Download Anatomy of the Pea Plant

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
Figure 1. Surface view of epidermal cells. (Bracegindle
and Miles. 1977)
the
One of the earliest adaptations of plants
arrangement, size, and number of these organs
was the production of a waxy cuticle by
and tissues, higher plants all have a similar
epidermal cells of stems and leaves. Although
internal structure.
the
cuticle
provides
protection
from
Different types of plant cells are
desiccation, it also blocks gas exchange, thus a
distinguished mainly by differences in the
complementary early adaptation was the
arrangement and/or chemical composition of
production of guard cells.
their extracellular matrices. When a section of
produced when two immature epidermal cells
the stem of a living pea plant is stained with
with adjoining end walls divide asymmetrically
toluidine blue a number of cell types can be
to produce two small, adjacent, frequently
distinguished
kidney-shaped cells. These are guard cells
because
their
walls
stain
different colors.
These cells are
(Fig. 2.). Early in their development the part of
the wall between adjacent guard cells comes
Epidermis
apart and a pore or stoma (pl. stomata) is
Epidermal tissue covers the surface of
formed which opens directly into the interior
most plant organs. The cells are frequently long
of the leaf. Guard cells control the size of the
and narrow in surface view (Fig 1.). The
stomata openings and hence the exchange of
margins of these cells vary in outline and may
CO2, oxygen and water vapor across leaf and
be straight or curvy, depending on the species.
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)
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
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
toluidine blue.
Most of the parenchyma cells in the
chloroplasts
Xylem tissue is unique in that when it
is mature it contains mainly dead cells. In
Parenchyma Tissue
above-ground
Figure 3. Parenchyma cells. (Bracegindle and Miles. 1977)
parts
and
of
are
plants
thus
contain
sites
of
photosynthesis. Both green and non-green
parenchyma cells store metabolites, notably
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
starch.
being sieve tubes and companion cells. In
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.
Anatomy of the Pea Plant
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
2
maturity. They are however, unique cells in
tube and intimately connected with it by
several respects. At maturity they lose their
numerous plasmodesmata (holes in the cell
nuclei (living, functioning enucleated cells are
wall which connect adjacent cells). The
rare in either plants or animals). Maturing
companion cell is alive at maturity and
sieve tubes produce enzymes which dissolve
contains
portions of their end walls to produce a sieve-
chloroplasts),
like plate (sieve plate) between adjacent cells
ribosomes and often many small vacuoles.
in each file of sieve tubes (Fig. 4.).
a
nucleus,
plastids
mitochondria,
(usually
abundant
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.
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.
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
In most plants (angiosperms) each sieve
tube has beside it a companion cell. These are
smaller in diameter and shorter than the sieve
References
Bracegindle and Miles. 1977. An Atlas of Plant Structure Vol.
1. London: Heinemann Educational Books Ltd.
Anatomy of the Pea Plant
3