Mind Mapping plant structure
All vascular plants will be composed of the same cells and cell types. What makes them different then, is
the arrangement of these cells and the tissues that they form, in the root, stem or leaf. Another factor is,
of course, the function of the tissue.
Fig. 2
Fig. 3
Fig. 1
Fig. 4
Figs. 1-4 above illustrate various aspects of structure in roots stems and leaves. What makes the images
different? The representative tissue systems. Vascular bundles (Fig. 1) can undergo secondary growth
– note significant production of secondary xylem (2X), increases the potential water transport capacity
many-fold, while the phloem tissue (P) seems limited in size. Why is this? Roots (Fig.2) may develop a
conspicuous endodermis. This layer is the innermost layer of the cortex. Immediately beneath it is the
pericycle. Like the interfascicular cambium illustrated in Fig 3, the pericycle is a meristematic zone
which is responsible for the formation of lateral meristems in higher plants. Leaves (Fig. 4) may contain a
high proportion of mechanical tissue as is the case in New Zealand Flax, where extensive U-shaped
sclerenchyma girders overarch the underlying vascular tissue.
All vascular plants contain elements of three
meristematically-active systems. All three are
produced in the shoot and root apical
meristems and sometimes also in the axis of
roots and shoots, These systems are the
dermal, fundamental/ground and vascular
systems respectively.
Although superficially the same the shoot
apical meristem (Fig. 5 left) differs from the root
apical meristem (Fig. 6 right) in several
fundamentally different ways – can you think of
at least three differences, that affect structure
and function?
A dicot stem
Fig. 5
Fig. 6
Red clover (Trifolium sp) is an example of a dicot. In this
micrograph one ring of vascular bundles appears, just
beneath the epidermis. The central region of the stem
consists of parenchyma cells. The cortex is very narrow
and is composed of chlorenchyma.
See The Virtual Plant The Stem, for more details.
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A Monocot Stem
The young Zea mays stem shown here, lacks the
characteristically-thickened sclerenchyma associated with the
outer ring of vascular bundles, seen commonly in old mature
tissues. Here, the tissues surrounding the vascular bundles are
thin-walled and primary. The vascular bundles are
characteristic of monocots, with normally two large
metaxylem vessels and one or two smaller protoxylem
vessels (or lacunae) towards the centre (endarch) are the
most obvious components of the xylem. Tracheids occur
between the metaxylem vessels. Note the large-diameter sieve
tubes in the phloem. Phloem is illustrated clearly in the stem
exercise of The Virtual Plant.
Zea mays typifies a monocotyledonous stem, and the bundles
are described as being closed as no further development of
the vascular system (i.e., secondary vascular tissue formation)
is not possible.
The two examples of stems given here are simply used to illustrate how cells form tissues in organs of
the plant. Some are functional (storage; transport) others function in support or synthesis.
THE TISSUE SYSTEMS
Dermal tissue system
function: produces
(a) protection - simple to complex epidermal cells (secrete wax/suberin protective) may develop
trichomes
(b) functional - gas exchange
Fundamental/ground tissue system
function: produces
(a) storage
(b) functional tissues (photo synthesis) (starch accumulation)
(c) mechanical support tissues
(d) is involved in short & long distance transport
Vascular tissue system
function: involved in short & long distance
transport; mechanical support
produces:
xylem elements [vessel members; tracheids
fiber tracheids, parenchyma].
phloem elements (Fig. 7 right) [sieve tube
members
(angiosperms);
sieve
cells
(gymnosperms);
companion
cells
(angiosperms);
albuminous
cells
(gymnosperms); fibers, parenchyma].
Fig. 7
Each has & occupies specific position, forms
specific cells, forms specific tissues
Location of systems This depends on
(a) the organ studied
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(b) the organ’s age (i.e., juvenile, mature, primary growth only, differentiation of secondary tissues,
mature secondary tissues).
Dermal Always exarch, single to multiple layers of cells simple tissue (cells all parenchymatous)
outermost layers of the vegetative plant body
Fundamental/ground tissue: Always beneath epidermis. In primary stems & roots, will form a cortex,
Can be simple (one cell type one tissue) or complex (several cell types, different tissues). May occupy
central region of the stem (called a pith).
In monocot roots, forms storage tissue, endarch of the vascular strands, can become lignified with age.
Sometimes patchy in young dicot & gymnosperm roots.
In stems: Ground tissue is delimited from the vascular tissue by a starch sheath which forms innermost
layer of the ground tissue.
In roots: Delimited from the vascular tissue by an endodermis which forms innermost layer of the
ground tissue.
In leaves: The bundle sheath forms the separation layer between the mesophyll (the equivalent of the
cortex in stems) and the vascular tissue.
Transport
Vascular: Vascular tissue main function is short- and long-distance transport. Systems are apoplasmic
(the xylem) and symplasmic (the phloem). Apoplasmic transport is driven by the mass flow of water to
leaves due to evapotranspiration. Symplasmic transport driven by mass flow, generated by increased
solute concentrations at a local source and reduction of this at a local sink.
The xylem: Transport channels are vessels and tracheids (tracheids only in Gymnosperms). Xylem
safety is a major consideration as cavitation effects reduced transport capacity.
The phloem Transport channels are sieve tubes (angiosperms) and sieve cells (Gymnosperms).
Companion cells (Angiosperms) and albuminous cells (Gymnosperms) are helper cells, regulating
metabolism and loading into the conducting elements. Parenchyma and in stems, sometimes
sclerenchymatous fibers complete the suite of associated cells.
A final word on structure. The structures that you can see in sections under an average microscope will
all fundamentally, contain the same cell and tissue systems. Using basic histochemistry allows you ro
recognize the cells (and hence the tissues) more easily. However position needs to be considered as
well.
CEJB 2/16/2016
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