Phyllotaxis, phenology and architecture in Cephalotaxus, Torreya and Amentotaxus (Coniferales)

Abstract

In Amentotaxus, Cephalotaxus and Torreya there is a regular seasonal alternation of foliage leaves and bud-scales, with foliage leaves largely preformed, i.e. initiated in the season before they expand. On most plagiotropic shoots phyllotaxis in the production of foliage leaves may be either bijugate (Cephalotaxus,Torreya ) or decussate (Amentotaxus). In bijugate phyllotaxis successive leaf pairs originate at an angle of about 68° to each other, i.e. approximately one-half of the «ideal» or Fibonacci angle of 137.5°. Secondary leaf orientation in Cephalotaxus and Torreya, by twisting of the leaf base, produces the dorsiventrality of plagiotropic shoots, whereas in Amentotaxus secondary orientation involves a twisting of the stem internode as well as the leaf base. In Cephalotaxus the bijugate condition is constant in the production of the numerous but imprecise number of bud-scales and in the production of foliage leaves. However, in Torreya the phyllotaxis changes from bijugate in the production of foliage leaves to decussate in the production of bud-scales, which are constant in number (about eight pairs). This allows a precise analysis of the biphasic production of leaf primordia in the seasonal cycle. The phyllotactic change inTorreya may not be the result of reported changes in shoot apex dimensions since Cephalotaxus, with its constant phyllotaxis, has a comparable seasonal change in apex dimensions. Information on architecture, chirality and cone morphology is also included.