Abstract Lycopsid megaspores processed from petrologically defined samples of the Low Barnsley Seam, Westphalian B, Upper Carboniferous have vertical distributions arranged in repeated successions of phases. A total of six phases was distinguished producing a more detailed pattern of succession than has previously been identified using miospores. Two important phases were recognized for the first time, designated: Phase 0, characterized by the species Cystosporites verrucosus Dijkstra, Lagenicula horrida Zerndt, Pseudovalvisisporites nigrozonalis (Stach & Zerndt) Lachkar and Cystosporites giganteus (Zerndt) Schopf, interpreted as a colonizing assemblage, and Phase 2, characterized by the species Triangulatisporites tertius Potonié & Kremp and Valvisisporites auritus (Zerndt) Potonié & Kremp which is considered to represent an association of non-arborescent lycopods. Phases are broadly related to petrographic type(s) but changes in phases and petrography are not necessarily coincident. Phasal changes of the kind described are believed to result from the formation of a raised bog.

Lycopod succession in coals: an example from the Low Barnsley Seam (Westphalian B), Yorkshire, England

SUMMARY (1) Seed samples of wheat and barley were collected in Nepal and Pakistan, from approximately 100 villages in each country. Nepal is said to be a Vavilovian 'centre of diversity' for barley and Pakistan for wheat. (2) The samples were grown in controlled conditions at Bangor, N. Wales. (3) The means, between plant variances and coefficients of variation for all characters, apart from awn length, were remarkably similar when comparisons were made both within and between the species of the two regions. Since the variation outside the previously defined centres of diversity was similar to that within them, the concept of centres of diversity must be reconsidered. (4) We conclude that the amount of variation for quantitative characters is determined largely by natural selection. Variation of the major environmental variable, altitude, is equivalent in Nepal and Pakistan. (5) Awn length was the only character to conform to Vavilov's description of variation: there was greater variation between barley plants from Nepal and between wheat plants from Pakistan. We conclude that domestication has reduced selection pressures in awn length thus allowing variation to be released and displayed in a Vavilovian fashion. (6) It is argued that a species, within its 'centre of diversity', is particularly variable only for conspicuous qualitative characters whose variation is not limited by natural selection. In the absence of strong selection pressures, such variation can be maintained between populations by the fixation and loss of major genes. (7) The barley collected from the rainfed agriculture of Nepal was more disease resistant than the barley collected from the irrigated agriculture of Pakistan. It is likely that wheat from Nepal is more disease resistant than wheat from Pakistan. (8) In the light of these results, there seems to be little advantage in collecting plant germplasm from within centres of diversity in an attempt to capture the maximum amount of genetic variation. Collections should be made as widely as possible, and cover areas of great ecological (climatic and edaphic) or cultural variation. The maximum amount of variation for disease resistance is likely to occur in areas where the incidence of disease is greatest.

Variation in cereals from the himalayas and the optimum strategy for sampling plant germplasm

Providing for the health services : proceedings of Section X (General) of the British Association for the Advancement of Science 139th Annual Meeting, 1977

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Providing for the health services : proceedings of Section X (General) of the British Association for the Advancement of Science 139th Annual Meeting, 1977