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Leaves
First posted June 11, 2004 Last updated June 19, 2009
The leaf is an exciting chemical factory! Learn more about them here.
Leaves are the main photosynthetic organs of most plants, that is, the sugars that are the main source of life-sustaining energy are made in the leaves. In form, leaves are made up of a blade (the broad thing that we usually think of as a leaf) and a small stalk called a petiole that connects the leaf to the branch. Leaves can be simple or compound, and of many different shapes.
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Simple leaf with parts labelled.
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| Leaves can be simple, or compound as above. The left leaf has all the blades coming off different parts of the stem (pinnately compound) and the one on the right all the blades come off the same point (palmately compound, from palm, since the blades radiate like fingers from your palm). |
The leaf has structures within it that allow it to be the energy-producing center for the plant. The outer surface is the upper epidermis, designed with tightly interlocking cells, somewhat like a jigsaw puzzle. It is the first line of defense again physical damage as well as against fungal and bacterial invaders.The outer layer of the epidermis is the cuticle, which is waxy and designed to keep water in but let sunlight through. The only passageways through the epidermis are the stomata (singular: stoma, Latin for "mouth"), which let in the carbon dioxide and let out oxygen. Water also leaks out of the stomata, which is bad for the plant which had to work very hard to draw the water up from its roots and send it up the stem to the leaves. Therefore the plant guards against water loss with the guard cells at the opening of the stomata. The middle of the leaf consists of the mesophyll cells, which have the chlorophyll for photosynthesis, the process of turning sunlight, water, and carbon dioxide into sugars and oxygen. The spongy mesophyll cells have a lot of air spaces to promote the gas exchange between the stomata and the palisade cells. The air spaces are particularly large around the stomata. The veins of the leaf are continuous with the veins of the stems, which in turn communicate with the veins of the roots.
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Parts of a leaf labelled. |
References
Grade 5 Science requirements regarding plant parts and leaves in Saskatchewan.