The following points highlight the two main types of permanent tissues of plants. The types are: 1. Simple Permanent Tissues 2. Complex Permanent Tissues.
Type # 1. Simple Permanent Tissues:
A simple permanent tissue is that tissue which 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.
i. Parenchyma:
(Gk. para- beside, engchyma-tissue):
Parenchyma is a simple permanent living tissue which is made up of thin-walled similar isodiametric cells. It is the most abundant and common tissue of plants. Typically the cells are isodiametric (all sides equal). They may be oval, rounded or polygonal in outline.
The cell wall is made up of cellulose. Cells may be closely packed or have small intercellular spaces for exchange of gases (Fig. 6.7 B). Internally each cell encloses a large central vacuole and a peripheral cytoplasm containing nucleus. The adjacent parenchyma cells are connected with one another by plasmodesmata. They, therefore, form symplasm or living continuum.
Parenchyma is morphologically and physiologically un-specialised tissue that forms the ground tissue in the non-woody or soft areas of the stems, leaves, roots, flowers, fruits, etc. The typical parenchyma is meant for the storage of food, slow conduction of various substances and for providing turgidity to the softer parts of the plants.
It is modified variously to perform special functions:
(a) Fibre-like elongated parenchyma is called prosenchyma. It is slightly thick walled and is meant for providing rigidity and strength.
(b) Cutinised parenchymatous cells form a protective covering layer or epidermis. Epidermis is single layered. Intercellular spaces are absent. The cutin also forms a distinct layer on the outer surface of epidermal cells. It is called cuticle (Fig. 6.7 E). It reduces transpiration.
(c) The young parts of the root are covered by a layer of un-thickened and un-cutinised parenchyma cells, some of which give rise to tubular outgrowths called root hairs. It is known as piliferous layer or epiblema. This layer is specialized to absorb water and mineral salts from the soil.
(d) Xylem parenchyma is made of small and often thickened cells. It helps in the storage of food and lateral conduction of water (Fig. 6.7 D).
(e) Phloem parenchyma is formed of thin-walled elongated parenchymatous cells. It takes part both in the storage and lateral conduction of food.
(f) Parenchyma cells containing chloroplasts are collectively termed as chlorenchyma. It takes part in the manufacture of food. Chlorenchyma of leaves is called mesophyll. It is differentiated into two parts, palisade parenchyma and spongy parenchyma (Fig. 6.7 F). Cells of palisade parenchyma are columnar in shape while those of spongy parenchyma are often lobed, rounded or irregular in outline.
(g) A special parenchyma tissue is found in the aquatic plants and some land plants (e.g., petiole of Banana, Canna). It is known as aerenchyma (Fig. 6.7 G). It consists of a network of parenchyma cells which enclose very large air cavities. These air cavities store gases and make the aquatic plants light and bouyant.
(h) Storage parenchyma is made of large sized vacuolate cells which store water, mucilage and food, e.g., Aloe, Opuntia Potato tuber.
(i) Idioblasts are specialized non-green large-sized parenchyma cells which possess inclusions or ingredients like tannins, oils, crystals, etc.
(j) Secretory cells are specialized parenchyma cells that produce nectar, oil, etc.
Functions:
(i) Storage of food,
(ii) Providing turgidity to softer parts,
(iii) Providing rigidity to tissues when prosenchymatous.
(iv) Protection and checking water loss in the form of epidermis,
(v) Formation of water absorbing epiblema in root,
(vi) Lateral conduction in the form of xylem and phloem parenchyma
(vii) Photosynthesis in the form of chlorenchyma.
(viii) Providing buoyancy and storage of metabolic gases in the form of aerenchyma.
(ix) Secretion.
ii. Collenchyma:
(Gk. kolla— glue, enchyma— tissue):
Collenchyma is a simple permanent tissue of retractile non-lignified living cells which possess pectocellulose thickenings in specific areas of their walls. The cells appear conspicuous under the microscope due to their higher refractive index.
The cells are often elongated. They are circular, oval or angular in transverse section. Internally each cell possesses a large central vacuole and a peripheral cytoplasm. Chloroplasts are often present. Wall possesses uneven longitudinal thickenings in specific areas.
Depending upon the thickening, collenchyma is of three types:
(i) Angular Collenchyma:
The thickenings are present at the angles (angular thickenings), e.g., stem of Tagetes, stem of Tomato (Fig. 6.8 B).
(ii) Lamellate Collenchyma:
The thickenings occur on the tangential walls (plate thickenings), e.g., stem of Sunflower (Fig. 6.8 A),
(iii) Lacunate Collenchyma:
The thickenings are found on the walls bordering intercellular spaces (lacunate thickenings), e.g., Cucurbita stem (Fig. 6.8 C).
Collenchyma is found below the epidermis in the petiole, leaves and stems of herbaceous dicots, forming either continuous layers or occurring in patches, especially in the region of ridges (e.g., Gourd).
Functions:
(i) It provides mechanical strength to young dicot stems, petioles and leaves,
(ii) While providing mechanical strength, collenchyma also provides flexibility to the organs and allows their bending, e.g., Cucurbita stems,
(iii) It prevents tearing of leaves,
(iv) Collenchyma allows growth and elongation of organs,
(v) Being living, its cells store food,
(vi) Its cells often contain chloroplasts and take part in photosynthesis.
iii. Sclerenchyma:
(Gk. scleros— hard, enchyma— tissue):
Sclerenchyma is a simple supportive tissue of highly thick-walled cells with little or no protoplasm. The cell cavities are narrow. The thickening of the wall may be made up of cellulose or lignin or both. A few to numerous pits occur in the wall. Sclerenchyma is of two types, sclerenchyma fibres and sclereids.
(a) Sclerenchyma Fibres:
The sclerenchyma fibres are highly elongated (1-90 cm), narrow and spindle-shaped thick-walled cells with pointed or oblique end walls. The fibres generally occur in longitudinal bundles (Fig. 6.9A) where the pointed ends of adjacent fibres get interlocked to form a strengthening tissue.
The adjacent fibres possess simple oblique pits (un-thickened areas with common pit membranes). Bordered pits also occur in some fibres. Pits do not perform any function in the mature fibres since the latter are empty and dead.
Living firbes occur in Tamarix aphylla. They possess nucleated protoplasts for several years. Fibres are septate in phloem of Grape Vine. Sclerenchyma fibres constitute the major mechanical tissue of the plants because they can bear compression, pull, bending and shearing.
The fibres occur in all those parts where mechanical strength is required, viz., leaves, petioles, cortex, pericycle, phloem, xylem and around vascular bundles (e.g., monocot stem). Commercial fibres obtained from plants are usually sclerenchyma fibres, e.g., Jute, Flax, Hemp.
(b) Sclereids:
They are highly thickened dead sclerenchyma cells with very narrow cavities. Sclereids are broader as compared to the fibres being isodiametric polyhedral, spherical, oval short or cylindrical. They may also be branched.
The thick cell walls have branched or un-branched simple pits (Fig. 6.10). Being elongated, the pits of sclereids are also known as pit canals. Sclereids may occur singly or in groups. They provide stiffness to the parts in which they occur.
The important types of sclereids are as follows:
(i) Stone Cells or Brachysclereids:
Un-branched, short and isodiametric with rami-form (branched) pits, e.g., grit of Guava, Sapota, Apple and Pear.
(ii) Macro-sclereids:
Elongated and columnar or rod-like, e.g., epidermal covering of legume seeds.
(iii) Osteo-sclereids:
Bone-like or columnar with swollen ends, e.g., sub-epidermal covering of some legume seeds.
(iv) Astro-sclereids:
Branched like a star, e.g., tea leaves, petiole of Lotus.
(v) Filiform Sclereids:
Fibre-like, sparingly branched, e.g., Olea.
(vi) Tricho-sclereids:
Very elongated hair-like and regularly once branched sclereids extending into intercellular spaces.
Functions:
(i) Sclerenchyma is the chief mechanical tissue of the mature plant organs,
(ii) It allows the plant organs to tolerate bending, shearing, compression and pull caused by environmental factors like wind,
(iii) It provides rigidity to leaves and prevents their collapsing during temporary wilting,
(iv) Sclereids provide strength to seed coverings.
(v) Dehiscence of many fruits is based on differential distribution of sclerenchyma fibres, e.g., pods,
(vi) Sclereids form stony endocarp of many fruits called stone fruits, e.g., Almond, Coconut,
(vii) A number of fibres are commercially exploited, e.g., Jute (Corchorus), Flax (Linum), Hemp (Cannabis).
Type # 2. Complex Permanent Tissues:
They are permanent tissues which contain more than one type of cells. All the types of cells of a complex tissue work as a unit. The common complex permanent tissues are conducting tissues, phloem and xylem.
i. Phloem:
(Gk. phlois- inner bark; Nageli, 1858):
It is a complex tissue which transports organic food inside the body of the plant. Phloem is also called bast (= bass, a vague term). It consists of four types of cells, viz., sieve tubes, companion cells, phloem parenchyma and fibres. Haberlandt (1914) uses the term leptom (e) for the conducting part of phloem.
(a) Sieve Tubes:
Sieve tubes are elongated tubular conducting channels of phloem. Each sieve tube is formed of several cells called sieve tube elements or members, sieve tube cells or sieve elements. Sieve tube members are placed end to end.
The end walls are generally bulged out. They may be transverse or oblique. They have many small pores or sieve pits. Due to the presence of sieve pits the end walls are commonly called sieve plates (Fig. 6.11 A).
In some cases the end walls of sieve elements possess more than one porous area. Such an end wall is called compound sieve plate, e.g., Grape Vine, Euphorbia royleana. The sieve plates connect the protoplasts of adjacent sieve tube members.
In non-flowering plants sieve cells remain separate. They are narrower but more elongated as compared to individual sieve tube members. The end walls are oblique. Porous areas are less conspicuous. They are borne on the lateral walls of the elongated sieve cells. They are called sieve areas.
Internally a sieve tube member or cell has peripheral layer of cytoplasm without any nucleus (Fig. 6.11 A). The nucleus is, however, present in the young cells. The central part is occupied by a network of canals which contain fibrils of p-protein. Sieve tube takes part in the conduction of organic food.
(b) Companion Cells:
Companion cells are narrow, elongated and thin walled living cells. They lie on the sides of the sieve tubes and are closely associated with them through compound plasmodesmata. They are squarish or rectangular in a transverse section.
The cells have dense cytoplasm and a prominent nucleus. It is supposed that the nuclei of the companion cells control the activities of the sieve tube through plasmodesmata (Fig. 6.11). Companion cells also help in maintaining a proper pressure gradient in the sieve tube elements.
Sieve tube member and its adjacent companion cells are derived from the same mother cell. Death of one results in death of the other as well. Companion cells are replaced by modified parenchyma cells (albuminous cells) in non flowering plants.
(c) Phloem Parenchyma:
They are ordinary living elongated parenchyma cells having abundant plasmodesmata. They store food, resins, latex, mucilage, etc. The cells help in slow conduction of food, especially to the sides. Phloem parenchyma is absent in most of the monocots and some herbaceous dicots.
(d) Phloem or Bast Fibres:
Sclerenchyma fibres found in the phloem are called phloem or bast fibres. They are generally absent in primary phloem but are quite common in secondary phloem where they occur more abundant in secondary phloem as compared to primary phloem.
The fibres occur in sheets or cylinders. Phloem fibres provide mechanical strength. The textile fibres of flax, (Linum usitatissimum), hemp (Cannabis) and jute (Corchorus species) are phloem fibres.
ii. Xylem:
Xylem is a complex tissue which performs the function of transport of water or sap inside the plant. Simultaneously, it also provides mechanical strength. Xylem is also known as wood. It consists of four types of cells, viz., tracheids, vessels (both tracheary elements), xylem or wood parenchyma and xylem or wood fibres. Out of these only tracheids and vessels take part in the transport of sap.
They are hence called tracheary elements. Vessels are the main tracheary elements of angiosperms. They are absent in gymnosperms and pteriodophytes. In the last two groups, conduction of sap is carried out by tracheids. The conducting elements of the xylem have been called hadrome by Haberlandt (1914).
(a) Tracheids:
The tracheids are elongated (5-6 mm dead cells with hard lignified walls, wide lumen and narrow end walls. In outline they are circular, polygonal or polyhedral.The inner walls of tracheids have various types of thickenings for mechanical strength.
The un-thickened areas allow the rapid movement of water from one tracheid to another. Tracheids constitute 90-95% of wood in gymnosperms while in angiosperms they hardly form 5% of the wood. Depending upon the thickenings, tracheids are of the following types (Fig. 6.12).
(i) Annular:
In this type the thickening material is laid down in the form of rings.
(ii) Spiral (Helical):
The thickening is deposited like a spiral or helix. Both annular and spiral thickenings are present in the first formed tracheids because they allow considerable stretching.
(iii) Reticulate:
Thickening is present in the form of a network. It is supposed that it is formed by the presence of several spiral bands of thickenings which cross one another.
(iv) Scalariform:
Here the thickenings give a ladder like appearance because they are laid down in the form of transverse bands.
(v) Pitted:
It is the most advanced type of thickening. The pitted tracheids are uniformly thickened except for small un-thickened areas called pits. In surface view they may appear circular, oval or angular. Pits often occur in pairs, that is, exactly at the same level on two adjacent elements. The pits are of two types, simple and bordered. The simple pits have uniform width of the pit chamber or cavity.
In bordered pits the pit cavity is in the form of a flask with a narrow aperture and a wide base. The area of the primary wall and middle lamella, which is present in a pit, is called pit membrane or closing membrane.
Actually it has many submicroscopic pores for the translocation of substances. A thickening called torus is present on the pit membrane of some gymnosperms for protecting the membrane from rupturing in case of unequal pressure on its two sides.
(b) Vessels:
Vessels take part, like tracheids, in the conduction of water or sap and provide mechanical support. They are much elongated tubes (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 or members.
The end walls of vessel elements are transverse or oblique (Fig. 6.13 B-C). They are often completely dissolved (Fig. 6.13 A). The condition is called simple perforation plate.
In a few cases the end walls remain intact and possess several pores in reticulate, scalariform or forminate forms. Such an end wall is called multiple perforation plate (Fig. 6.13 D), e.g., Liriodendron, Magnolia. Vessels help in quick movement of water in the plant.
The walls of the xylem vessels are lignified. They are thickened variously— annular, spiral, reticulate, scalariform and pitted. The pitted condition is more common. In outline the vessels are rounded in monocots and angular in dicots.
Vessels are absent in gym no-sperms and pteridophytes with the exceptions of a few (e.g., Selaginella species, Gnetum). Their tracheary elements comprise tracheids only. Flowering plants possess both vessels and tracheids but the latter are comparatively fewer.
(c) Xylem or Wood Parenchyma:
It is made of generally small thin or thick walled parenchymatous cells having simple pits. The wood parenchyma stores food (starch, fat) and sometimes tannins. It helps in the lateral conduction of water or sap. Ray parenchyma cells are specialised for this.
(d) Xylem or Wood Fibres:
They are sclerenchyma fibres associated with xylem. Xylem fibres are mainly mechanical in function. They are aseptate but can be septate.
Xylem fibres are of two types:
(i) Libriform Fibres. Typical fibres with thick walls having simple pits and obliterated central lumens,
(ii) Fibre Tracheids.
Intermediate between fibres and tracheids having thin walls and pits with reduced borders.
iii. Protoxylem and Metaxylem:
Depending upon the time of origin in relation to the growth of the plant organ, the xylem is of two types, protoxylem and metaxylem. Protoxylem (Gk. protos— first, xylem— wood) is the first formed xylem, where lignification begins before the completion of elongation.
It is made up of small tracheids and vessels which possess annular or spiral thickenings. They are capable of being stretched. The later formed xylem is described as metaxylem (Gk. meta— after, xylem— wood).
It consists of bigger tracheids and vessels which have reticulate, scalariform or pitted thickenings. Lignification occurs in them after completion of elongation. Depending upon the position of protoxylem in relation to metaxylem, xylem can be of four types— exarch, mesarch, centrarch and endarch.
In exarch (L. ex— without, Gk. arche— beginning) type, protoxylem lies towards the outside of metaxylem. It is inner in the endarch (Gk. endon— within, arche— beginning), middle of metaxylem in the mesarch xylem (Gk. mesos— middle, arche— beginning) and centre of metaxylem in centrarch xylem.
Protophloem and Metaphloem:
Protophloem is the first formed part of phloem which develops in parts that are undergoing enlargement. It consists of narrow enucleate sieve elements which may occur singly or in groups amongst cells that often grow later into fibres. Companion cells may or may not be associated with protophloem. During elongation the protophloem elements (sieve elements) get stretched and become non-functional.
Metaphloem is part of primary phloem that differentiates in plant organs after they have stopped enlargement. The sieve elements are wider and longer. Companion cells are regularly associated. Fibres are absent but parenchyma cells may later become sclerified.