Introduction to leaf
Leaves are green flat structures borne on the stem or on the branches of plants. These arise from nodes and have buds in their axils. Leaves increase the surface area for the absorption of sunlight.
A typical foliage leave consists of three parts. The lamina is green expanded portion of the leaf. It performs photosynthesis. The leaf stalk is called petiole. It raises the lamina towards sunlight. It is absent in some leaves. So, it is directly attached to stem.
The leaf base is the part of leaf which attaches to the stem. It forms sheathing in many cotyledons. The leaf bases in many Dicotyledons bear two outgrowths called stipules.
Evolution of Leaf
Early vascular land plants did not have true leaves or roots. They were small in size, with dichotomously branched erect smooth aerial parts and equally strong below ground anchoring and absorptive rhizome. Cooksonia had the exact same structural design i.e., a naked stem without leaves.
Such plants started to form leaves as small scale-like outgrowths. These outgrowths were not provided with vascular tissues, for that reason they were not considered true leaves. Lycopods were the first plants that formed the true leaves and roots.
Microphylls
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.
About 380 million years ago, plants with vascular tissue first evolved a special type of leaf, referred to as a microphyll. A microphyll typically has a single midvein and occurs from a stem that does not have leaf gaps, in regions of parenchyma (i.e., unspecialized) tissue where the vascular strand leads into the leaf base.
The microphyll may have come from as an outgrowth of a vascularized stem, or by evolutionary simplification of a complex branch system. The leaves of certain modern plants in the Lycopodophyta (Lycopods) and Sphenophyta (Horsetails) are classified as microphylls. Although the microphylls of these modern plants are rather little, a few of their fossilized relatives had huge microphylls.
Megaphylls
Large leaves having divided veins and veinlets with an expanded leaf blade or lamina are called megaphylls.
About 350 million years ago, plants first progressed megaphylls, the leaf kind of contemporary seed plants and ferns. A megaphyll generally has a complicated venation pattern and emerges from a stem that has leaf spaces, or areas of parenchyma tissue where the vascular hair leads into the leaf base. One theory proposes that megaphylls, along with other plant organs, evolved by modification of branch systems.
To put it simply, megaphylls may have developed by the flattening of a three-dimensional branch system, and connection of the flattened branches with a cellular webbing.
It is presumed that the development of a megaphyll consisted of series of succeeding evolutionary actions which are as follows:
Overtopping
The dichotomously branched aerial portion of the stem showed unequal branching. Some branches remained short while others grew and broadened at a much faster rate. All these branches grew in different planes. Such an unequal development of various branches is called overtopping.
Planation
The next essential action was the arrangement of unequal dichotomies in one plane. This procedure is described as planation.
Fusion/Webbing
Overtopping and planation were followed by a process known as fusion or webbing. The space in between the overtopped dichotomous branches was occupied by a sheet of parenchyma cells which connected these branches forming a flat lamina or leaf blade kind of structure, having actually lots of dichotomously branched veins.
Throughout the course of development fusion of the vascular hairs resulted in the net or reticulate venation pattern. The process of evolution of leaf was very sluggish and progressive which finished in more than 15-20 million years.
Numerous botanists believe that the 4 different whorls of a flower (sepals, petals, stamens, and carpels) originated by evolutionary modification of the megaphylls of a free-sporing plant. Many modern plants have actually progressed complex and highly specialized leaves.
For example, the insect-eating organs of carnivorous plants, such as Venus Flytrap, Sundew, Pitcher Plant, and Bladderwort, are all highly specialized leaves.