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The two arborescent birch species recognised within the morphologically variable Betula alba complex in Britain are B.pendula Roth (2n=28) and B.pubescens Ehrh. (2n=56). Reports of the occurrence of an intermediate cytotype (2n=42) within a mixed B.pendula/B.pubescens population in Glen Gairn, Deeside (Brown and Al-Dawoody, 1977) raise interest in view of the extremely rare occurrence of natural hybrids in Europe (Vaarama,1969). An integrated cytological and biochemical approach was taken with the aims of developing a reliable method of separating the two variable species of the Balba complex, identifying intermediate-cytotypes and determining their genetic origin. A programme of artificial crosses and self-pollinations was undertaken to provide material of known genetic background for comparison with natural populations and information about the crossing behaviour of the three Glen Gairn cytotypes. The results of the crossing programme showed that hybridisation between B.pendula and B.pubescens is possible under artificial conditions, that interspecific crosses between parents of different ploidies are more successful when the lower ploidy tree is used as female parent, and that individuals vary in their success as parent trees. Betula species show high levels of self-compatibility but this is more variable in intermediate-cytotype and 56-cytotype (B.pubescens) individuals and some self-pollinations produced a small proportion of germinable seed. The performance of intermediate-cytotype trees in inter-cytotype crosses, selfing and open-pollination indicated a closer relationship with B.pubescens than with B.pendula. The most useful method for routine counting of somatic chromosome numbers was found to be squash preparation of leaf-bases stained by a modified Feulgen technique. Somatic counts were made of adult trees and seedling progenies from open-pollination and controlled crosses. Despite some experimental error in counting small and numerous chromosomes, real variation was recognised within and between cytotypes. The occurrence of an intermediate cytotype in the Glen Gairn population was confirmed but only recognisable in progeny somatic counts. Higher-cytotype adult trees, some of which gave rise to intermediate-cytotype progenies and some to 56-cytotype progenies, showed aneusomatic variation, possibly due to some inherent mitotic instability. Intermediate cytotypes [special characters omitted] recognised in populations which did not contain B.pendula. A major proportion of this thesis is concerned with the development of a method for the extraction of birch leaf enzymes and overcoming the problems of denaturation of enzymes by tannins. A suitable extraction medium was devised after investigation of various buffers and additives to counteract "tanning". The most useful extraction medium to date, based on a Tris-HCl buffer, includes reagents to complex with tannins, to break down tannin/protein complexes and to inhibit the activity of phenoloxidases which catalyse the polymerisation of phenolic compounds to form tannins. An efficient method for the mechanical breakdown of leaf tissue was sought and starch and polyacrylamide gels, in tube and slab form, were compared for efficiency and clarity in separating isozymes by electrophoresis. Systems were compared by measuring the concentrations of protein in solution and/or the number of general protein or specific isozyme bands which could be detected after electrophoresis. The vertical slab polyacrylamide gel system was found to be the most useful electrophoretic system for routine analysis of birch leaf enzymes. Using this method peroxidase isozymes were shown to be variable and species specific patterns were recognisable. Preliminary results showed that chemotaxonomic relationships between B.pendula, B.pubescens and B.nana were in agreement with classical taxonomic divisions. A naturally occurring hybrid from Glen Prosen was identified by combined cytological and biochemical evidence. Information from suitable polymorphic enzyme systems may now be used to compare artificial and naturally occurring hybrids with intermediate cytotypes of uncertain origin. Isozyme analysis is now seen to have the potential to replace the more limited methods of cytological and morphological analyses in measuring genetic variation within and between populations of birch. The genetic origin of the intermediate-cytotype birches in Glen Gairn remains uncertain but, on balance, cytological evidence and information from the crossing programme indicate that recent hybridity is not involved and that the intermediates are derived from a B.pubescens genome.

This study of the reproductive biology of some emergent species - in the family Dipterocarpaceae forms part of the collaborative research programme between the University of Aberdeen and the University of Malaya on the reproductive biology of tropical rain forest tree species. It is intended that such a study will contribute to a better understanding of the mode of speciation prevalent among rain forest trees which has led to the diversity of species in our present day rain forest. General flowerings of dipterocarps are intense involving nearly all species. Blooming is copious and markedly synchronized for a given species. Among closely related species, flowering is staggered, though with some amount of overlapping. Within Shorea section Muticae, flowers are similar in morphology with low pollen production and sickly sweet scent. Their characteristic floral morphology was shown to be well adapted for thrip pollination. There is a high incidence of self-incompatibility among most species studied. Of all species studied, only Dipterocarpus oblongifolius is self-compatible. During a general flowering, mortality of buds and flowers is partially due to predation. The major cause of mortality during this stage is nevertheless caused by defective pollination. Despite disparity in flowering times among species in the section Muticae, mature fruit fall is concurrent. Fruit production in S. leprosula is variable among individuals comprising a clump but nevertheless is much greater than that produced by isolated .individuals. It is suggested that it is these clumps which maintain high 'reproductive pressures' and are the main breeding groups. Fruit dispersal in dipterocarps is inefficient and the demonstrated clumped distributional pattern seems therefore to be due to poor fruit dispersal. There is 110 consistent correlation of survival during seed germination, seedling establishment and after successful establishment with initial seed density or distance from the tree butt. Seedling and sapling mortality in S. leprosula is extremely high for about the first 20 years and the presence of gaps in the canopy largely determine their survival though other factors may play a role. For the following 30 years mortality rate appears to be insignificant but increases again thereafter. Leaf morphological studies on 55 adult individuals of S, leprosula showed significant heterogeneity that is independent of edaphic influences. Regression analysis of leaf morphological variation with between-tree distance showed, significant linear correlation, It is inferred that gene exchange is restricted, Phytogeographical evidence suggests -that speciation among dipterocarps occurs after initial isolation of breeding populations. Evidence presented here is largely compatible with this view and suggestion that this is enhanced by the ineffectual means of pollen and fruit dispersal.

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No description available.

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No description available.