Early vascular land plant did not have true leaves or roots. They were small in size, with dichotomously branched erect smooth aerial parts and equally strong subterranean anchoring and absorptive rhizome.
Compression fossil of cooksonia approximately 350 million years ago
Reconstruction of cooksonia to show vegetative and reproductive parts
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 sere not supplied with vascular tissues, therefore they were not regarded as true leaves. Lycopods were the first plants that formed the true leaves and roots.
However in ly copods (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. Megaphy lls 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:
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 different planes. Such a unequal development of various branches is called overtopping.
Next important step was the arrangement of unequal dichotomies in one plane. This process is termed as planation.
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 flat lamina or leaf blade type of structure, having many dichotomously branched veins ( fig. 9.13).
During the course of evolution fusion f the vascular strands resulted in net ofr reticulate venation pattern. The process of evolution of leaf was very slow and gradual which completed in more than 15-20 million years.