| Leaves are the primary site of photosynthesis in plants. They are capable of producing large amounts of carbohydrates, which can be shuttled throughout the plant as needed. Deciduous trees (those that lose their leaves in the fall) are able to move carbohydrates to be stored in the roots over winter. In the spring, they can mobilize those materials as their metabolism increases. |
The primary area of photosynthesis in a leaf is in a layer of specialized parenchyma cells called mesophyll. There are 2 types of mesophyll cells. Palisade mesophyll cells are generally rectangular and found in rows, while spongy mesophyll is more unevenly shaped with air spaces in between the cells. Usually palisade mesophyll is found on the upper surface, while spongy mesophyll is found on the lower surface. The outermost layer of a leaf is the epidermis, which is covered by a waxy cuticle to help prevent water loss.
For descriptions of all cell types and their functions, click here.
Note: You can identify various tissues by scrolling over the region with your mouse.
How do plants produce carbohydrates? Photosynthesis! Carbon dioxide is taken in through tiny pores on the leaves, called stomata, and is utilized to produce various sugars. Each stomate is flanked by two guard cells that can open and close in response to changing environmental conditions. Because stomates must be open to take in CO2, they also allow water to escape through evaporation (transpiration). This can cause problems during periods of drought, as plants may lose more water through evaporation than they can take in; this leads to wilting, and if the water loss is prolonged and severe, the plant could die.
Plants have adapted a variety of ways to minimize water loss and also to live in aquatic environments. The leaf shown below is an example of a mesophyte leaf found in temperate climates.
In order to live submerged in water, aquatic plants have adapted a variety of structures. Their parenchyma cells have large air spaces; this special type of parenchyma is called aerenchyma. Stomata are found on the upper surface only. Aquatic plants are also known as hydrophytes.
How can plants live in extremely dry regions such as deserts? Cacti, for example, have very well-developed root systems that allow them to reach water. Also, their stems are thick and can store water very effectively. In many xerophytes (plants adapted to hot and dry conditions), the leaves have a very thick cuticle. Bundles of sclerenchyma cells provide support
A variety of environmental factors can effect the size and thickness of leaves. In many species, leaves grown under high light intensity (sun leaves) are smaller and thicker than those grown under low light intensity (shade leaves).
Increased thickness of sun leaves is due to greater development of palisade parenchyma.
Acer (maple): shade leaf
Acer (maple): sun leaf
In leaves, vascular bundles (xylem and phloem) are found in the veins. There are 3 basic patterns of venation as shown in the pictures below.
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Left: Palmate venation occurs when veins radiate from a common point, as in this maple leaf.
Center: Pinnate venation occurs when veins branch off of a main vein in the center, as in this elm leaf.
Right: Parallel venation occurs when all the veins run parallel to the edge of the leaf.
Which of the leaves are dicot leaves? Monocot leaves?