Leaves are the most obvious part of typical plants. Leaves are what you see that cover the crown of the tree, that cover a surface that is spread by vines, or that cover the ground in turf grasses. Leaves are typically green and carry out photosynthesis. They produce a range of chemicals from photosynthesis that is critical to the function of a plant. Other functions are perhaps less-obvious.
Leaves can be either deciduous or evergreen. Deciduous leaves typically have a lifespan of just one season. In the autumn of the year, such leaves change color in a process known as senescence (aging) culminating in leaf abscission (falling off). While the plant recovers minerals and other chemicals from the leaf during senescence, the falling leaf may carry away significant toxins from the plant. Thus abscission may be a mechanism functionally equivalent to excretion for some plants. Evergreen leaves typically have a lifespan of several seasons. Because new evergreen leaves are produced each year, the plant is never completely bare of leaves...hence the name evergreen. In fact evergreen trees have leaves that do senesce and abscise. We often fail to notice the process, but the litter on the forest floor in a pine forest tells us that it happens. Closer examination in autumn reveals that needles of pines that are two years (older in some species) old turn yellow and all fall from the trees at the same time as maple trees!
Leaves come in a myriad of shapes and sizes. From the thin, linear shape of a spruce needle, leaves can be as broad and as long as a person in some palms. The leaf can be simple with just one blade on the petiole, or compound with two or more blades per petiole. Compound leaves can be palmate (with all the blades attached at the end of the petiole) or pinnate (with the blades distributed along the edges of the petiole). The shape of each blade is generally fixed for a particular species, but some plants are heterophyllous (have multiple leaf shapes on the same plant). For example all the blades on a maple tree have the same basic shape, but Sassafras has four leaf shapes on the same plant! The blade in this plant can be ovate, left-handed mitten, right-handed mitten, or tri-lobate. In many heterophyllous plants one of the shapes is juvenile (produced when the plant is young or on the immature regions of the plant) and the other is mature (produced when the plant or region of the plant is nearing reproductive maturity). The many shapes and compositions of leaves permit a taxonomist to use leaves as important characteristics to distinguish plant species.
Leaves can be arranged along the stem in different ways. Leaves attach at specific locations along the stem called nodes. When only one leaf attaches at the node we call the leaf arrangement alternate. When there are two leaves attached at the node, the arrangement is opposite. If there are more than two leaves attached at a single node, the arrangement is whorled. In the laboratory we will look at kidney bean plants which demonstrate both opposite and alternate leaves, as well as simple and compound leaves, and petiolate and sessile leaves!
The leaf originates as a leaf primordium, a small pile of cells, at the shoot apical meristem. Although the leaf primordium is cylindrical at first, just as stem and root, it later develops marginal growth to produce a flattened blade. Intercalary growth allows the tiny developing leaf to achieve its final size. Most leaves are determinate and have a specific final size (example: Maple), but there are leaves that are indeterminate and just keep expanding and expanding (example: Welwitschia).
The upper and lower leaf surfaces are covered with a layer of cells called epidermis. The upper epidermis faces the sun and it has mostly window functions (permit light entry, prevent gas and water loss). The lower epidermis faces the soil and typically it is fitted with openings called stomata for gas exchange. Of course clearly the one epidermis becomes the other as it wraps around the edge of the leaf!
Between the epidermis layers is the mesophyll. The mesophyll is divided into two layers; just under the upper epidermis is the palisade mesophyll. This layer is responsible for most of the photosynthesis as it has the best sun exposure and the densest population of chloroplasts in each cell. The lower layer of mesophyll is the spongy mesophyll. This layer also carries out photosynthesis, but it is in the shadow of the palisade layer, so the spongy layer is more important for evaporative cooling and gas exchange than it is for photosynthesis. Evaporation occurs into the gas space between all the cells of the mesophyll.
Running between the palisade and spongy mesophyll are veins. These consist of the two conducting tissues: xylem and phloem. The xylem which was on the interior of the stem, connects outward to the leaf and therefore ends up facing the upper epidermis and palisade layer. The phloem which was toward the exterior of the stem, connects outward to the leaf and therefore ends up facing the spongy mesophyll and lower epidermis. The xylem and phloem are sometimes surrounded by bundle sheath cells that together comprise the vein. Veins ramify (branch) into a complex network in the leaf blade. This network is called veination. In dicot leaves the veination is often called netted because the network is quite intricately interconnected to form a net. In many monocot leaves, most of the veins run parallel to each other...although close inspection reveals a more closely-interconnected network.
Photosynthesis. The leaf in typical plants is the site of photosynthesis for the plant. The overall equation for this complex process appears deceptively simple:
|CO2 + H2O||light|
|O2 + CH2O|
Evaporative Cooling. The flow of water in the xylem continues from root to the leaves via the stem. The water leaves the xylem in the leaf and evaporates from the internal cells into the atmosphere. This evaporation accomplishes three functions...it cools the leaf which might otherwise bake in the hot sun, it draws more water up through the xylem in the stem below, and it concentrates the mineral nutrients supplied by the root. To help restrict water evaporation, the epidermis is often coated with cutin, a kind of wax. The Carnauba palm tree has leaves thickly coated with wax, which we boil off, cool to harden, and polish our cars with. We also add dyes to carnauba wax and and polish our shoes with it. The stomata of a leaf permit the leaf to control evaporative cooling and water loss. When the two guard cells on either side of the stoma are fully pumped up with water, they push each other apart and the stoma is open. When these guard cells lose water and turgor pressure, the stoma collapses shut. The stomata are most numerous on the lower epidermis. It makes sense from a water conservation perspective, that stomata are located primarily on the cooler side of the leaf.
Export nutrients. Sugars and amino acids from photosynthesis in the leaf are loaded into its phloem and exported to the rest of the plant. This flow will be upwards from leaf to apical bud, to flower, or to fruit as well as downwards from leaf to root.
Storage of water, etc. In the leaves of many succulent plants water can be stored. Of course this water needs to be protected from herbivory in desert situations. So leaves may possess strong secondary chemistry to poison animals.
Defense. leaves have evolved into spines in cacti and other plants to mechanically protect the plant from herbivory. In other species, such as Erythoxylon coca and Cannabis sativa, the leaf produces potent chemistry to deter and/or kill herbivores. In these two examples the drugs are cocaine and delta-9-tetrahydrocannabinol, respectively.
Anchorage. As you have observed in peas in the laboratory, leaves sometimes produce tendrils at their tips. These can assist a plant climb obstacles in the environment or other plants to gain a competitive edge.
I will show you some examples of leaves that do unusual things for us. You will learn to recognize Verbascum thapsis which we employ as a kind of Native American Charmin. We will see epidermal glandular hairs in Cannabis that supply defense against herbivores (if "fun" for Homo sapiens). We will see the floating leaves of Victoria that have their stomata in the upper epidermis and huge gas spaces for floatation. These leaves float well enough that the weight of a young woman in a formal gown, distributed evenly by a sheet of plexiglass on the blade, can be supported by these "lily pads." We will also consider some strange adaptations of leaves to obtain minerals (normally a root function) in carnivorous plants. These plants have leaves modified to trap and digest insects to mine out their calcium and other minerals. Examples will include Dionea (the Venus flytrap), Drosera (the sundew), and Sarracennia (the pitcher plant).
This page © Ross E. Koning 1994.
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