Morpho-anatomical studies on the change in the foliage of two imbricate-leaved New Zealand podocarps : Dacrycarpus dacrydioides and Dacrydium cupressinum

Abstract

Many extant gymnosperms have strongly reduced leaves. While this is usually treated as adaptation to drought, other habitat factors must also be involved. This subject and the related change from juvenile needle leaves to adult imbricate leaves are poorly understood. Here we explore these topics by describing the leaf morphology and leaf anatomy of seedlings and adult plants of Dacrydium cupressinum and Dacrycarpus dacrydioides. In both species, the cotyledons, primary leaves, and juvenile leaves are needle like. Then D. dacrydioides foliage becomes partly imbricate. In D. cupressinum, the needle leaves gradually become smaller and more close-set, with some fully imbricate leaves appearing on old trees. It follows from very high rainfall and low soil phosphorus of the habitat that their very small leaf size is likely to be a scleromorphic character, not a xeromorphic one. Confusion in the literature in the use of scleromorphy and xeromorphy is pointed out and clarified.