Growth and development of Betula pendula Roth. and Betula pubescens Ehrh.
1. Betula pendula Roth. and B. pubescens Ehrh. are found widespread throughout northern Europe and the U.S.S.R. Their current utilization is highlighted by the plywood industry of Finland but throughout their range they are normally only considered as being of minor importance to forestry. The work reported here has run parallel to the development of a small, breeding and selection programme for B. pendula. 2. During the raising of open pollinated, progenies, seedlings from B. pendula mother trees, were found that were considerably different from their half sibs. These atypical seedlings have been studied closely for several years and their stable morphological characteristics led to the conclusion that they were natural hybrids. Reports of such hybrids have been frequent in local and national floras, but firm evidence for their production is very limited in scientific literature. 3. A technique for- rooting cuttings of young birch was used to study both the propagation process and the clonal growth of B. pendula, B. pubescens and atypical seedlings from B. pendula mothers. The results of the growth analysis showed that in almost all characters assessed, the atypical clones were more closely associated with B. pubescens than B. pendula. This series of observations indicated the paternal influences of B. pubeacens on physiological characteristics of the atypical clones. 4. A study, on the shoot and branch growth of B. pendula and B. pubescens seedlings developed into a larger scale investigation into the crown development of B. pendula trees in the progeny test. The measurement of simple parameters of shoot growth has revealed; two aspects of growth control, one acting on currently produced; lateral branches and the other on the controlled development of bud. outgrowth. The shoot growth parameters assessed have been shown to decrease with increasing orders of branches and this has been represented by the statistic, mean internode extension. Although trees of up to four years of age have been studied, 3* branches show dimensional characteristics similar to the pendulous, seed producing clusters of branches, characteristic of B. pendula. 5. Following the observations made on outside trees, a glasshouse trial was used to examine the sequences of branch growth on small seedlings. Records were kept from the first signs of the expansion of dormant buds to the establishment of an elementary crown. Early bud expansion was correlated with subsequent shoot growth. 6. Records of the incidence of flowering were analysised for two stands of trees. B. pubescens appears to be an earlier seed producer than B. pendula, but the atypical seedlings showed a high incidence of flowering. Possible causes for these observations have been presented. Two progenies of B. pendula have been identified as precocious seed producers. Within the progeny test observations were made on the development and occurrence of a pronounced, abnormal shoot development. These have been termed hyper active meristem and their possible causes examined. Reports on these structures have not been found in the literature but the continuation of their study appears to be set with difficulties due to their unpredictable occurrence.