Leaves are a good place to start when looking at plants because of the leaves importance in photosynthesis. Also in leaves we get to see all the main tissue types in plants and some of their functions. First we look at a generalized leaf structure and then at some of the adaptations that have evolved in plants from this basic structure.
Leaves serve a number of functions for most plants. First of all and perhaps most obvious, leaves are the principle site for photosynthesis. Second, leaves are the main site of evaporation of water from the plant. This may seem to be a bad thing as all land creatures must deal with problems associated with water loss. But this evaporation, called transpiration, form the leaves provides most of the energy necessary to draw water and minerals up from the roots of the plant. In addition to transpiration, gas exchange carbon dioxide for photosynthesis and oxygen from photosynthesis and for respiration are usually exchanged through the leaves. Next, if you think about it, plants compete with one another for sun light and leaves serve a function here by shading potential competitors. Finally leaves may serve other functions such as protection and reproduction in certain plants. For example most thorns appear to be highly modified leaves that protect the plant from herbivores, and leaves of many plants have specialized hairs which release defensive chemicals to deter leaf feeding insects.
Leaf cross section and tissue types.
The picture is of a privet leaf cross section. Privet is a common ornamental shrub. Click on the 'hot spots' to get an explanation of each of the tissue types found in the leaf, or review them using the menu. Privet shows common leaf structures, but as we shall see these structures and the overall shape of the leaf have often been modified by natural selection in different environments.
A plant has three basic types of tissue and leaves typically have all three. Thus a leaf is an organ rather than a tissue.
Dermal tissue is a kind of complex tissue consisting both of flattened cells covering the upper and lower surfaces of the leaf, and of specialized cells called guard cells. Guard cells regulate gas exchange between the environment and the interior of the leaf by controlling the size of the stomata, openings through which gas exchange takes place. The upper and lower layer of dermal tissues are respectively called the upper and lower epidermis.
The epidermis serves both to protect the plant and for water conservation as the outer surface of the epidermal cells are covered with a waxy waterproof cuticle. In addition the epidermis often has specialized hair like cells called trichomes. Trichomes often secrete chemicals which deter feeding by insects and may also mechanically prevent insects from feeding. In addition they may also serve to keep the leaf cooler by shading the surface of the leaf.
The guard cells are specialized dermal cells that regulate the size of the openings or stomata(singular stoma or stomate) in the epidermis of the leaf. Each stoma is surrounded by two guard cells that either take up or release water to the surround cells.
When the guard cells release water to the surrounding epidermal cells, the guard cells become flaccid which causes the two cells to close off the stoma. This prevents water loss from the leaf. Conversely, when the quard cells take up water from the surrounding cells, the guard cells swell(become turgid) which causes then to bow out, opening the stoma. This allows gas exchange and an increase in water loss from the leaf, transpiration.
Water is not directly pumped into our out of the guard cells. Instead, the guard cells actively transport potassium ions and the water follows by osmosis.
When the guard cells close their stomata, this conserves water, something which is important when water is in short supply, but this also means that carbon dioxide cannot be taken up by the leaf for photosynthesis and excess oxygen produced by photosynthesis removed.
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Vascular tissue is responsible for transporting nutrients through the plant. Xylem transports water and minerals up from the roots into the leaves. Phloem transports sugars and other organic molecules from the leaves to the roots for storage or from the roots to other parts of the plant as needed. Vascular tissue in leaves and non woody parts of plants is typically arranged in vascular bundles containing both xylem and phloem.
Both xylem and phloem are complex tissues in that like dermal tissue there is typically more than one type of cell in the tissue. Phloem for instance consists of specialized cells called sieve tube members which conduct the sugar and water. Sieve tube members either lack a nucleus or have just the remains of the nucleus. Associated with the sieve tube members are specialized companion cells move food into and out of the sieve tube members.
Xylem likewise consists of specialized cell types. For example the main water conducting xylem cells are called tracheids and vessel members, both of which are dead when mature. Indeed the term xylem comes from the Greek word for wood, for what we call wood is indeed xylem!
Ground tissue has a wide range of possible functions depending on its location. In leaves there are two types of ground tissue collectively called mesophyll:
Palisade mesophyll is the region of the leaf specialized for photosynthesis, especially the light dependent reactions. This makes sense considering that it is just below the transparent upper epidermis.
Spongy mesophyll is an open and spongy layer specialized for gas exchange. The sponginess provides plenty of surface area through which gas exchange can take place. The cell walls of spongy mesophyll are exceptionally thin.