#evolutionofleaf #visiblescience #alevelbiology #mdcatbiology #fscbiology this lecture is about EVOLUTION OF LEAF Early vascular land plants did not have true leaves or roots. They were small in size, withdichotomously branched erect smooth aerial parts and equally strong subterranean anchoring and absorptive rhizome. Fig. 9.12 Cooksonia : an early vascular plant bearing sporangia at the tips of the branches. Cooksonia (Fig. 9.12) had the same structural layout i.e. naked stem without leaves. Such plants started to form leaves as small scale like out growths. These out growths were not supplied with vascular tissues, therefore they were not regarded as true leaves. Lycopods were the irst plants that formed the true leaves and roots. However in lycopods (e.g. Lycopodium) the leaves are small in size. Each leaf has a single undivided vein (vascular supply). Such a leaf is called microphyll. Large leaves having divided veins and veinlets with an expanded leaf blade or lamina are known as megaphylls. Megaphylls are characteristic for ferns and seed plants. It is suggested that evolution of megaphylls started from a dichotomous branching system in some primitive psilopsids approximately 350 million years ago. It is assumed that evolution of a megaphyll included series of successive evolutionary steps (Fig. 9.13) which are as follows Overtopping The dichotomously branched aerial portion of the stem showed unequal branching. Some branches remained short while others grew and expanded at a much faster pace. All these branches grew in diferent planes. Such an unequal development of various branches is called overtopping. Planation Next important step was the arrangement of unequal dichotomies in one plane. This process is termed as planation. Fusion/Webbing Overtopping and planation was followed by a process known as fusion or webbing. The space between the overtopped dichotomous branches was occupied by a sheet of parenchyma cells which connected these branches forming a lat lamina or leaf blade type of structure, having many dichotomously branched veins (Fig. 9.13). During the course of evolution fusion of the vascular strands resulted in net or reticulate venation pattern. The process of evolution of leaf was very slow and gradual which completed in more than 15-20 million years.

• published: 12 Feb 2023
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