Leaves - Shape Part 3
Leaves are the engines of life on this planet; their shapes, flat or round, smooth or rough, thick or feathery in infinite variation, create the form of our plants to our delight or despair. The answer to why there is such variation lies in the past, in the present and inside the leaves themselves.
Evolution
Over some five hundred million years, plants evolved from water dwelling one-cell structures to early land dwelling, non-vascular plants resembling mosses and liverworts. Plant pioneers developed who could thrive in dry air; morphing their forms with incredible innovations in methods of transporting water and food through their systems; procreating, first with spore and then with seed, using flowers to lure pollinators instead of relying on wind; successfully occupying about every ecological niche on the planet. In the process, about ninety percent of everything that has ever lived has died. The extraordinary diversity we have today is a drop in the sea of life that has come and gone.
Over the ages, some leaf shapes have remained fairly constant and are recognizable in fossils today. They evolved to best support the life of the plant to which they belong.
It's not simply the survival of a leaf shape, but the survival of the plant that must be considered.
Each plant - with its leaf form - followed a different path of evolution, accounting for many variations in form related to type of habitat.
For instance, coniferous (cone bearing) plants have adapted to the northern parts of the world, south of the tundra but north of deciduous forests and grasslands. Coniferous leaves, sometimes needle shaped, generally remain on the plant all year, sometimes for more than one year. They are flexible to withstand the weight of snow, with waxy coatings to reduce water loss in both cold and arid climates.
The three examples illustrate the similarities as well as the differences in coniferous leaves, the spruce being needled, the juniper awl-shaped and the Chamaecyparis scale-like - variations on a theme reflecting different evolutionary paths from some original form.
Deciduous plants are better suited to temperate climates, taking advantage of mild conditions during spring and summer for growth, while losing their foliage when weather becomes bitter. Their leaves exhibit a greater variety of shapes than those of coniferous plants.
Variation
Plants, unlike animals, are rooted where they grow, unable to move if conditions become too hot or too cold; too wet or dry. They have developed means of responding fairly rapidly to environmental cues to allow them to grow in ways that maximize their chances of surviving the environmental hazards they face.
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