Sources
Raven, Evert, and Eichhorn, Biology of Plants [Sixth Edition], W.H. Freeman and Company/Worth Publishers, 1999
The leaves in the phyla Ginkgophyta, Coniferophyta, and Anthophyta exhibit many similar and many different characteristics. Ginkgophyta and Coniferophyta are gymnosperms, while Anthophyta are angiosperms.
Ginkgophyta
The only living member in the phylum Ginkgophyta is Ginkgo biloba. On the exterior, the leaves are fan-shaped with a dichotomous venation pattern. The leaves growing on short shoots are entire, while those growing on long shoots tend to be deeply lobed. Ginkgo biloba is also deciduous, which is rather unique for gymnosperms.
The leaves of Ginkgo biloba contain vascular bundles that are arranged closer together than those found in dicot Anthophyta leaves (fig. 1 and fig. 6). Yet, the vascular bundle arrangement in Ginkgo biloba is much more similar to those in monocot Anthophyta leaves (fig. 5).
Coniferophyta
The phylum Coniferophyta contains many species and the leaf characteristics are rather diverse. This discussion will only look at leaf characteristics in the genus Pinus. Leaves, commonly known as needles, are arranged in fascicles (bundles) containing one to eight leaves. The fascicles are short shoots where apical meristem activity has been suspended. Rarely, the activity may resume resulting in a new shoot that can grow roots and subsequently grow into a new tree.
The leaves in Pinus (and most conifers) have many drought-resistant characteristics. The epidermis is covered by a thick cuticle and beneath these cells, there is a tightly packed hypodermis layer (fig. 2). The stomata are recessed in small cavities, which is an advantage in harsh climates. Resin canals are found in the mesophyll. The resin canals secrete aromatic and antiseptic resin that protects the plant against the development of fungi and insect attack. The vascular bundles are surrounded by transfusion tissue (composed of short tracheids and parenchyma cells), which aid in the movement of materials between the vascular bundles and the mesophyll.
Leaves are generally retained on Pinus for 2-4 years. Pine species typically inhabit areas that have a short growing season; thus the retention of leaves for 2-4 years allows the plants to begin optimal photosynthesis rather than produce new leaves first.
Figure 2 Pinus leaf
Anthophyta
Leaf characteristics in the phylum Anthophyta are organized into dicot (165,000 species) and monocot (65,000 species) divisions. Leaf venation in dicots is typically reticulate (netlike), while in monocots it is generally striate (parallel). In reticulate venation, there are smaller veins (minor veins) branching from larger veins (major veins). The midvein found along the middle of the leaf is the largest major vein. In striate venation, all the veins are typically the same size. In monocot and dicot leaves, there are vascular bundles, a spongy mesophyll, a palisade mesophyll, and an epidermis (fig. 3 and fig. 4).
