Plant Anatomy
Introduction
“The study of gross internal structure of plant
organs by the technique of section cutting is called plant anatomy (ana
= asunder ; temnein = to cut)”. Plant Anatomy deals with the study of internal
structure of the various organs of the plant. It includes the structure of
cell, which makes the basic unit of all living organisms including plants. The
cells constitute tissues and their detailed study is called histology.
To study plant anatomy one should be quite familiar with the study of tissues,
i.e., histology. In modern days, with the discovery of transmission electron
microscope and the scanning electron microscope many interesting features have
been discovered.
Historical Sketch
The study of the plant anatomy began in the seventeenth century in the hands of two men who worked quite independently of each other; Marcello Malpighi (1628-94) and Nehemiah (Grew 1641-1712). It is also remarkable that preliminary treatises by the two men were presented to the Royal Society of London, of which Grew was Secretary, on the same day, 7th December 1671. Sherwin Carlquist writes in his article plant Anatomy’s 300th Anniversary. “As the fore page of Grew’s The Anatomy of Vegetables Begun indicates, Grew’s paper was read on November 9, 1671, before the Royal Society of London. Sachs (1890) tells us that Grew presented his manuscript in May 1671, and that Malpighi’s work was received by the Royal society on December 7, 1671. However, Malpighi’s (1675) Anatome Plantarum gives the date and place of writing as November 1, 1671 and Bologna. Grew’s The Anatomy of Vegetables Begun was published on December 7, 1671, according to Sachs, although the title page bears the date 1672. We know that both Malpighi and Grew were studying aspects of plant Anatomy several years before 1671. The various 1671 dates cited above are all close, however, and we may, along with Sachs, cite 1671 as the year in which the "Anatomy of vegetables’ was indeed ‘begun’, although I am citing his book here as Grew (1672)".
Grew never reached a correct idea of cell structure and believed that the plant was composed of lace-work of interwoven threads (hence the term “tissue”), which were no doubt the profiles of the* cell walls as seen in sections.
Sherwin
Carlquist quotes further in his article, “Hooke 's Micrographia of 1667 cannot
be called the first volume to be published in the field of plant anatomy,
despite Hooke's figuring of cork cells, and giving them that term.”
In eighteenth century Kaspar Friedrich Wolff (1733—94), studied meristems and tried to formulate a theory of apical development, and Sir John Hill (1716-75), published a book on the structure of timber in 1770.
In
the beginning of nineteenth century C.F. Brisseau-Mirbel (1776-1854), published
a theory of plant organization in 1802. J.J. Bernhardi (1774-1850), published
his work on angiospermic vessels. The other workers of nineteenth century are
Kurt Sprengel (1766-1833); K.A. Rudolphi (1771-1832), H.F. Link (1767-1851) and
L.C. Treviranus (1779-1864). Hugo von Moh! (1805-72) and Carl von Naegeli
(1817-91) were held responsible for modern outlook founded on a clear
preception of cellular structure, based upon the cell theory, as it was
elaborated by Schleiden and Schwann.
The Plant Body And Its Development
The plant body consists of a number of organs, i.e., root, stem, leaf and flower. The flower consists of sepals, petals, stamens, carpels and sometimes also sterile members. Each organ, is made up of a number of tissues. Each tissue consists of many cells of one kind.
The complex multicellular body of the seed plant is a result of evolutionary specialization of long duration. This specialization has given rise to the establishment of morphological and physiological differences between the various parts of the plant body and also caused the development of the concept of plant organs.
Fig. 1.1. Different stages in the germination of the seed leading to the formation of young seedling (A-G); G, seedling with well developed roots, two young leaves, hypocotyl, cotyledons, stem and young apical bud.
The
organization of the plant body of the oldest known land plants, the
Psilophytales, suggests that the differentiation of the vegetative plant into
leaf, stem and root is a result of evolutionary development from an originally
simple axial structure (Arnold, 1947; Eames, 1936). As regards the morphologic
nature of the flower it is thought that the flower is homologous with a shoot
and the floral parts with leaves. .
Fundamental parts of the plant body
The axis, consists of two parts—that portion which is normally aerial is known as the stem, and the portion which is subterranean is called the root. There are three types of appendages arising from the axis. 1. Leaves—The strands of vascular tissue pass through the leaves. The leaves are characteristic of the stem and do not occur on the root. The leaves are found to be arranged on the stem in a definite manner, and bear an intimate structural relation to the skeleton of the axis. The leaf is looked upon as the lateral expansion of the stem, continuous with it. All fundamental parts of the stem are concerned with the formation of the leaf. 2. Emergences—In the appendages of the second rank only the outermost layers of stem, the cortex and the epidermis, are usually present which are known as emergences. The prickles of the rose make a good example of it. 3. Hairs—The appendages of the third rank are hairs. These are projections of the outermost layer of the cells. The emergences and hairs occur on both axis and leaves, usually without definite arrangement.
Development Of The Plant Body
A vascular plant begins its existence as a morphologically simple unicellular zygote (2n). The zygote develops into the embryo and thereafter into the mature sporophyte. The development of the sporophyte involves division and differentiation of cells, and an organization of cells into the tissues and tissue systems. The embryo of the seed plant possesses relatively a simple structure as compared with the mature sporophyte. The embryo bears a limited number of parts—generally only an axis bearing one or more cotyledons. The cells and tissues of this structure are less differentiated. However the embryo grows further, because of the presence of meristems, at two opposite ends of the axis, of future shoot and root. After the germination of the seed, during the development of the shoot and root, the new apical meristems appear which cause a repetitive branching of these organs, After a certain period of vegetative growth, the reproductive stage of the plant is attained.
Primary And Secondary Growth
As mentioned above,
this first-formed plant body is known as the primary plant body, since it is built up by means of first or primary growth. The tissues of this
first-formed body are known as primary
tissues ; for example the first-formed xylem is called primary xylem. In most vascular cryptogams and monocotyledons, the
entire life cycle of the sporophyte is completed in a primary plant body.
The gymnosperms, most dicotyledons, and
some monccotyledons show an increase in thickness of stem and root by means of
secondary growth. The tissues formed as the result of secondary growth are
called secondary tissues. Generally the new types of cells are not formed by
means of secondary growth. The bulk of the plant increases because of secondary
growth. Especially the vascular tissues are developed which provide new
conducting cells and additional support and protection. The secondary growth
does not fundamentally change the structure of the primary body. The primary
growth increases the length of the axis, forms the branches and builds up the
new or young parts of the plant body. Thus, a secondary body composed of
secondary tissues is added to the primary body composed of primary tissues.
A special meristem, the cambium, is concerned with the secondary thickening. The cambium arises between the primary xylem and the primary phloem, and lays down new xylem and phloem adjacent to these. Thus the secondary masses of xylem and phloem are found entirely within the central cylinder and between the primary xylem. The newly formed secondary xylem cloaks and ultimately surrounds the primary xylem and the pith. During this process the primary structure is not changed but engulfed intact within secondary xylem. The primary phloem and all other tissues outside the cambium are forced outward by secondary growth and ultimately crushed or destroyed.
Fig. 1.4. T.S. through the shoot apex of Ranunculus acris.
In-addition, a cork cambium or phellogen commonly develops the peripheral region of the axis and produces a periderm, a secondary tissue system assuming a protective function when the primary epidermal layer is disrupted during the secondary growth in thickness.
The primary growth of an axis is
completed in a relatively short period, whereas the secondary growth persists
for a longer period and in a perennial axis the secondary growth countinues
indefinitely.
The stem apex like the
root apex consists of a meristematic zone of cells that remain in a continuous
and rapid state of division. This is called promeristem having cells with very thin walls.
Immediately beneath the
promeristem there is zone of
determination which has no visible boundary with the promeristem. In the
dicots, this zone has a group of conspicuous cells with dense cytoplasm. These
cells in a transverse section are arranged in a circle (fig. 1.4.). It is a
remnant of a primordial meristem, which remains behind in a maturing segment,
and it retained its activity to divide. Due to its circular appearance it is
also called the ring meristem. The
cells in the centre are protopith and those that are external to the ring
meristem are the protocortex. The
cells of protocortex and protopith divide and build up the mass of ground tissue. The cells of the ring
meristem divide longitudinally and form elongated cells that later on develop
in vascular bundles and are known as procambical strands. The first formed
phloem elements and so the xylem elements differentiate from the procambial
strands.
Fig. 1.5. Primary and
secondary growth in a dicot plant A, longitudinal view of plant; B, transverse
section of stem; C, transverse section of root.
Internal Organization Of Vascular Plant
The cells or the morphologic units of the plant body are associated in various ways with each other forming tissues. In the plant body the cells are of several kinds and their combinations into «tissues are such that different parts of the same organ may differ from one another. The larger units of tissues may show topographic continuity or physiologic similarity, or both together. Such tissue units are called tissue systems. Thus the complex structure of the plant body results from variation in the form and function of cells and also from differences in the type of combination of cells into tissues and tissue systems. As pointed out by Sachs (1875), the plant body of a vascular plant is composed of three systems of tissues—(1) the dermal (2) the vascular and (3) the fundamental or ground system (See details in chapter 7, The Tissue System).
The three vegetative organs, i.e.,
stem, root and leaf, are distinguished by the relative distribution of the
vascular and ground tissues. The vascular system of the stem is found between the
epidermis and the centre of the axis. In such type of arrangement the cortex
(ground tissue) is found between the epidermis and the vascular region and the
pith in the centre of the stem (Fig. 1.6 B,C). In the root, the pith may be absent
(Fig. 1.6 E), and the cortex is generally shed during secondary growth (Fig.
1.6 D). The primary vascular tissues are commonly being arranged in the form of
a ring of bundles as seen in transverse section of stem (Fig. 1.6 B). During
secondary growth the original primary vascular system may be obscured by secondary
vascular tissues between the primary xylem and the primary phloem (Fig. 1.6 C).
In the leaf the vascular system consists of many interconnected strands
(bundles) found in the ground tissue. In the case of leaf the ground tissue
consists of photosynthetic parenchyma, and is known as mesophyll (Fig. 1.6 G).
The above mentioned tissue systems of
the primary plant body are derived from the apical meristems (Fig. 1.6 F,H).
The partly differentiated derivatives from these meristems may be classified as—protoderm,
procambium and ground meristem. They make the meristematic precursors of the
dermal, vascular and fundamental (ground) systems, respectively. The vascular tissue
system enlarges by secondary growth which takes place in the vascular cambium.
The periderm may be derived from a separate meristem, the phellogen or cork
cambium (Fig. 1.6 D).
Cell Types And Tissues
The
cells of a plant derived from a meristem acquire their distinctive
characteristics through developmental stages, and they become specialized to
varied degrees. The distinctions among cells and tissues are summarized here:
Fig. 1.6.
Organization of a vascular plant. A, habit sketch of linseed plant (Linum usitatissimum) in vegetative
state; B-C, transverse sections of stem; D-E, transverse sections of root; F,
L.S. of shoot apex with apical meristem and developing leaves; G, transaction
of leaf lamina; H, L.S. of root apex with apical meristem and other root
regions.
Epidermis
Fig. 1.7. Epidermis. Stereoscopic view of epidermis. The outer walls of the cells are generally thickened and convex as seen in this fig.
The
epidermal cells make a continuous layer on the surface of the plant body in the
primary state. Very often they are tabular in shape. Other specific epidermal
cells are guard cells, various trichomes and root hairs. The cuticle is present
on the outer wall of the epidermal cells of the aerial parts of the plant. The
epidermis is commonly supplanted by the periderm after secondary growth of
stems and roots.
Periderm
The
periderm consists of phellem (cork),
phellogen (cork cambium) and
phelloderm. The phellogen develops in
the epidermis, the cortex, the phloem or
the root pericycle and produces phellem
toward the outside and phelloderm
toward the inside. Commonly the cork
cells are tabular in shape. The cells
of phelloderm are usually parenchymatous.
Fig. 1.8. Parenchyama. Stereoscopic view of pith parenchyma, with three of the cells in tangential aspect.
Parenchyma
The parenchyma cells
are characteristically living cells
capable
of growth and division. The cells vary
in
shape and are generally polyhedral or
rounded. These cells form continuous tissues in the cortex of
stem and root and in the leaf
mesophyll. Parenchyma
cells are concerned with photosynthesis,
storage, wound healing, and
origin of adventitious structures.
Collenchyma
Fig.1.9. Collenchyma. Stereoscopic view of collenchyma, with two cells in tangential aspect.
Collenchyma cells make a living tissue
closely related to parenchyma. It is regarded as a form of parenchyma
specialized as supporting tissue of young organs. The cells are
prismatic to much elongated in shape. The presence of unevenly thickened primary walls is the most distinctive character. The cells occur in strand or continuous cylinders near the surface of the cortex in stems and petioles and along the veins of leaves.
Sclerenchyma
The sclerenchyma cells posses thick, secondary lignified walls and lack protoplasts at maturity. They make strengthening elements of mature plant parts. They may occur in continuous masses or in small groups or individually among other cells. Two forms of cells, i.e., sclereids and fibres are distinguished.
Xylem
Xylem cells make a complex tissue
which, in association with the phloem, is
continuous throughout the plant body.
It may be primary or secondary in origin. It is concerned with’ water
conduction, storage and support. The principal conducting cells are the
tracheids and the vessel members.
Phloem
Phloem cells make a complex tissue.
This tissue occurs throughout the plant body,
in association with the xylem. It may
be primary or secondary in origin. It is mainly concerned with translocation of
solutes and storage of food. The principal conducting cells are the sieve cells
and “sieve tubes. Sieve tubes are associated with companion cells.
Laticifers
They contain latex and are
multinucleate. There are two kinds of laticifers—
articulated and non-articulated. The
articulated laticifers are formed through union of cells in which parts of the
walls are dissolved. The non-articulated laticifers are single cells, usually
much branched. The articulated laticifers may be primary or secondary in origin
whereas the non-articulated are primary in origin. They are restricted to
certain angiospermic families.
References
Pandey, B.P. Plant Anatomy, Latest Ed., S. Chand & Company
Fahn A (1982). Plant Anatomy. Pergamon press
Dickison W.C. 2000. Integrative Plant Anatomy. Harcourt Academic
Press. ISBN-13: 978-0-
12-215170-5
Botany Knowledge
All PDF which are
provided here are for Education purposes only. Please utilize them for building
your knowledge and don’t make them Commercial. We request you to respect our
Hard Work. We are Providing Everything Free Here. BotanyKnowledge.com Will
Not Charge Any Cost For Any Service Here.
E-mail: ranjanshaweditor@gmail.com
BotanyKnowledge.com does not own this book, neither created nor scanned. we just providing the links already available on Internet. if any way it violates the law or has any issues then kindly contact us. Thank you.
