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.
Reducing desiccation. To help restrict water losses to manageable rates, the leaf epidermis layers are usually coated in cutin a waxy substance. The layer of this wax over the epidermis is sometimes called cuticle. The Carnauba palm leaves have a very heavy layer of cuticle. We boil up the leaves in a vat, strain out the cooked leaves, let the vat cool and remove the floating wax. This carnauba wax is used directly as an automotive polish, with dye added it can be used as shoe polish, if pure enough if can be used as a mold release for candy making. Check the label on your next box of Ju JuBes at the theater. The epidermis may have epidermal hairs to provide a boundary layer of humid air over the leaf surface to retard evaporation. Virtually all leaves have stomata, openings, in the epidermal layers to regulate water loss and gas exchange.
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 stem 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, particularly in desert situations.
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.
The leaf originates as a leaf primordium at the shoot apical meristem. This origin is exogenous rather than endogenous as in lateral roots. The photomicrograph below shows that these primordia are attached to stem at zones of little elongation growth known as nodes. The leaf primordia tend to arch over the zone of cell division in the stem to protect this tender meristematic tissue from herbivory and desiccation.
This is a longitudinal section of a shoot tip:
This view of a shoot apex is as diagrammed below:
Although the leaf primordium is cylindrical at first, just as stem and root, it later develops marginal growth to produce a flattened organ. Intercalary growth of the cells in this tiny primordium then expands the blade to full-size. Because most leaves have a particular final size, we call them determinate. Stem and root are indeterminate in size for many species. The leaf of Welwitschia is indeterminate in size it just keeps elongating throughout the life of the plant.
Anatomically, the protoderm matures to become the epidermal system. The epidermis facing the sun is called the upper epidermis and it has mostly window functions (permit light entry, prevent gas and water loss). The epidermis facing the soil is called the lower epidermis; it is fitted with openings called stomata for gas exchange. Specialized guard cells in the epidermis regulate the size of the stomata and thereby control water loss and gas exchange. Of course clearly the one epidermis becomes the other as it wraps around the edge of the leaf, so the separate names are somewhat artificial. However, some leaves have a far higher density of stomata in the lower epidermis than in the upper. Thus while the epidermis is a single continuous layer, it is asymmetric in many species.
The ground meristem matures into 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 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. This evaporation of water cools all of these interior surfaces.
The provascular tissue in leaves forms xylem and phloem as expected. 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 packaged into veins and ramify 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. But of course, close inspection will reveal vascular cross-connections between these parallel veins in monocots. But it is also true that some monocots do not show parallel veination at all well in their leaves. A Dieffenbachia leaf is a good example.
Leaves come in many sizes and shapes. Sometimes there are many shapes of leaves on the same plant; Sassafras for example has four leaf shapes on the same plant! On this plant, leaves can have lobes, so the blade can be ovate, left-handed mitten, right-handed mitten, and tri-lobate.
In many plants the leaves are simple (one blade per petiole). In other species, the leaves are compound (more than one blade per petiole). Some species have the multiple blades attached to the end of the petiole (palmately compound), others have them attached along the edges of the petiole (pinnately compound). Yet other leaves have no petiole at all (sessile).
Leaves are attached to the stem at a position called the node. The number of leaves that attach to the node determine the leaf arrangement. If only one leaf is attached at each node, the arrangement is alternate. If two leaves attach at a node, they are in an opposite arrangement. If more than two leaves attach at the node, then the leaves are whorled.
The many shapes, compositions, and arrangements of leaves permit a taxonomist to use leaves as important characteristics to distinguish plant species. However a leaf shape, composition, and arrangement is more likely to lead to a genus identification than it is to lead to a specific identification.
Leaves also vary in lifespan. In deciduous species, the leaves last less than a single year, and in the autumn these all fall off, leaving the bare stems. In evergreen species, the leaves last more than one year, so there never is a time that the plant has bare stems. However, even in these evergreen species, older leaves change color and fall from the branches each season, and often in autumn. A walk under some pine trees will reveal much leaf-litter to you. The color change is a result of biochemical breakdown of materials in the aging leaves. The subunits of those materials are loaded in the phloem of the leaf and are transported to the rest of the plant for use in the next season. The plant is not recycling hydrocarbons in this senescence process; it is recycling mineral and other nutrients. The chlorophyll binding proteins have much nitrogen that needs to be retained. The chlorophyll contains nitrogen in the porphyrin ring system, and a Magnesium ion that is also worthy of retention. So the green color disappears. Anthocyanins and carotenoid pigments (red and yellow) are hydrocarbons or less...so they are dispensable to a plant. So abscission is initiated as the last of the minerals have been mined from the leaf. An abscission zone in the petiole of the leaf produces the enzyme, pectinase, and the cells come "unglued" to each other in this layer of the petiole. The weight of the blade then causes the separation of leaf from the stem.
Time permitting, I will show you some examples of leaves that do unusual things. 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 bigger 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 Sarrascennia (the pitcher plant).