focusing on both polar faces. Reference, based upon the criteria indicated below, has been made into three categories. c/c Grains with a simple network on both polar areas with about 12^1 meshes to a 12 /^(m line across the centre of the polar area: the meshes usually without a central spot. p/p Grains with a complicated network on both polar areas with about 12 meshes to a 12 /an line across the centre of the polar area: the large meshes tending to polygonal and usually with a central spot of contrasting refractive index. c/p Grains with T. cordata characters on one face and 7'. platyphyllos characters on the other. sp In four samples shown in Table i a proportion of the grains showed exceptionally large meshes of 'platyphyllos' type (10 to a 12 ^m line) on one or both polar areas. There is of course a measure of subjectivity in the assessments, and it is recognized that the classification of the opposing faces is an arbitrary one: none the less the results seem evidently significant. Consideration of the tabulated results makes it clear that reference material of T. platyphyllos trees has yielded pollen with very high frequencies of T. platyphyllos type, and that either no pollen with T. cordata morphology occurred or it was in very low frequency, i.e. upon this score the tree and pollen phenotype identifications correspond, although in two instances (, 17) the pollen shows the extreme T. platyphyllos characters (sp) in high frequency. Of reference material described as T. cordata, three reference slides have pollen % or more of purely T. cordata type and the grains c/p in low frequency (1, 21, 22). The other three reference slides from material of similar attribution show large to very large frequencies of c/p grains and large to low frequencies of c/c grains (23,, 13) whilst one of them has 4 and one 1% of p/p grains (13, ). Thus if pollen criteria had been solely used for recognition of numbers 13, and 23, these would have been considered to be of hybrid origin. Likewise numbers, 9 and 12, regarded by Pigott as hybrids near to T. cordata, have as much as 234% of p/p grains. The other reference slides are of material from trees regarded as hybrid and they show a strong preponderance of c/p grains (7, 1, 19) or a preponderance of p/p grains with

4 R. ANDREW considerable frequencies also of c/p and c/c grains (io, ii, 14). Two of tlie presunied hybrid trees indeed have given a high proportion of pollen with extreme T. platyphyllos characters, one described as hybrid without qualification (10) and the other (11) as a hybrid close to T. platyphyllos. Thus, although we have shown that the material referred to T. platyphyllos consistently yields platyphyllos type pollen, the same is not true for T. cordata material, some of which yields pure cordata type pollen and some has pollen showing a considerable platyphyllos component. It is as though tlie trees attributed to 'T. cordata' include some liybrids, but that hybridity expresses itself only in the pollen. The sanie tendency is apparent in the material from the putative hybrid trees, in which the pollen reveals a stronger 'platyphyllos' character than did the parent trees. The situation suggests the presence in Britain of pure T. cordata and pure T. platyphyllos together with hybrid populations containing a proportion of individuals that seem to be pure 'cordata' or more largely 'cordata' than the pollen evidence indicates. What can scarcely be in dispute is that in material of accepted or presumed hybrid origin the pollen shows a remarkable range of morphological type often with large percentages of grains of pure T. cordata and pure T. platyphyllos type as well as the intermediates. One naturally enquires how our results affect conclusions based on the study of pollen of Tilia found fossil in Britain. When large populations of fossil pollen occur, all or almost all referable to the T. cordata type, as commonly happens in British Flandrian material, we can be sure that pure T. cordata existed. We do not find assemblages of fossil material with pollen exclusively of T. platyphyllos type, but at certain levels the type p/p occurs along with the c/p and c/c types. This implies the presence of hybrid limes and by inference tlie presence of T. platyphyllos at no great distance in time or space, a conclusion that is supported in a few instances by identification of macroscopic plant remains, along with grains of T. platyphyllos type. ACKNOWLEDGMENTS The author wishes particularly to thank Professor C. D. Pigott for his kindness in supplying type material of Tilia pollen and Professor Sir Harry Godwin for his assistance in drafting this paper. She is especially grateful to Professor HansJurgen Beug who kindly arranged for her to visit Gottingen to examine material and discuss palynological matters of common interest: from this visit it emerged that there was agreement between us on the main criteria for distinguishing pollen of Tilia cordata and T. platyphyllos. Finally the author wishes to acknowledge the consistent help and support of Dr R. G. West, head of the Subdepartment of Quaternary Research in. REFERENCES BiUG, H. J. (1971). Lcitfaden der Pollenhcstinnnung, Teile II/HI. Gustavfischer, Stuttgart. CnAMBEiiS, T. C. & GODWIN, H. (191). The fine structure ot the pollen wall of Tiliii ptatyphyllos. New Phytot., 0, 393. CiiAMBiiRS, T. C. & GODWIN, H. (1971). Scanning electron microscopy of Tilia pollen. New Pliytol., 70, 7. FAKGRI, K. & IVHitSEN, J. (194). Textbook of Pollen Analysis. Munksgaard, Copenhagen. PiGorr, C D. (199). The status of Titia eordata and T. platyphyllos on the Derbyshire limestone. J. EeoL, 7, 491.

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