THE WITHIN POPULATION VARIANCE IN GENECOLOGICAL TRIALS

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1 THE WITHIN POPULATION VARIANCE IN GENECOLOGICAL TRIALS BY D. J. HARBERD Scottish Plant Breeding Station, Pentlandfield, Rosliu, Midlothian (Received 21 September 1956) (With I figure in the text) INTRODUCTION In genecological trials, the significance of difference between populations is normally assessed for each of the various recorded characters by the standard statistical procedure of comparison of the variance between means of populations with that within populations. Clearly, the size of the within population variance is of great importance in trial work any justifiable procedure which will reduce it, will correspondingly increase the precision of the experiment. The within population variance can be regarded as being composed of two parts, the one genetic and the other non-genetic. The genetic part derives from the different genotypes making up the population; while such factors as non-uniformity of the trial ground and errors of recording contribute to the non-genetic part. Thus the genetic part is primarily a characteristic of the population, and imposes a lower limit to the size of the within population variance, whereas the non-genetic part is essentially a feature of the trial and could perhaps be beneficially reduced to an insignificant quantity by suitable adjustments of the experimental technique. Although the genetic part of the within population variance is more characteristic of the population, yet it is not independent of the trial. For instance, a group of different genotypes might yield closely similar phenotypes under one environment, but clearly distinguishable ones under another. Cooper (1951) has shown that populations of Loliuin are not genetically uniform in their daylight requirements. Thus S.2 grown under continuous daylight contains both heading and non-heading plants while all head under normal daylength. Italian ryegrass under continuous daylight has a wide variation of leaf number before heading; under natural daylength it is much more uniform. Nevertheless, it is probably safe to assume that the genetic part of the within population variance is reasonably constant from trial to trial so long as the environmental conditions do not vary too drastically. There is apparently no published information on the relative sizes of the constituent parts of the within population variance in a typical genecological trial. Yet the interpretation of such a trial would be quite diff'erent in the two extreme cases of its being almost entirely made up of the one or other constituent. If the non-genetic part were by far the most important, then the populations would be more uniform than they appear, and hence the differences between them more clear cut. It might be possible to reveal this by repeating the trial under more carefully standardized conditions. If on the other hand the non-genetic part is unimportant, then any overlap between the populations is likely to be genetical. This could have an important bearing on theories of the adaptive values of the characters recorded. N.P. 269

2 20 D. J. HARBERD The present paper is an account of an experiment designed to investigate the relative importance of the constituent parts of the within population variance. If a number of plants representing several populations are laid out in a random trial, then the within population (or error) variance is simply b + e, where b is the genetic and e the nongenetic part and the two parts cannot be separated. If, however, each plant in the trial is cloned to give r ramets and the trial is laid out in r replications, each plot containing each genotype, then the analysis is of the split plot type and yields two error variances: the one is an estimate of e, while the other is an estimate of rb + e. The non-genetic part of the within population variance can itself be attributed to two main groups of causes those concerned with the trial ground, and those concerned with measuring and recording. In the former group would come variations of soil and microclimate, attacks by pests and diseases, competition from weeds, damage during cultivations, etc. In the latter group, selection of material for measurement, inaccuracy of measurement and mistakes in recording can play an important part. Visitors to this station have occasionally remarked on the somewhat crude method of measuring such characters as stem height. In big trials speed can often be more important than accuracy. However, it cannot be denied that recording techniques might introduce serious errors. The two parts of the non-genetic variance can be separated if the character is recorded a second time in an independent examination. This again leads to a split plot analysis with two error variances, the one being a direct estimate of the one part, and the other a compound. THE TRIAL The species used in the trial was Festuca ovina agg. chosen because it is easily cloned, and because it was already known (P. J. Watson, unpublished data) that it is genecologically differentiated. The material was gathered by the author and Dr. Watson in July Although designed primarily for the examination of the within population variance, the trial does also give some information on population differentiation. Eighteen sites were visited and these are briefly described in Table i. The location of the sites is shown in Fig. i. About a quarter of the plants from each population were examined for chromosome number in the first instance, though later complete counts were made of populations, D., G., WM., Arrow and Snettisham. Nine populations had the diploid chromosome number zn = 1, and seven were tetraploid, 2n = 2. The collections from two sites, G. and WM., were mixed, containing about one third diploid and two thirds tetraploid. For purposes of analysis these two collections were each treated as two populations. During the course of the trial, some of the characters recorded were found to be useful for distinguishing diploids and tetraploids. At the completion of the trial eight plants whose measurements suggested that they might have been misclassified were counted, and it was found that their initial placing had been correct. There appears to be an ecological separation of the diploid and tetraploid races of F. ovina in East Anglia. All of the dry acid sites visited contained the diploid form, which is by far the rarer of the two m the region. Most such sites were exclusively diploid. It may be of interest that G. and WM. differed from the rest of the dry acid sites not only in supporting both races but also apparently in the absence of Airaflexiiosa. It appears from Dr. Watson's very extensive unpublished work on the distributional relationships of the two chromosome races that this ecological separation does not hold very well for the rest of Britain.

3 Variance in genecological trials 21 A. Aa. B out Bin C. D. Arrow. G. WM. Bridgham. Holt low. Holt high. Buxton dry. Buxton wet. Sandringham. Snettisham. Ashdown. Cannock. Table I. Origin of the populations used in this study Collected from "Om grassland prasrlflnd A ^Wfitt (Watt, Tr^^rt^ 190) in tkf^ the Rt-^r-l-Unrl Breckland. Surface Q.i. soil calcareous, vegetation calcicolous. Plants tetraploid. Collected from a blown sand desposit close to A. Surface soil acid, soil at 12 in. calcareous. Thick cladonia mat with Agrostis, Rumex acetoseiia. Plants tetraploid. Collected from Watt's grassland B. Surface soil neutral. Plants tetraploid. Vegetation heavily grazed. Collected from within a small rabbit proof enclosure erected on grassland B in Vegetation very tussocky. Plants tetraploid. Collected from Watt's grassland C. Surface soil slightly acid. Plants tetraploid. Collected from Watt's grassland D. Surface soil acid. Plants tetraploid. Collected from bombing marks of chalk rubble laid in 191 near grasslands E-G. Appearance very like grassland A. Surface soil calcareous, soil at in. acid. Plants tetraploid. Collected from Watt's grassland G. Surface soil very acid. Plants mixed, diploid and tetraploid. Collected from Weather Heath, Elevedon, Suffolk from a site very similar to G, and some four miles distant from it. Surface soil very acid. Plants mixed. Collected from Bridg:ham Heath, Norfolk. Surface soil very acid. Bent-fescue with bracken. Plants diploid. Collected from a heath, near Holt, Norfolk. Surface soil acid. Bent-fescue with bracken, gorse and heather. Sample taken from a patch of grazed grassland. Plants diploid. Collected from the Holt heath from under dense gorse and heather. Plants diploid. Collected from Buxton Heath, Norfolk. Surface soil acid. Bent-fescue with bracken, gorse and heather. Plants diploid. Collected from a large wet hollow in Buxton Heath. Surface soil acid. Molinia, Nardus, Juncus, Sphagnum. Plants diploid. Collected near Sandringham, Norfolk. Surface soil acid. Edges of dense heather where it gives over sharply to a bent-aiva community. Plants diploid. Collected from Snettisham Common, Norfolk. Surface soil very acid. Open heatherbent-fescue. Plants diploid. Collected from Ashdown Forest, Sussex. Surface soil very acid. Acid grasslands with heather and gorse. Plants diploid. Collected from Cannock Chase, Stafford. Surface soil acid. Grass patches in heathen community. Plants diploid. Each of the 55 wild plants gathered were cloned to five pieces and a piece of each genotype was planted in each of the five replications. The cloning was carried out in the greenhouse during April 195, and the ramets were planted in randomly allocated positions in the boxes so that the field planting would be straightforward. The trial was planted out on 23 and 2 June 195. The late planting, rabbit damage before the trial was netted, and the severe winter of combined to give a heavy death rate. Nearly nine per cent of the ramets were missing when the trial finished. The losses were particularly heavy among the diploids. Among the survivors, variation in vigour from plant to plant seemed to be high, and the appearance of the trial suggested that it was badly grown. The analyses reported are limited to those genotypes recorded in all five replications. During the course of the trial five characters were recorded for analysis date of ear emergence, length of stem, length of panicle, length of leaf and habit of growth. In addition to these five, two characters were recorded as a guide to the vigour of the ramets. These were a spring score, varying from o for an apparently dead plant to 3 for 'normal' plants, and an inflorescence count. Weak ramets, having low scores for these two characters, tended to difter from their better grown counterparts in the expression of the other characters. They were later emerging, and shorter in both stem and panicle, so that they biased the mean and increased the within clone variance for these characters. In the case of habit of growth, weak ramets were less accurately classified they do not bias the mean, but they do increase the within clone variance. Leaf length does not appear to be affected by the vigour scores. This is probably due to the fact that the vigour scores are a reflection of the condition of the ramets in the spring, while leaf

4 22 D. J. HARBERD length was recorded in the autumn, after the plants had had the growing season to even up. In addition to analyses of all available data some selection of records, using the vigour scores for guidance, was practised among the tetraploids. The aim of the selection was not to throw out the weak genotypes, but to avoid those genotypes in which one or two ramets were weak compared to the rest. It is suggested that the analysis of selected records gives an indication of the situation in more satisfactory trials Scale of Miles Fig. I. Sketch map of Southern Britain showing location of sites mentioned in Table i. RESULTS Ear emergence All five ramets of 22 genotypes were recorded for ear emergence. The scores were given according to the day on which inflorescences were flrst observed to be emerging from the sheaths. Observations were taken three times a week for six weeks. The mean emergence score for the tetraploids is 6.2 (approximating to May) while that for diploids is 12.5 i^^ May). This difference is highly significant having a t value of 11. fo'' '^ degrees of freedom. Dr. Watson found a similar difference in one of her trials. The analysis of diploids (Table 2) shows highly significant population differentiation. On examining the population means (Table 3) it seems clear that Ashdown and Cannock differ from the rest: if these two populations are omitted from the analysis, the significance of difference between populations disappears.

5 Variance in genecological trials 23 The residual error item with a mean square of 2.25 is an estimate of e, the non-genetic part of the within population variance. The highly significant item for between genotypes within populations, with a mean square of is an estimate of 5b+ e, so that b, the genetic part of the within population variance, is approximately 2.1. Thus the genetic and non-genetic parts of the within population variance are about equal in size. Table 2. of ear emergence in diploid populations Item Populations io 65.*** Genotypes within pops *** Plots 16.0*** Plots-Pops Error In this and following tables asterisks are used to indicate the significance of the F values: *for P = o.c;; **, o.oi; ***, o.ooi. Three analyses of ear emergence in tetraploids are printed in Table, and the population means in Table 5. The first analysis is of all genotypes recorded in all five ramets, the second is based on genotypes selected for their more uniform vigour scores, while the Table 3. Mean emergence scores in diploid populations G. WM. Bridgham Holt High Holt Low Buxton Dry Buxton Wet Sandringham Snettisham Ashdown Cannock II.1 third is based on a more rigorous selection. Points of interest in this series are (a) the decreasing size of the error mean square, (b) the increasing significance of the item 'between genotypes within populations', (c) the decreasing population means, and (d) the loss of the small significance between populations. The first three points are clearly related and the fourth point may also be related, to the later emergence of weak ramets; the delay in weak ramets being greater in some populations than in others. Whether or not the significance of populations in the first analysis is indicative of population differentiation, it is clear that the differences are very small compared for instance to those found in the diploids. Table. Three analyses of ear emergence in tetraploid populations Item Populations Genotypes within pops. First * 35.96*** Second 92.'\nalysis go.23* 33.12*** Thi 1 rd *** Plots Plots-Pops. Error ** *

6 2 Populations A Aa B out B in C D Arrow G WM. Table 5. D. J. HARBERD emergence scores in tetraploid populations First VIean 5. 5-' n Second Mean s n Third Mean n 6 Estimates of the two main parts of the within population variance are tabulated in Table 6. The genetic part is expressed in the third column as a percentage of b+e, the error variance of a randomized trial not including cloned material. This value is about 50 per cent as with the diploids, but it gradually increases in the more evenly grown material. Table 6. Structure of the within population variance for ear emergence First Second Third b 6.13 S-5-5 e h b + e 5-56% 56-93% 5-61",, Habit of growth This character was scored on 21 July, 21 genotypes being scored for all five ramets. The score is an estimate by eye, in conjunction with a series of standard diagrams of the angle from the vertical (in units of io") which includes the bulk of the inflorescences. Scores varied from 2, the most erect, to, the most prostrate. Item Populations Genotypes within Pops. Plots Plots-Pops. Frror Table. Analyses of habit of growth Diploids! Tetraploid Populations *** 3.96*** First d.f *** 2.16*** Second 92 " *** 1.5*** Third 1 ' *** 1.3*** There was no significance of difference in this character either between chromosome races or populations. There were highly significant differences within populations, and between plots (see 1 able ). The estimates of the genetic and non-genetic parts of the within population variance are set out in Table. As in ear emergence, the three analyses in tetraploids form a series with decreasing error mean square, increasing significance between genotypes within populations and increasing b over b + e percentage. Table. Structure of the within population variance for habit of growth First. Second Third Diploids b e h b~+e 3-5o"o 55-29", " %

7 Variance in genecological trials 25 Stem length Stem lengths were measured to the nearest centimetre, and recorded in all five ramets for 266 genotypes. The score was taken on 21 July and repeated on the 22nd, so that the error of recording could be examined. The analysis of all populations is set out in Table 9. Stem length is another character which seems not to differ between races or populations, though there is marked variation within populations. Table 9. of stem length, all populations Item Chromosome Races Populations within races Genotypes within pops. Plots Plots-Pops. Error Scores Error I I *** 603.1*** *** 15-3** 1-3 The double scoring of the trial makes possible the separation of e, the non-genetic part of the within population variance into its two parts, ei and &,_, the former being due to errors of the trial ground and the latter of the recording. The final error variance of 1.3 is a direct estimate of e. and indicates that records are distributed around their Item Populations Genotypes within pops. Plots Plots-Pops. Error Scores Error Table 10. Analyses of stem length I 6 Diploids M.S *** 3.22*** *** 9-2* 1-5 Eirst * 3.69*** 21.91*** *** Tetraploid Populations Second I 5 0.3* 39.2*** 16.5*** *** 6.iO 1.6 Third *** ** *** true value with a standard deviation of approximately 1.3 cm. The significance of difference between scores is due to the mean for the second score being 0.15 cm. shorter than that for the first. No explanation is forthcoming for this intriguing but unimportant point. The second error variance of.93 is an estimate of 2ei ^e^ so that Population A Aa B out B in c D Arrow G WM. Table 11. Mean stem lengths of tetraploid populations Eirst Mean n Second Mean n Third. Mean

8 26 D. J. HARBERD e[ is Clearly the value of 1.3 for e, is of little importance in comparison with ei great accuracy in measuring and recording would lead to very little improvement in the precision of trials. The item for between genotypes within populations is an estimate of ^i + e. so that b is approximately Table 12. Structure of the zvithin population variance for stem length First Second Third Diploids b e, e h b+e 65-3 ^ -3,; 1.09 ; 5-96 /< The analyses of diploids and tetraploids taken separately are given in Table 10, the means of tetraploid populations in Table 11, and the estimates of b and e in Table 12. The three analyses in the tetraploids again form a series with decreasing error mean square (the second error item), increasing significance within populations and increasing b over b + e percentage. The weak ramets tend to score lower than their better grown counterparts so that there is an increase in population means in the series. As in ear emergence, the small significance between populations in the first analysis is not found in the third. Panicle length Panicle length was measured on 25 July to the nearest millimetre in all five ramets of 259 genotypes. The tetraploids had a mean panicle length of.16 cm. while the diploids had a-mean of 5.19 cm. The difference is highly significant having a t value of.9 for 1 degrees of freedom. There were no differences between populations within races, but Item Populations Genotypes within pops. Plots Plots-Pops. Error Table 13. Analyses of panicle length Diploids *** First *** Tetraploid Populations Second *** Third *** considerable differences between genotypes within populations. The analyses are printed in Table 13 and estimates of b and e in Table 1. As in the earlier characters the three analyses of the tetraploids show a series with decreasing error mean square, increasing significance of genotypes within populations, and increasing relative size of the genetic part of the within population variance. Table 1. Structure of the within population variance of panicle length First Second Third Diploids Tttraploids b e b b + e 5-% 50.6% 5-5 'o 63.OO"r,

9 Variance in genecological trials 2 Leaf length Leaf length was measured to the nearest centimetre on 23 September, 31 genotypes being recorded for all five ramets. The diploids had a mean leaf length of 1.1 cm., the tetraploids of 15.5 cm.; the difference having a t value of 3.9 for 1 degrees of freedom has a probability of less than Table 15. of leaf length in diploid populations Item Populations io 1.3*** Genotypes within pops *** Plots 69.1*** Plots-Pops Error There was no population differentiation among the tetraploids, but the diploids show considerable differences. The analysis of the diploid populations is given in Table 15, and the population means in Table 16. It can be seen that the diploids fall into two Table 16. Mean leaf lengths of diploid populations G. WM. Sandringham Snettisham Bridgham Holt High Holt Low Buxton Dry Buxton Wet Ashdown Cannock groups: G., WM., Sandringham and Snettisham have a leaf length not significantly different from that of the tetraploids, while the rest vary around a mean of 1. cm. An analysis shows the difference between the groups to be highly significant, while there are no significant differences within the groups. Table 1. Analyses of leaf length in tetraploid populations First Second Third Item Populations Genotypes within Pops *** *** 1 1.9*** Plots 0.9*** 31.9*** 23.2*** Plots-Pops Error The three analyses of tetraploids, and estimates of b and e are given in Tables 1 and 1. The most striking feature of these tables is the contrast with those for the earlier characters. There is in the series no reduction of the error mean square, nor increase of the within population significance, nor of the relative size of b. It has already been noted that ramets scored weak in the spring did not tend to differ consistently from the stronger ones in the leaf length score, and this is held to be the explanation of the situation. Table 1. Structure of the within population variance of leaf length b e Diploids ;, First o Second.5 3-i3 60.2"^, Third o

10 2 D. J. HARBERD DISCUSSION AND SUMMARY Differences between populations Highly significant differences (P <o.ooi) were found in panicle length (two classes, means of t;.2 and.1 cm.), ear emergence (three classes, mean scores 6., 10.i and 13.0), andleaf length (two classes, means of 15.6 and 1. cm.). Using these characters together with chromosome number the populations can be arranged in four groups (Table 19). The differences between groups are highly significant for at least one character; the differences within groups do not attain significance, except among the tetraploids where a 0.05 significance was found in two characters. Reason was given for doubting the importance in these cases. The difference in ear emergence between the chromosome races agrees with an earlier finding by Dr. Watson (unpublished). By contrast, however, in her trial which was based on a different series of populations of wider geographical origin, the tetraploids significantly exceeded the diploids in stem length, panicle length and leaf length. Table 19. Grouping of populations chromosome Number Panicle Length Ear Emergence Leaf Length 2 short early short 1 long medium long 1 long late long 1 long late short Populations A Aa Bout Bin C D Arrow G (tet) WM. (tet) Ashdown Cannock Bridgham Holt High Holt Low Buxton Dry Buxton Wet G. (dip.) WM. (dip.) Sandringham Snettisham One feature of Table 19 of great interest is that the populations gathered from neighbouring sites which differed markedly in their ecological characteristics are placed in the same group. By contrast, ecologically similar sites well separated geographically, did not necessarily yield similar populations. This finding appears to be contrary to the current theories of genecology (Bradshaw, 195). But it must be remembered that failure to demonstrate a difference between populations is not proof that they are identical. The most convenient interpretation of Table 19 is that the characters recorded have little selective value and that the Breckland tetraploids, for instance, are one breeding unit. The population differences between groups would then be a result of isolation. The characters recorded in the present work are ones in which population differentiation has been recorded in several species and in some cases the inference of adaptive value is most tempting (Bradshaw, 195; Clausen, Keck and Hiesey, 190; Gregor, 193; Gregor and Watson, 195; Turesson, 1922, 1930, 1931; etc.). Differences within populations All five characters, in both races, were highly significant within populations: the populations examined have a high genetic variability. This observation is in full agreement with other authors (Clausen, 1953; Turrill, 193), and is not surprising if the characters have no adaptive value. A simple extension of the theory of natural selection would suggest that a population from a uniform habitat should be uniform for adaptive characters, but such a system is probably much simpler than that found in practice. With

11 Variance in genecological trials 29 changmg conditions of environment those genotypes which fair ill one season, but manage to survive, might be the ones which fiourish the following season. Considerable variation within a population is then to be expected. The within population varia?ice Even in a badly grown trial, such as the one under discussion, some 50 per cent or more of the within population variance is of genetic origin in most characters. Evidence is given that in better trials the genetic part would be relatively much greater; it attains 6 per cent in one of the cases quoted. In one character the constitution of the nongenetic variance was examined; errors of recording seem to be of very little importance compared to those resulting from non-uniformity of the trial ground. Spurious significance in genecological trials The low relative size of the non-genetic part of the within population variance can be a serious source of error in genecological work. Repeated isolation of the same genotype in a population sample will decrease the standard error, and so make differences between populations apparently more significant. This can be illustrated using the figures for the first analysis of leaf length in tetraploids (Table 1), in which differences between populations are found to be non-significant. If all items excepting the one between populations are combined, we get one mean square of 9.09 with 51 degrees of freedom. Using this as an error item, the differences between populations are highly significant (F = 3., P -< o.ooi). Such a procedure is quite inadmissible in the present case, and the significance is erroneous, because we know that the genotypes in the trial are represented more than once. But how often is it known in trial work that no genotype is represented more than once? We know very little about the spread of a single genotype in natural communities, or the number of genotypes to be found in, say, a square yard. We can guess that isolates every two yards are less likely to repeat genotypes than isolates at one yard spacing; and we can guess that isolates at one yard spacing are more likely to repeat genotypes in the rhizomatous Festuca riibra than in the tufted Festuca ovina. (It is hopefully assumed that no repeated genotypes occur in the populations in the present study.) But our knowledge does not enable us to be certain that all plants gathered from a single population are distinct genotypes. Clearly it is possible to gather too large a sample from a small area, and this will lead to an erroneous significance, which cannot be distinguished from a genuine one. The relationship between population studies and genecology Since the classical foundation of the subject by Turesson, genecology and population work have been largely treated as one phenomenon. It has been supposed that population differences may be due to selective or non-selective influences (e.g. Sewell- Wright effect) and much debate has gone into the distinction between the two. It is now demonstrated that some apparent population differences may be due to another nonselective infiuence the repeated isolation of single genotypes. Without belittling the value of population studies, it is suggested that the relationship between morphology and selection could be more usefully studied in a scheme which does not use the within population variance as an error item. Two possible schemes are the correlation between population means and environmental factors; and the use as error item of the variance between populations from ecologically similar sites. In contrast to the tendency to restrict the number of populations in a trial in order that each can be represented by a

12 2o D. J. HARBERD large sample, these schemes require large numbers of populations, any restriction being in sample size. While expressing my thanks to the many kind people without whose help, guidance and advice this study could not have been completed, I should like in particular to mention Dr. A. S. Watt for the field demonstration of his work on the Breckland and his permission to sample from his type localities; and Dr. D. J. Finney for statistical advice. REFERENCES BRADSH.^W, A. D. (195). Local population differences in,^ios?w *fh«is. Caryologia. Vol. suppl CLAUSEN, J., KECK, D. D. & HIESEY, W. I\I. (190). Experimental Studies on the Nature of Species, i. The effect of varied environments on Western North American Plants. Carnegie Institution of Washington, Publication 520. CLAUSEN, J. (1953). The ecological race as a variable biotype compound in dynamic balance with its environment. I.U.B.S. Symposium on Genetics of Populations. Pavia, Italy. COOPER, J. P. (1951). Studies on growth and de\'elopment in Loliiim. II. Pattern of bud development of the shoot apex and its ecological significance. J. EcoL, 39, 22. GREGOR, J. W. (193). Experimental Taxonomy. II. Initial population differentiation in Plantago maritima L. of Britain. Nezi: PhytoL, 3, 15. GREGOR, J. W. & WATSON, P. J. (195). Some observations and reflexions concerning the patterns of intraspecific differentiation. New PhytoL, 53,291. TuRESSON, G. (1922). The genotypical response ofthe plant species to the habitat. Heieditas, 3, 211. TuRESSON, G. (1930). The selective effect of climate upon the plant species. Hereditas, 1,99. TuRESSON, G. (1931). The geographical distribution of the alpine ecotype of some Eurasiatic species. Hereditas, 15, 9. TuRRiLL, W. B. (193). Material for the study of taxonomic problems in Taraxacum. Report of Botanical Exchange Club for 193, 50. WATT, A. S. (190). Studies in the Ecology of the Breckland W. The Grass Heath. J. EcoL, 2, 2.

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