Leaf Functions:

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 accomplished 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.

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.

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 Origin of the Leaf

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 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:

Anatomy of a Leaf

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. Of course clearly the one epidermis becomes the other as it wraps around the edge of the leaf.

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 exchanges than it is for photosynthesis. Evaporation occurs into the gas space between all the cells of the mesophyll.

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.