New observations on morphology of beech growth units (Fagus sylvatica L.) - Shoot symmetry, reflection of tree vigor

Beech has a rhythmic growth pattern, each annual shoot being made of a spri ng growth unit (u.c.), sometimes followed, a few weeks later, by a summer g rowth unit. Spring u.c. shows an alternate distic foliar distribution, wher eas summer u.c., according to some authors, shows an alternate spiralized f oliar distribution. Following morphological and histological observations o f the buds formed at different periods of the year, it appears that the fol iar distribution pattern in beech does not evolve from one u.c. type to the other: foliar primordia are always distributed along two parallel straight lines, close to one side of the stem, following a "plagiotropic distic" pa ttern. However axial symmetry of a u.c. can change during its elongation, e volving from a bilateral symmetry toward a secondary radial symmetry. This evolution results from a nodal reorientation produced by a twisting of the internodes, the most significant shift of foliar organs coming from the lon gest u.c.'s at the level of their longest internodes. It is possible to lin k u.c.'s degree of development with tree architecture. The less vigourous t rees (very young or growing under a dense forest canopy) show a more or les s leaning main axis, formed by less developped u.c.'s, with a bilateral sym metry; main axis is plagiotropic bearing a flat crown. On the other side, t he most vigourous trees (adults or on favourable forest sites) show a verti cal main axis, formed by relatively long u.c.'s, with a radial symmetry; ma in axis is orthotropic with a well and widely distributed crown.