Evolution of the transport process

Water and nutrients flow through conductive tissues (xylem and phloem) in plants just as the bloodstream distributes nutrients throughout the bodies of animals. This internal circulation, usually called transport, is present in all vascular plants, even the most primitive ones.

The importance of transport processes in plants increased as multicellular plants evolved and became larger and their tissues acquired specialized functions. As land plants developed, long-distance transport assumed an important role; not only are carbohydrates transported from the organs in which they are formed (the leaves) to other parts—such as reproductive organs (flowers and fruits), stems, and roots—but water and minerals must be transported to leaves, which are not submerged in water (as are those of most primitive nonvascular plants) but are in a relatively dry air environment. Highly developed land plants have two types of tissues specialized for long-distance transport: the xylem and the phloem. Water and dissolved mineral nutrients ascend in the xylem (the wood of a tree, such as an oak or a pine), and products of photosynthesis, mostly sugars, move from leaves to other plant parts in the phloem (the inner bark of a tree).

Evolving land plants faced not only the problem of transport but also the problem of supporting their weight. Aquatic plants are supported by their buoyancy in water and do not need a rigid stem; flotation devices such as gas-filled stomata and intercellular spaces hold them upright and enable them to grow toward the water surface and obtain sufficient sunlight for photosynthesis. On land, a rigid, self-supporting structure is necessary for plants; this structure, the xylem, consists of tiny rigid tubes through which water and dissolved mineral nutrients can move. The rigidity of the tubes within a stem is sufficient to make it self-supporting.

Land plants take up water from the soil through the roots; some exceptions, such as some desert plants that grow in dry soil and epiphytes, which grow in tree canopies, rely on adaptations that enable them to obtain water from the air. In most plants, then, water ascends through the xylem, the tiny capillaries of the woody stem tissue, into all plant parts but primarily into the leaves, from which it is transpired (evaporated) into the air. In this way, the mineral nutrients are transferred from the soil to all above-ground plant parts.

Plants living in humid habitats, such as the small and primitive mosses and liverworts, do not have a well-developed xylem, but rather have similar cells called hydroids that lack true lignin. Similarly, water plants that have returned from land to an aquatic habitat during evolution have a reduced xylem; such plants, which have readapted to an aquatic environment, are not woody, because they need neither water-conducting tissues nor a self-supporting structure. On the other hand, tall land plants such as trees, vines, and lianas have the most highly developed long-distance transport systems. Vines and lianas differ from trees in that their xylem serves primarily for water conduction; they depend, for the most part, on other plants for support. Certain vines are of great length (a few hundred metres) and have extremely highly developed tissues for transporting water and nutrients.

Most of the material that composes a plant’s dry weight is a consequence of photosynthesis, in which light energy is converted into chemical energy used to synthesize organic substances. Carbon dioxide from the air and water, which the plant takes from the soil, are utilized during photosynthesis, which occurs mostly in green plant parts—especially the leaves. Since plants shed their leaves either continuously or periodically but still increase in size, it is clear that many photosynthetic products must be transported out of the leaves and carried to all other plant parts; this process takes place primarily in the phloem.

The discovery of the functions of xylem and phloem was made following that of the circulation of blood in the 17th century. By the early 19th century, it had been established that water ascends from roots into leaves through xylem and that photosynthetic products descend through phloem. Experiments now called girdling experiments were performed, in which a ring of bark is removed from a woody plant. Girdling, or ringing, does not immediately interfere with upward movement of water in the xylem, but it does interrupt phloem movement. In some plants surgical removal of phloem is difficult; in this case phloem may be killed by using steam (steam girdling). Xylem conduction is normally not affected by such treatment, and movement in the two transport tissues can thus easily be distinguished. Girdling experiments, however, are not entirely foolproof. The question as to whether or not mineral nutrients can ascend in the phloem illustrates the kinds of difficulties that may be encountered. Much smaller amounts of mineral nutrients reach the leaves in girdled plants than in ungirdled ones. From this observation it might be concluded that some nutrients ascend in the phloem of ungirdled trees; girdling, however, interrupts the flow of sugars into roots. Roots are thereby starved and take up fewer mineral nutrients; the reduced flow of mineral nutrients to the leaves of girdled plants can thus be explained as a secondary effect.

Structural basis of transport

Two features of plant cells differ conspicuously from those of animal cells. In plant cells the protoplast, or living material of the cell, contains one or more vacuoles, which are vesicles containing aqueous cell sap. Plant cells are also surrounded by a relatively tough but elastic wall. Water entering the vacuole by osmosis (i.e., movement of water across a membrane from regions of higher water concentration into regions of lower water concentration that normally contain dissolved substances, such as cell interiors) expands the protoplast and consequently the cell wall until the internal pressure is balanced by the elastic counterpressure of the wall. Spaces between and within cell walls are sufficiently large to permit water to flow around all cells. The space available for free water flow is called apoplast. Water in apoplast originates from the roots and contains nutrients taken up by them. Nutrients enter a cell by crossing the outer cytoplasmic membrane (plasma membrane).

Most of the metabolic activities of the cell—the chemical reactions of living systems—occur within protoplasts. Substances can enter a protoplast by their cytoplasmic connections between neighbouring cells (plasmodesmata) or by active transport mechanisms requiring energy and a group of enzymelike compounds called permeases. Plasmodesmata may penetrate neighbouring cell walls at areas called primary pit fields. Also, some substances pass out of cells into the apoplast and are transported by energy-requiring processes into the protoplast of another cell.

Cell-to-cell transport takes place in all plants, but it is a slow process; the higher plants evolved the specialized tissues, xylem and phloem, for rapid long-distance transport. The woody tissue, xylem, contains highly specialized cells for water conduction. The cells are long and reinforced by strong, woody (lignified) walls; their protoplast breaks down and dissolves after wall growth is completed, so that the entire inside of the cell becomes available for rapid water conduction. In other words, the water-conducting cells of xylem are dead when functional. In the more primitive conifers the xylem consists largely of spindle-shaped cells called tracheids, which have a diameter around 0.04 millimetre (0.0016 inch) and a length of about 3 millimetres (0.12 inch). Flowering plants have a more highly specialized xylem, in which the mechanical function and the water-conduction function have been separated during evolution. Tracheids, the primitive conducting cells, have evolved into fibres for mechanical strength and vessels for water conduction, particularly in angiosperms. Vessel elements are barrellike cells with widths of up to 0.5 millimetre (0.02 inch) in some plants. Vessel elements are arranged end to end; their end walls are partly or wholly dissolved, and rows of such cells thus form long capillaries (tubes) up to several metres in length. These tubes are the vessels.

Numerous vessels of limited length thus provide a certain protection against injury—that is, since water pressures in the xylem are often well below zero (i.e., the water is under tension), air will be sucked into any injured xylem vessel and spread immediately throughout it but cannot pass through the wet pit membranes into the uninjured units. Damage is thus confined to the units that are injured and cannot easily spread. In addition, the smaller the conducting unit, the more confined is the damage. Plants with large, highly efficient vessels are much more vulnerable to injury, as is evident, for example, from the vulnerability of the elm, which has large vessels, to Dutch elm disease, in which the water-conduction vessels are injured by beetle activity and fungal growth. In general, both the less efficient but safer coniferous wood and the more highly efficient but more vulnerable wood of flowering plants have been successful during evolution. Very tall trees occur in both groups—e.g., Sequoia among the conifers and Eucalyptus among the flowering plants.

The conducting elements of the phloem underwent evolutionary changes somewhat similar to those of the xylem. The conducting elements of conifers, called sieve cells, are similar in shape and dimensions to tracheids. They do not have a woody wall, however, and they are alive at functional maturity even though their cytoplasm may be highly specialized and the cells have usually lost their nucleus during development. In flowering plants the conducting elements in the phloem are called sieve elements and consist of sieve cells and sieve-tube members, the latter differing in having some sieve areas specialized into sieve plates (generally on the end walls). Sieve-tube members are arranged end to end to form sieve tubes, a name derived from the sievelike end walls through which passage of food from one cell to the next occurs. Sieve elements are almost invariably accompanied by special companion cells believed to control, to a certain extent, the metabolism of the nucleus-free conducting cells.