When you stop to think about it, the variety in leaf shapes in the plant kingdom is rather astonishing. Why are there so many different leaf shapes in the first place? And, why do the leaves vary in shape on single species? The answers to these questions are both simple and extremely complex.
To understand why leaves have assumed certain shapes, we need to have a basic understanding of how a leaf actually functions. We gardeners are more observant than non-gardeners when it comes to seeing plant leaves, but I wager that most of us don't really know what goes on inside that generally thin, flat green object.
The Inside Story
The inner life of plants, in general, and leaves in particular is a complex interaction of many cells, each performing specific tasks.
This is a leaf section through a typical dicot plant. Dicots are plants who start out with two seed leaves as opposed to monocots (like grasses) who start with only one seed leaf and have a different internal arrangement..
Leaves, no matter their size or shape, are green factories that are responsible for life on our planet. If it weren't for plants, there would be no oxygen and none of the other living creatures (including us!) who require oxygen would survive.
I always think of plants breathing in carbon dioxide and breathing out oxygen - processes called transpiration and respiration. Stomata are tiny holes, mostly in the underside of leaves, through which they do their breathing...plant noses, if you want to get excessively cute about it. Each of these stomata is flanked by guard cells that help them open and close. What they're really doing is more complex than this mental vision.
Respiration is the process of converting stored food into useable energy - like burning wood - and requires oxygen just as a wood fire does. Plant respiration releases water and carbon dioxide, just as we do when we breathe out. Unlike photosynthesis, which requires light, respiration occurs whether it's light or dark. It's really the opposite of photosynthesis because it uses food and oxygen to turn the plant's chemical energy into heat and a form of energy that the plant can use, for example, in forming a sprout or a bud.
Transpiration is how plants carry nutrients through their structure. Water passes out through the leaf stomata, in the process pulling more water up through the plant from the roots.
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