Heterophyllous Shoots in Betula papyrifera

Abstract

LEAVES of two morphological types occur on long shoots of tree crowns in many species of Betula. Those at the shoot base frequently differ from leaves closer to the shoot tip in venation, size, toothing and other characteristics. The first group of leaves (early leaves) appears at or shortly after bud opening, and the second (late leaves) later in the growing season after the first leaves are well expanded. Early leaves are also found on short shoots, that is, shoots the internodes of which fail to elongate more than a few mm. Late leaves are lacking on short shoots. In a report on leaf dimorphism in Populus trichocarpa, Critchfield1 stated that differences in leaf form were due to differences in ontogeny between early and late leaves. Early leaves formed in the growing season preceding their expansion. Their development was arrested and they over-wintered as embryonic leaves, continuing development the following spring. Late leaves over-wintered as leaf primordia or were not initiated until the growing season in which they expanded. Consequently their formation was not interrupted in the embryonic stage and their form differed from that of early leaves. These investigations have now been extended to an evaluation of bud contents, shoot growth, and heterophylly in Betula papyrifera Marsh. growing in north central Wisconsin (U.S.A.).