QUANTITATIVE CHANGES OF SOME ANATOMICAL CHARACTERS DURING BARK DEVELOPMENT IN QUERCUS-ROBUR, ULMUS-GLABRA, POPULUS-TREMULA AND BETULA-PENDULA

Quantitative changes of certain anatomical characters during bark deve lopment of Quercus robur L., Ulmus glabra Huds., Populus tremula L. an d Betula pendula Roth were analysed. Generally, bark thickness increas es continuously with age. The time of rhytidome formation varies consi derably between species and individuals and does not show a regular re lation to bark thickness or stem diameter. The quantity of sclereids i ncreases with age in most trees. The development of conduction and sto rage tissue varies considerably, although in most trees its quantity r emains more or less constant after a certain age. In elm the quantity of secretory cells decreases with age. Oak shows an increase of phloem ray height with age and an increase of phloem ray width of some rays due to fusion of uniseriate rays. In elm uniseriate phloem rays are mo re frequent in young bark. The length of secondary phloem fibres signi ficantly increases in oak and poplar. The sieve tube members do not ex hibit a regular developmental trend with regard to cell length, althou gh they tend to be slightly longer in old bark. The tangential diamete r of sieve tubes considerably increases up to a certain age in most tr ees. It is impossible to determine precisely the time range of those c hanges; they vary between species and individuals. However, some chang es are restricted to the transition from primary to secondary growth, others seem to stabilise after approximately 10 years and some last lo nger.