On the other hand, Anatomy (Ana-as under and tommen- to cut) deals with the ontogeny, structure and distribution of various types of tissues. It is the branch which deals with the study of gross internal structure of plant organs as observed after section cutting. Histology (Greek histos web, logos science) is the study of tissues, their composition, structure and organisation as observed with the help of a microscope. For plants, the terms, anatomy and histology are synonymous and interchangeable.
PLANT ANATOMY
Plant anatomy is the branch which deals with the study of gross internal structure of plant organs as observed after section cutting.Study of this branch started in 1671, when Marcello Malpighi and
N. Grew independently studied the anatomy of vegetable plants. N. Grew is known as Father of
Plant Anatomy
A group of cells having a common origin and co-operating with one another to perform a similar function (or a Set of similar functions) is described as a tissue. All the plant organs viz., roots, stem, leaves, flowers and fruits are made up of different kinds of tissues to perform different functions. Based on the capacity to divide, the plant tissues have been classified into two fundamental types, meristematic and permanent.
Meristematic tissues
The regions of indefinitely dividing capacity or regions of theoretically unlimited growth are called meristems or meristematic tissues.
On the basis of origin, meristems are of two types, primary and secondary. Primary meristems are those that originate from the embryonic meristems or promeristems. They cause the meristematic nature throughout the life of the plant. They are located at the tips of stems, roots and appendages. The secondary meristems originate as new meristems from the permanent tissues which have already undergone differentiation. They do not have their Own promeristem Depending upon their position, meristems are of three types: apical, intercalary and lateral. Apical meristems are present at the tips of stem, root and their branches.They cause growth in length. Intercalary meristems are meristematic regions which are derived from the apical meristems. These help in elongation of the organs and also allow the fallen stems of cereals to become erect.
These are commonly found at the bases of leaves, above the nodes (e.g., grasses) or below the nodes (e.g., mint). Both apical meristems and intercalary meristems are primary meristems because they appear early in life of a plant and contribute to the formation of the primary plant body.
Lateral meristem occurs on the sides and takes part in increasing girth of the plant. They are cylindrical meristems and give rise to secondary tissues that constitute secondary growth. The common examples are fascicular vascular cambium, interfascicular cambium and cork cambium
or phellogen.
Permanent tissues
Permanent tissues are those tissues which have lost the power of cell division. Cells of permanent tissues are matured, assuming a definite shape, size and function. The permanent tissues which develop from primary apical meristem are called primary permanent tissues. The Permanent tissues which develop secondary meristem are called secondary permanent tissues. On the basis of constituent cells permanent tissues are classified into three types: simple tissues, complex tissues and special
Simple permanent tissues
A Simple permanent tissue is made up of similar permanent cells that carry out the same function or similar set of functions. Simple permanent tissues are of three types parenchyma, collenchyma and sclerenchyma.
Parenchyma is phylogenetically and ontogenetically the primitive tissue. The tissue mainly consists of thin-walled living cells which have intercellular spaces between them and their cell wall is made of cellulose or calcium pectate. Each cell possesses a prominent nucleus and
vacuolated cytoplasm.
Collenchyma is a simple permanent tissue of refractile ,non lignified living cells which possess pectin cellulose thickenings in specific areas of their walls. Internally each cell possesses a large central vacuole and a peripheral cytoplasm. Chloroplasts are often present. Depending upon The thickening, collenchyma is of three types - angular ,lamellate and lacunar collenchyma.
Sclerenchyma consists of thick-walled dead cells and possesses hard and extremely thick secondary walls due to uniform deposition of lignin. Sclerenchyma i1s of two types, sclerenchyma fibres and sclereids. The sclerenchyma fibres are highly elongated narrow and spindle-shaped, thick-walled cells with pointed or oblique end walls. The fibres occur in all those parts where mechanical strengths are required, viz., leaves, petioles, cortex and around vascular bundles.
Complex permanent tissues
+The permanent tissues which contain more than one type of cells and work as a unit are called complex tissue .The common complex permanent tissues are conducting tissues, phloem and xylem.
Phloem or Leptome
Phloem is a complex tissue which transports organic food inside the body of the plant. Phloem is also called bast and consists of four types of cells, viz., sieve tubes, companion cells, phloem parenchyma and fibres. Haberlandt (1914) used the term leptome for the conducting part of phloem. The first formed primary phloem consists of narrow Sieve tubes and is referred to as protophloem and the later formed phloem has bigger Sieve tubes and is referred to as metaphloem.
Sieve tubes are elongated tubular conducting channels formed of several cells called sieve tube members or Sieve tube cells. Sieve tube members are placed end to end and have many small pores or sieve pits lined by a layer of callose. Due to the presence of sieve pits these end walls are commonly called sieve plates. On the other hand, a sieve cell is a special kind of cell which possesses Sieve areas in its lateral walls and there is no specialized sieve plate in it. The sieve cells are usually found in pteridophytes and gymnosperms. The sieve tubes are round in angiosperms.
Companion cells are narrow, elongated and thin walled living cells on the sides of the Sieve tubes and are closely associated with them through compound plasmodesmata. The companion cells are absent in pteridophytes and gymnosperms. They are present in angiosperms (both in
monocots and dicots). Sieve tube member and its adjacent companion cells are derived from the same mother cell. Thus death of one results in death of the other as well. Phloem parenchyma are ordinary Living elongated parenchyma cells having abundant plasmodesmata and store food, resins, latex, mucilage, etc. Ihe cells help in slow conduction of food, especially to the sides.Phloem fibres are also called bast fibres and mostly occur in secondary phloem. The fibres have both cellulose and lignified thickenings
.
Xylem or hadrome
Xylem is a complex tissue which performs the function of transport of water or sap inside the plant and also provides mechanical strength. The term xylem (Greek, xylos= wood) was introduced by Nageli (1858). Xylem is also known as wood. It consists of four types of cels, viz., tracheids, vessels (both tracheary elements), xylem or wood parenchyma and xylem or wood fibres. Conduction of sap is carried out by tracheids. The conducting elements of the xylem have been called hadrome by Haberlandt (1914). Tracheids are dead, lignified and elongated cells with tapering ends. Tracheids are commonly found in pteridophytes, gymnosperms and xylem of dicotyledons. Tracheids possess annular, scalariform, spiral, reticulate and border pitted thickenings on their walls. Vessels are present in angiosperms and elongated tubes (2 metres in Quercus) and (3-6 metres in Eucalyptus), which are closed at either end and are formed by the union of several short, wide and thickened cells called vessel elements. Vessels are absent in pteridophytes and gymnosperms. Vessels help in conduction of water and dissolved salts from roots to the different parts of the shoot and provide mechanical support.
Parenchyma assoCiated with xylem is called xylem parenchyma or wood parenchyma. It is made up of simple pits. It helps in lateral conduction of water or sap and stores food (starch, fat) and sometimes tannins. Xylem fibres are sclerenchyma (dead cells) fibres associated with xylem. Xylem fibres add to the mechanical strength of the plant body.
First formed xylem is protoxylem and later formed xylem is metaxylem. Depending upon the position of protoxylem in relation to metaxylem, xylem can be of four types: exarch, mesarch, centrarch and endarch. In each type, protoxylem lies towards the outside of metaxylem. It is inner in the endarch, in the middle of the metaxylem in the mesarch xylem and at the centre of the metaxylem in the centrarch xylem.
Special tissues
Cells concerned with secretion or excretion of materials together constitute secretory tissue or special tissue. They secrete substances like enzymes, nectar, mucilage, Oll, etc.
They occur in different parts of the plant body. Secretory tissues are of two types: glandular tissues,laticiferous tissues. Glandular tissues consist of glands.A gland is a specialised group of cells, capable of secreting some substances. These glands may be external or internal e.g., digestive glands are external glands present in insectivorous plants like, Drosera, Nepenthes, ete. While hydathodes are internal glands.
Laticiferous tissues or laticifers are common in tropica plants. They secrete latex which is an emulsion of proteins, sugars, alkaloids, enzymes, rubber and other substances in the matrix of watery fuid. Most commonly latex is milky white in colour. Laticifers are of two types, latex cells and latex vessels.
TISSUE SYSTEM
A group of tissues performing some common function irrespective of their position in the plant body is called a tissue system. Sachs classified the tissue systems into three types namely epidermal tissue system, ground tissue system and vascular tissue system derived from protoderm, ground tissue system from ground meristem and vascular tissue system from procambium.
Epidermal tissue system
Epidermal tissue system consists of epidermis and epidermal outgrowths. Epidermis is the superficial layer covering the entire surface of the primary plant body. All the epidermal cells are living (parenchymatous) and contain vacuolated protoplasms (a large central vacuole and a peripheral thin cytoplasm). The epidermis 1S cOvered with a cutting layer called cuticle on its outer surface. it is absent in young roots. Root epidermis 1S called rhizodermis or epiblema or piliferous layer.The epidermis of aerial parts usually bears a number of minute pores called stomata.
The spec1alised green epidermal cells present around the stomata are called guard cells.Guard cells in dicots are kidney (bean) shaped and in monocots (grasses) are dumb- bell shaped. Outer wall of guard cells is thin but the inner wall 1s thick and inelastic. Moreover, the cellulose
micelles in guard cell walls are oriented radially rather than laterally to help in opening and closing of stomata. The guard cells are surrounded by two or more epidermal cells called subsidiary cells. When subsidiary cells lie above the guard cells, the stomata are called sunken. Stomatal aperture, guard cell and subsidiary cells together constitute a complex called stomatal apparatus.
Epidermal outgrowths are of two kinds trichomes and emergences. The unicellular or multicellular outgrowths formed from epidermis only are called trichomes. Trichomes may be of various types as hairs, scales, etc. Hair are elongated outgrowths of epidermis present in almost unicellular hair, multicellular hair. The multicellular outgrowths formed from both epidermis and outer most cortical cells are called emergencies. Prickles in Rosa are emergencies. They are sharp and stiff outgrowths. Prickles do not have vascular supply. They protect the plant from excessive transpiration and grazing animals.
Ground tissue system
Ground tissue systems, also known as fundamental tissues, contain various types of tissues excluding epidermis and vascular tissues. Ground tissue system is derived partly from periblem and partly from plerome. Ground tissue system is of two types: an extrastellar ground tissue system and an interstellar ground tissue system. Extrastellar ground tissue system is also called the cortex. Interstellar ground tissue systems include pericycle, pitch and medullary rays.
Vascular tissue system
Central column of the axis (root and stem) is called stele which is made up of a number of vascular bundles, which constitute the vascular tissue system. Each vascular bundle comprises xylem, phloem and cambium (if present).
ANATOMY OF DICOT ROOT
Dicot root consists of a single layer of epiblema orpiliferous cells which bears unicellular root hairs. Some Cells of the epiblema give rise to thin-walled tubular outgrowths called root hairs, also called trichoblasts. Cortex lies below the epiblema and is made up of many layers of thin walled parenchyma cells which enclose intercellular spaces for diffusion of gases. The cells of the cortex store food and conduct water from the epiblema to the inner tissues. Cuticle or stomata is absent in the root.
Endodermis is made up of a single layer of barrel shaped cells without intercellular spaces. The cells are rich in starch grains. Casparian strips, bands of thickenings are present along the tangential and radial walls of endodermis. Due to the presence of casparian strips, the endodermal cells do not allow wall to wall movement of substances between cortex and pericycle. Substances must enter the cytoplasm of endodermal cells. As a result, endodermis functions as a biological
check post.
Endodermis is followed by one (sunflower) or more (mulberry) layers of pericycles. Inner to pericycle lies radially arranged vascular bundles 2-6). Xylem and phloem are equal in number and lie on different radii. In between the two adjacent xylem bundles is found a phloem bundle which is oval in outline. Phloem and xylem bundles are separated from each other by one or more layers of small thin walled cells called conjunctive parenchyma or tissue.
Xylem is an exarch. Later on the conjunctive tissue becomes meristematic to form vascular cambium. Generally pith is absent in root or if present is very small.
ANATOMY OF MONOCOT ROOT
Monocot root is similar to dicot root in having epilema,cortex, endodermis and vascular tissues. In older roots the outer one (e.g., Smilax) or more (e.g., maize) layers of the cortex become thick walled and suberised and constitute the exodermis. It is protective and to some
extent absorptive in function.The number of radial vascular bundles are more than
that in dicot roots. Xylem is exarch and polyarch. The Conjunctive tissue present in between xylem and phloem,stores food and provides mechanical strength but does not form cambium. The centre of the monocot root is occupied by a large well-developed pith.
ANATOMY OF DICOT STEM
Epidermis of dicot stem is made up of compactly arranged, elongated parenchymatous cells, having distinct cuticle, stomata and trichomes (e.g., sunflower). Inner to epidermis 1S present 3-4 layers of collenchymatous hypodermis. Collenchyma cells are green and enclose small intercellular spaces. Inner to hypodermis is the cortex. In the younggreen stem, the outer cortical cells possess chloroplasts (chlorenchyma) and manufacture food. However, the major function of the cortex is storage of food. Endodermis lies at the innermost boundary of cortex and is called starch sheath. Casparian strips are absent in the stem. Inner to endodermis is present a few layers of heterogeneous pericycle (parenchyma and sclerenchyma). A large number of vascular bundles are arranged in a ring. The 'ring arrangement of vascular bundles is a characteristic of dicot stem. Bundle sheath is usually absent. Vascular bundles are conjoint, collateral and open. Cambium helps in secondary growth. Xylem is endarch and fascicular cambium is present. Medullary rays are present in between vascular bundles. The central portion is occupied by pith.
ANATOMY OF MONOCOT STEM
Monocot stem is characterised by single layered epidermis, 2 -3 layered hypodermis and undifferentiated ground tissue system. The outer walls of epidermal cells possess deposition of silica and cutin. Hypodermis is made up of thick walled lignified, non-green sclerenchyma fibres & acts as heat sereen and provides rigidity and mechanical strength to the stem. Ground tissue does not show distinction into cortex, endodermis, pericycle, pith and
pitch rays.
Vascular strand is atactostele (numerous and scattered). Vascular bundles are conjoint, collateral, closed and endarch. A sclerenchymatous bundle sheath is generally
present on the outside of each vascular bundle. Phloem parenchyma is absent. Secondary growth is usually bundle by the dissolution or separation of some protoxylem
vessels and parenchyma cells lying nearby.
ANATOMY OF DICOT LEAF
Dicot leaf shows differences between upper (dorsal) and lower (ventral) surfaces, in the number of stomata and texture etc. Such leaves are called dorsiventral leaves.
Dicot leaf is characterized by upper and lower epidermis, cuticle, mesophyll cells and vascular bundles. The outer walls of epidermal cells are coated with cuticles. Usually the stomata are present more in lower epidermis as compared to upper epidermis. Mesophyll is present in between upper and lower epidermis. Mesophyll is differentiated into palisade parenchyma and spongy parenchyma. The palisade parenchyma lies below the upper epidermis which consists of 1-3 layers of vertically elongated, parallel and closely placed columnar or cylindrical cells. The cells are rich in discoid chloroplasts. They are, therefore, the main site of photosynthesis .The spongy parenchyma or spongy mesophyll lies between the lower epidermis and the palisade parenchyma. They enclose large cavities or intercellular spaces which are connected with the atmosphere through the stomata. Thusa large substomatal cavity lies below each stoma. Vascular bundles (VBs) are scattered in spongy parenchyma and conjoint, collateral and closed. Each vascular bundle is surrounded by a bundle sheath of parenchymatous cells. Xylem is present towards the upper epidermis and phloem towards the lower epidermis.
ANATOMY OF MONOCOT LEAF
.Monocot leaves show similar appearance on both the surfaces. Such leaves are called isobilateral leaves. Monocot leaf is somewhat similar to dicot leaf in having epidermis and cuticle. Stomata are present on both the surfaces of epidermis. The leaf is therefore, Mesophyll is undifferentiated into palisade and spongy parenchyma. Instead it is similar to spongy tissue. The mesophyll cells are chlorenchyma toys and contain a number of chloroplasts. Therefore, mesophyll constitutes the photosynthetic tissue of the leaf. Each vascular bundle is surrounded by a bundle sheath of compactly arranged chlorenchyma cells. Larger vascular bundles bear bundle sheath extensions, which are sclerenchymatous and provide mechanical strength to the leaf. Vascular bundles are conjoint, collateral, closed with phloem towards lower side and xylem towards upper side.The wide midrib does not contain any mesophyll. Above and below larger vascular bundles are present patches of sclerenchymatous cells.
SECONDARY GROWTH
The formation of secondary tissue which leads to increase in girth is called secondary growth. Secondary tissues are formed by two types of lateral meristems - vascular cambium (formed from conjunctive parenchyma and pericycle) and cork cambium or phellogen (formed from pericycle).
Cork cambium or phellogen produces phellem (cork cells) on the outer side and phelloderm on the inner side. Phellem, phellogen and phelloderm together constitute the periderm.
The fascicular cambium and the interfascicular cambium together constitutes the vascular cambium. Vascular cambium produces secondary vascular tissues. The cambial ring cuts off towards the centre and secondary phloem towards the periphery. Secondary xylem consists of vessels, xylem parenchyma and few fibres and secondary phloem consists of sieve tubes, companion cells, phloem parenchyma and fibres. Amount of secondary xylem is more than secondary phloem. Due to secondary growth, stem increases in girth. Cambial cells are of two types: fusiform initials and ray initials. Fusiform initials give rise to medullary rays vessels and tracheids. Ray initials give rise to rays in secondary tissue.
The wood formed in a single year is called annual rings. Spring wood is the secondary xylem formed during spring when the cambial activity is more. Wood elements are larger in size and have wider lumen. Autumn wood (or late wood) is the wood element formed during winter when cambial activity is less. Wood formed is lesser in amount and has a narrow lumen. The age of a tree can be determined by counting annual rings; the process is known as dendrochronology.
In perennial woody trees, the central portion is dark, hard and tough due to deposition of resins, tannins, or dead elements and does not conduct water but provides mechanical support. This central region is called heartwood (duramen). The outer or peripheral portion is soft and lighter in colour consisting of living cells. It is called sap wood (alburnum) and helps in conduction of water and minerals. Wood of gymnosperm is called nonporous or soft wood (absence of vessels and fibres) and that of dicots is called porous or hardwood. Tyloses are balloon-like structures, produced due to ingrowth of xylem parenchyma into the lumen of xylem vessels through pits. Bark is all the tissues outside vascular cambium. There are certain loosely arranged areas in the periderm formed due to rapid activity of phellogen, called lenticels. Lenticels help in gaseous exchange and transpiration.