Anatomy of the Pea Plant Background Information - SLC Home Page

Anatomy of the Pea Plant
Modified from a lab developed by Margaret E. McCully, Professor of Biology, Carleton University.
Background Information
You will be introduced to plant
anatomy by becoming familiar with the
anatomy of just one plant, the pea (Pisum
sativum). Compared to the anatomy of most
animals, the anatomy of vascular plants is
relatively simple; there are only four major
organs (leaves, stems, roots and flowers) and
four major types of tissues (epidermis,
parenchyma, xylem, and phloem). Although
there is considerable variation in the
arrangement, size, and number of these organs
and tissues, higher plants all have a similar
internal structure.
Different types of plant cells are
distinguished mainly by differences in the
arrangement and/or chemical composition of
their extracellular matrices. When a section of
the stem of a living pea plant is stained with
toluidine blue a number of cell types can be
distinguished because their walls stain
different colors.
Epidermis
Epidermal tissue covers the surface of
most plant organs. The cells are frequently long
and narrow in surface view (Fig 1.). The
margins of these cells vary in outline and may
be straight or curvy, depending on the species.
Figure 1. Surface view of epidermal cells. (Bracegindle
and Miles. 1977)
One of the earliest adaptations of plants
was the production of a waxy cuticle by
epidermal cells of stems and leaves. Although
the cuticle provides protection from
desiccation, it also blocks gas exchange, thus a
complementary early adaptation was the
production of guard cells. These cells are
produced when two immature epidermal cells
with adjoining end walls divide asymmetrically
to produce two small, adjacent, frequently
kidney-shaped cells. These are guard cells
(Fig. 2.). Early in their development the part of
the wall between adjacent guard cells comes
apart and a pore or stoma (pl. stomata) is
formed which opens directly into the interior
of the leaf. Guard cells control the size of the
stomata openings and hence the exchange of
CO 2 , oxygen and water vapor across leaf and
stem surfaces.
Anatomy of the Pea Plant 1

guard cell
stomatal pore
cytoplasm
nuclei
vacuole
chloroplasts
nucleus
chloroplasts
Figure 2. Stomata and their guard cells. (Bracegindle
and Miles. 1977)
Parenchyma Tissue
Most of the plant cells you will see in
your thin sections will be parenchyma tissue.
These cells always contain one or more large
vacuoles and their cytoplasm is limited to a
narrow peripheral layer and thin cross strands
(Fig 3.). The walls of these cells are thin and
are composed mainly of cellulose and pectin.
Check the staining key (Table 2, Hand Cut
Sections) to guess what color they stain with
toluidine blue.
Most of the parenchyma cells in the
above-ground parts of plants contain
chloroplasts and are thus sites of
photosynthesis. Both green and non-green
parenchyma cells store metabolites, notably
starch.
The vascular tissues: xylem and phloem
Another early plant adaptation to land
was the development of vascular tissue which
allows the transport of nutrients and water, as
well as provides support.
Figure 3. Parenchyma cells. (Bracegindle and Miles. 1977)
Xylem tissue is unique in that when it
is mature it contains mainly dead cells. In
general, xylem is composed of long, narrow,
tubular cells. When the cells are young (and
still alive) their walls (called primary wall) are
quite thin and stain pink with toluidine blue
When these cells mature (i.e. they are no
longer enlarging), they produce a secondary
wall which is often laid down in a spiral
pattern. Eventually, lignin appears in xylem.
This hard, decay-resistant polymer provides
strength to plant organs. The ultimate
development of xylem occurs in trees where
most of the wood is composed of dead xylem
tubes. Lignified walls stain green with
toluidine blue.
Phloem is a complex tissue composed
of a number of cell types, the most important
being sieve tubes and companion cells. In
some plants the phloem contains long heavywalled
phloem fibers. Phloem is found
throughout a plant in association with xylem.
Sieve tubes are narrow, elongated,
roughly cylindrical cells which remain alive at
Anatomy of the Pea Plant 2

maturity. They are however, unique cells in
several respects. At maturity they lose their
nuclei (living, functioning enucleated cells are
rare in either plants or animals). Maturing
sieve tubes produce enzymes which dissolve
portions of their end walls to produce a sievelike
plate (sieve plate) between adjacent cells
in each file of sieve tubes (Fig. 4.).
Figure 4. Phloem tissue. (Bracegindle and Miles. 1977)
The side walls of sieve tubes are thicker
than those of parenchyma cells but unlike
those of xylem cells they are of uniform
thickness and do not become lignified. The
sieve tube walls are rich in cellulose but
contain rather little pectin so are relatively
unstained by toluidine blue.
In most plants (angiosperms) each sieve
tube has beside it a companion cell. These are
smaller in diameter and shorter than the sieve
tube and intimately connected with it by
numerous plasmodesmata (holes in the cell
wall which connect adjacent cells). The
companion cell is alive at maturity and
contains a nucleus, plastids (usually
chloroplasts), mitochondria, abundant
ribosomes and often many small vacuoles.
Phloem fibers are narrow, highly
elongated cells with tapered ends which are
dead at maturity. Before death these cells
secrete a uniformly thick secondary wall which
becomes lignified. The lignin of phloem fibers
appears to be different chemically from that of
xylem walls since it stains a bright, light blue
color with toluidine blue.
See your textbook for more photos and
information on plant anatomy.
Note:
• tissues are composed of more than one
type of cell
• organs are composed of more than one type
of tissue
References
Bracegindle and Miles. 1977. An Atlas of Plant Structure Vol.
1. London: Heinemann Educational Books Ltd.
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