GROUND TISSUE

The ground tissue system arises from a ground tissue meristem and consists
of three simple tissues: parenchyma, collenchyma, and sclerenchyma (Figure
5). The cells of each simple tissue bear the same name as their respective
tissue.

Figure 5: Cell types and tissues.Encyclopædia Britannica, Inc.

Parenchyma, often the most common ground tissue, takes its name from the
Greek para, meaning beside, and egchnma, meaning the contents of a pitcher
(literally, something poured beside), indicating its ubiquitous nature throughout
the plant body. It forms, for example, the cortex and pith of stems, the
photosynthetic tissue layer within the epidermis of the leaves (mesophyll), the
cortex of roots, the pulp of fruits, and the endosperm of seeds. Parenchyma is
composed of relatively simple, undifferentiated parenchyma cells. In most
plants, metabolic activity (such as respiration, digestion, and photosynthesis)
occurs in these cells because they, unlike many of the other types of cells in
the plant body, retain their protoplasts(the cytoplasm, nucleus, and cell
organelles) that carry out these functions.
Parenchyma cells are capable of cell division, even after they
have differentiated into the mature form. They can therefore give rise to
adventitious buds and roots at some distance from the apical meristem at the
tips of shoots and roots. Parenchyma cells are also capable of further
differentiation into new cell types under appropriate conditions, such as after
trauma. (For information concerning the development of bark during the
secondary growth of tissues, see below Vascular tissue.) Parenchyma cells
are active in secretion, photosynthesis, and water and food storage
(especially in fleshy fruits). They have large fluid-filled vacuoles that maintain
cell turgidity; when a plant wilts, for example, it is because the vacuoles in the
parenchyma cells have lost water and have become flaccid. Thus,
parenchyma also functions in plant support. However, parenchyma cells do
not have a secondary cell wall at maturity and thus remain flexible and
capable of elongation.
Prosenchyma cells are starch-containing parenchymal cells whose cell walls
have become lined with lignin, as occurs in the stems
of Bougainvillea (Nyctaginaceae). A specialized type of parenchyma cell,
called a transfer cell, is involved in the short-distance movement of solutes by
cell-to-cell transfer. Transfer cells occur in association with veins in leaves and
stems and also in many reproductive parts.
Collenchyma tissue (Figure 5) consists of collenchyma cells that also have
retained their protoplasts. They are closely related to parenchyma, although
they have thick deposits of cellulose in their primary cell walls, and the two
types often intergrade in areas of continuity.
Dennis William Stevenson
Collenchyma is found chiefly in the cortex of stems and in leaves. For many
herbaceous plants it is the chief supporting tissue, especially during early
stages of development. In plants in which secondary growth occurs, the
collenchyma tissue is only temporarily functional and becomes crushed as
woody tissue develops. Collenchyma is located along the periphery of stems
beneath the epidermal tissue. It may form a complete cylinder or occur as
discrete strands that constitute the ridges and angles of stems and other
supporting structures of the plant.
Collenchyma cells, polygonal in cross section, are much longer than
parenchyma cells. The strength of the tissue results from the thickened cell
walls and the longitudinal overlapping and interlocking of the cells. The wall is
not uniformly thick in all cells, and thickening may occur predominately in
longitudinal strips at the corners of the cell, on the tangential (i.e., outer,
toward the stem exterior) surface of the cell, or around the spaces
between adjacent cells. Pits are present in the cell wall and provide a
mechanism for intercellular communication. An important feature of
collenchyma is that it is extremely plastic—the cells can extend and thus
adjust to increase in growth of the organ. Because collenchyma cells are alive
at maturity, these thickenings may be reduced when meristematic activity is
resumed as in formation of a cork cambium or in response to wounding.
Sclerenchyma tissue (Figure 5) is composed of sclerenchyma cells, which are
usually dead at maturity (i.e., have lost their protoplasts). They
characteristically contain very thick, hard secondary walls lined with lignin;
consequently, sclerenchyma provides additional support and strength to the
plant body.
The two principal types of sclerenchyma cells are sclereids and fibres.
Sclereids vary in shape and size and may be branched. They are common
in seed coats and nutshells. Apart from providing some internal support for
various plant organs, sclereids deter desiccation of hard seeds, such as
beans, and discourage herbivory of certain leaves.
Fibres are slender cells, many times longer than they are wide. They are
highly lignified cells with tapering (oblique) end walls. The side walls of fibres
are often so thick that the centre of the cell (the lumen) is often occluded.
Fibres have great tensile strength and yet are also elastic. These qualities are
significant in the flexible support of the stems of large herbs and leaves of
many monocotyledons, such as palms. Leaf fibres are the source of abaca, or
Manila hemp (Musa textilis; Musaceae), sisal (Agave
sisalana; Asparagaceae), and many other fibre products. Fibres are found in
various parts of the plant and are particularly common in the vascular tissues
(see below).
Martin Huldrych ZimmermannDennis William Stevenson
Vascular tissue
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.

Tillandsia aeranthosTillandsia aeranthos.Eric J. Gouda

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
energyused 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 elementsare 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.

eucalyptus treeEucalyptus tree.Peter Firus, Flagstaffotos

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 elementsand 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.
Martin Huldrych Zimmermann