Leaf dimorphism in Archaeopteris roemeriana (Progymnosperm): further early fossil evidence of shoot dorsiventrality1

Additional information on the morphology of the vegetative ultimate branches of Archaeopteris roemeriana was obtained by uncovering compression specimens collected in Upper Devonian deposits in Belgium. For the first time, anisophylly, previously inferred from anatomical studies, is demonstrated on one species of the genus. Small leaves less than half the size of those more readily seen in unprepared compressions are borne on the adaxial surface of the ultimate branches. The phyllotaxis is discussed and the eventual adaptation to light interception of the combination of anisophylly and shoot dorsiventrality is put forward.