In this article we will study about permanent and secretary tissues found in plants.
Study of Permanent Tissues:
The derivatives of meristematic cells gradually become differentiated, lose the capability of undergoing divisions and form permanent tissues.
In plants, different types of permanent tissues are found:
1. Simple Permanent Tissues:
These are composed of similar cells and so are homogeneous in nature:
A. (a) Parenchyma (Fig. 4.1A):
Material:
Stem of sunflower.
Cut T.S. and L.S. of the stem. Stain in chlor-zinc-iodine solution. Mount in 50% glycerine both stained and unstained sections. All parenchyma cells having cellulose walls turn blue. Note the distribution of parenchyma tissues. Study the features of parenchyma cells mainly from cortex and pith.
Parenchyma is made up of cells with cellulose cell walls, cell walls uniformly thin, with intercellular spaces, cells isodiametric or slightly elongated longitudinally, cells with vacuolated protoplast.
(b) Aerenchyma (Fig. 4.1 B):
Material:
Petiole of banana or Nymphaea.
Cut T.S. of the petiole. Mount in 50% glycerine. Note parenchyma cells enclosing large air-cavities. This is called aerenchyma.
(c) Chlorenchyma (Fig. 4.1C):
Material:
Leaf of tuberose.
Cut T.S. of the leaf. Mount in 50% glycerine. Study the mesophyll tissue. The cells are parenchymatous with chloroplasts. This is called chlorenchyma.
B. Collenchyma (Fig. 4.2):
Material:
Stem of Leonurus, Calotropis and sunflower.
Cut T.S. and L.S. of sunflower stem and only T.S. of Leonurus and Calotropis stems. Stain in chlor- zinc-iodine solution. Mount both stained and unstained sections in 50% glycerine.
Note the presence of collenchyma in the hypodermis of sunflower and Calotropis stem and at the corners of the square stem of Leonurus — also in the hypodermis. In the stained sections their walls have turned blue, because collenchyma cell walls contain, as in parenchyma, cellulose which stains blue with chlor-zinc-iodine solution.
The most distinctive feature of collenchyma is the cell wall which is unevenly thickened due to unequal deposition of cellulose and pectic materials. In Leonurus stem the thickening is restricted to the corners of the cells, the sides remaining thin.
This is called angular collenchyma (Fig. 4.2A). In Calotropis stem intercellular spaces are present in-between collenchyma cells and thickening is localised to the regions of the walls bounding the intercellular spaces. This is known as tubular or lacunate collenchyma (Fig. 4.2C).
In sunflower stem the collenchyma cells are compactly set and thickening is restricted to the tangential walls of the cells. This is called lamellar or plate collenchyma (Fig. 4.2B). Collenchyma cells contain chloroplasts and are elongated along the long axis of the plant organ.
C. Sclerenchyma (Fig. 4.3):
Material:
Stems of sunflower and Cucurbita.
Cut T.S. and L.S. of the materials. Stain with chlor-zinc-iodine solution (lignin containing sclerenchyma cell walls turn yellow) or phloroglucin and conc. HCl (lignified walls turn red). Mount both stained and unstained sections in 50% glycerine.
Locate the sclerenchyma as bundle caps (a patch of sclerenchyma) on the outer side of each vascular bundle in sunflower stem and as a continuous ring of a few layers of sclerenchyma cells in the cortical region of Cucurbita stem.
Sclerenchyma appears polygonal in T.S. and long narrow fibre-like with pointed and interlocked ends in L.S. (hence they are called sclerenchyma fibres). Cell walls are very thick and lignified with simple pits. Cell lumens are narrow and sometimes almost obliterated. The cells are dead and devoid of protoplasm.
N.B.: Individual fibres can be clearly observed from macerated tissue. There it is possible to distinguish extraxylary and intraxylary fibres. Extraxylary fibres have simple pits. Intraxylary fibres, i.e., fibres associated with xylem (wood fibres), are of two types: libiriform fibres and fibre-tracheids.
Libiriform fibres are similar to other fibres. Fibre-tracheids possess bordered pits and so are regarded as intermediate between fibres and tracheids. They may be aseptate or septate.
D. Sclereids (Sclerotic cells) (Fig. 4.4):
Material:
Pear fruit, Phdseolus seed, Pisum seed and Nymphaea petiole.
Cut cross-sections of the above materials. It is advisable to soak the seeds of Phaseolus and Pisum in water for a few hours, in order to soften the seeds a little. This will facilitate section cutting. It will be sufficient to have only the seed coat in sections.
As sclereids have lignified walls, stain the sections with chlor-zinc-iodine (lignin turns yellow) or phloroglucin and conc. HCl (lignin turns red) or aniline sulphate (lignin turns bright yellow) and mount in 50% glycerine.
Sclereid or sclerotic cells are a type of sclerenchyma cells with highly lignified walls. Lignification is so extensive that the cell lumen is often almost obliterated. The cells are dead and usually isodiametric or slightly elongate.
In pear fruit the sclereids are isodiametric with a small central cell lumen from which narrow canal like simple pits radiate in all directions. These are called ramiform pits. The cells are irregularly scattered. These are called Brachysclereids or stone cells (Fig. 4.4A).
In the seed coat of Phaseolus the sclereids form the epidermis. They are rod-shaped elongated cells and form a palisade-like layer. There are Macrosclereids (Fig. 4.4B). In the seed coat of Pisum, the sclereids are bone-like, consisting of columnar cells which are dilated at the two ends.
These are known as Osteosclereids (Fig. 4.4C). In the petiole of Nymphaea the sclereids have irregularly branched arms and have a somewhat stellate appearance. They have reniform pits. These are called Astrosclereids. In the petiole of Nymphaea the sclereids have a typical star-like appearance and are called trichosclereids (Fig. 4.IB, Fig. 4.4D).
2. Complex Permanent Tissues:
The complex permanent tissues are heterogeneous in nature, having different types of cell elements. Xylem and phloem are the two complex tissues of plants and are also referred to as vascular tissues. Together they constitute the vascular bundles.
(i) Xylem (Fig. 4.5):
Material:
Stems of sunflower, Xanthium, maize, etc. and macerated wood elements. Xylem is composed of tracheids, tracheae or vessels, fibres (xylem or wood fibres) and parenchyma (xylem or wood parenchyma).
Cut T.S. and L.S. of the materials. For studying xylem elements, sections alone are not sufficient; Along with sections, macerated wood elements also should be observed in order to get an idea about the shape, size and nature of wall-thickening of individual xylem elements.
Tracheids (Fig. 4.5A, B, C, D):
Tracheids are narrow elongated dead cells with fairly large lumens and blunt chisel-like ends; the last two features enable them to be distinguished from fibres. They are round or polyhedral in cross- section.
The lignified secondary cell wall shows annular, spiral, scalariform, reticulate or pitted thickening. Very long and narrow tracheids, with transversally elongated simple pits, give them a scalariform appearance — these are characteristic offer rhizomes.
In gymnosperms the tracheids have almost circular bordered pits with the borders looking very prominent. The pits are often uniseriate, although multiseriate condition also is found. In many conifers the bordered pit pairs are accompanied by transversely oriented thickenings called crassulae or bars of sanio.
In angiosperms, the pit pairs may be simple, bordered, or half-bordered. Very often a mixture of the three types of pits are found on the same element. The borders are sometimes poorly developed and less prominent than those of gymnosperms.
A typical tracheid differs from fibre in having blunt ends, large lumen and relatively thin secondary wall with more prominent pit-pairs. An intermediate type of cell element with reduced bordered pits is found in some plants. These are called fibre-tracheids (Fig. 4.5 I & J). Such cell elements having some transverse partition walls are called septate fibre-tracheids.
Vessels or Trachea (Fig. 4.5E, F, G):
These are long tube-like elements open at both ends. An apparent vessel is composed of a number of vessel segments joined end to end, the end walls (cross-walls) being usually perforated. However, perforation can also occur on the lateral walls. The walls which become perforated are called perforation plates.
The perforation plates may be multiple or simple. In primitive plants (Magnoliceae etc.) the walls are not completely dissolved. The perforations remain in more or less parallel series, like cross or transverse lines. These are called scalariform perforation pl&tes-(Fig. 4.5E).
The perforations may also occur in the form of a network (reticulate perforation) or in the form of a group of circular holes (foraminate perforation). In advanced angiosperms, the entire end wall becomes dissolved forming a single large hole. This is known as simple perforation plate (Fig. 4.5F, G).
The vessels are dead cell elements and have lignified secondary cell wall with all the types of thickening patterns as noted in tracheids. However, vessels with annular or spiral thickening are more frequent in the veins of leaves and those with pitted and other types of thickening are more commonly found in stems and roots. The pit pairs are simple, bordered or half-bordered. Often the borders are not well-developed.
Vessels are commonly found in angiosperms, although they are absent in some members of Ranales and some xerophytes, parasites and hydrophytes. In many members of Gramineae and Cyperaceae also they are absent. They are normally absent in pteridophytes and gymnosperms. But Pteridium and Selaginella among pteridophytes and Gnetum among gymnosperms have vessels.
Xylem Fibres (Wood fibres):
Sclerenchyma fibres which remain associated with xylem elements are called xylem fibres or wood fibres. These are of two types: libiriform fibres and fibre-tracheids which may be septate also (Fig. 4.51, J).
Xylem Parenchyma (Wood parenchyma):
Parenchyma cells which remain associated with xylem elements form xylem parenchyma or wood parenchyma. These cells may be thin- or thick-walled. The thick-walled parenchyma cells have lignified secondary wall with pit pairs. The pit pairs between a parenchyma cell and an adjacent non-parenchymatous xylem element may be simple, bordered or half-bordered, the last type being more frequent (Fig. 4.5H).
(ii) Phloem (Fig. 4.6):
Material:
Stem of Cucurbita or other members of Cucurbitaceae. Phloem is composed of sieve elements (sieve tubes and sieve cells), companion cells, phloem parenchyma arid phloem fibres (last fibres).
Cut T.S. and L.S. of Cucurbita stem; also take macerated element of the stem. Maceration should be done in KOH solution. Stain the sections in 1% aqueous aniline blue solution (callose is stained blue), or in 1% rosolic acid (Corallin) in 4% aqueous sodium carbonate (Callose turns red), or in 1: 2,500 solution of resorcin blue for 15 minutes (callose turns brilliant blue).
Mount in 50% glycerine and observe. To obtain a permanent preparation transfer the stained the section to a slide, add a drop of levulose syrup (10 g of levulose to 8 ml warm distilled water) and cover with a cover glass. After the syrup evaporates slowly and becomes thickened, seal the slide.
Sieve Elements:
The sieve elements are of two types — sieve tubes and sieve cells. The former is found in angiosperms and the latter in gymnosperms and pteridophytes. Sieve tubes are comparable with vessels and sieve cells with tracheids of xylem.
Sieve tubes are long tube-like bodies arising from a longitudinal row of cells. The end walls are perforated in a sieve-like manner. These perforated end walls are called sieve plates and the perforations are the sieve areas. The sieve areas are comparable with the pit-fields, although they are much bigger in area. Cytoplasmic continuity is maintained between adjacent cells through the sieve areas.
Callose, an insoluble polysaccharide, is impregnated into cellulose or replaces cellulose forming a case around each sieve area. It is this callose which takes up stain and helps to identify the sieve elements.
A sieve plate is called simple when it has all the sieve areas grouped in one unit, as in Cucurbita. The sieve plate is compound when the sieve areas are grouped into several units. Such sieve areas are often found on the side walls or oblique walls (Fig. 4.6A, B, D).
Sieve cells found in pteridophytes and gymnosperms are narrow elongated cells with closed end walls without conspicuous sieve plates (Fig. 4.6C). Sieve areas are scattered in groups along the longitudinal walls of the cells and are more numerous near the ends.
The walls of sieve elements are primary and composed of cellulose, so xylem and phloem takes up different stains when double staining is performed, the xylem elements being lignified. When callose is stained to identify the phloem elements, sometimes it is noted that the stained callose which, normally, appears as a ring around each sieve area, appears instead as a pad over the entire sieve area. This is called callus pad.
The callus pad may be seasonal (dormancy) callus or definitive callus. The former appears with the approach of the inactive or resting season (winter) and disappears with the arrival of the active season (spring). The definitive callus pad is a permanent structure and appear in old and functionless sieve elements. The formation of callus pad cuts-off cytoplasmic connection between adjacent sieve elements.
Companion Cells (Fig. 4.6B):
In T.S. companion cells appear as small triangular, rectangular or polyhedral cells with dense protoplast. Actually, these are the smallest cells of phloem and always lie in direct contact with the sieve tubes. In L.S. these are found to lie on one side of the sieve tubes.
The length of a sieve tube is usually covered by a few companion cells. These cells have dense cytoplasm and prominent nuclei. Starch is never present in companion cell and this may be confirmed by iodine test. Companion cells are found only in angiosperms. They are replaced by albuminous cells in pteridophytes and gymnosperms.
Phloem Parenchyma (Fig. 4.6B):
In addition to companion cells or albuminous cells, another kind of parenchyma cells are found in phloem. These are called phloem parenchyma. In T.S. they appear bigger than the companion cells and, in L.S., longitudinally elongated. These cells are thin-walled (made of cellulose), have vacuolated protoplast and are rich in starch which may be confirmed by iodine test. Phloem parenchyma is absent in monocotyledons.
Phloem (Bast) Fibres (Fig. 4.6D):
Sclerenchyma fibres remain associated with the phloem of many plants, such as, jute, sunflower, etc. In sunflower the bundle cap which is present on the outer side of each vascular bundle is regarded as a part of phloem, Jute fibre of commerce is best fibre.
Study of Secretory Tissues:
Cells concerned with the secretion of essential oils, resins, mucilage, latex, nectar, etc. form the secretary or special tissue. Secretory tissues are often organised into glands which are named according to the substance which they secrete, such as digestive gland, oil gland, etc. and may be located superficially or internally in plants.
1. Nectaries:
Material:
Inflorescence (cyathium) of Poinsettia (Euphorbia) pulcherrima.
On the green involucre of the inflorescence there are a number of yellow-coloured large glands. Take one such gland along with the subtending involucral tissue and cut vertical sections. Mount in water or 50% glycerine and observe.
The gland is composed of a row of large columnar cells having dense cytoplasm, large nuclei and inconspicuous vacuoles.
2. Resin Ducts (Glands):
Material:
Stems of pine and sunflower.
Cut T.S. and L.S. of the stems, mount in 50% glycerine and observe under microscope.
Locate the ducts in the cortical region. The ducts form extensive canals. In L.S. they look like long tubes and in T.S. they appear as small and circular areas having a lining of small parenchyma cells with dense protoplast. These cells are called epithelial cells. They are schizogenous in origin and secrete resin.
3. Oil Duct (Gland):
Material:
Rind (skin) of Citrus fruit.
Cut V.S. through the rind (skin) of the fruit, mount in 50% glycerine and observe under microscope.
Numerous oval or round cavities are present in the section. These are oil glands or cavities and contain an essential oil. These glands are lysigenous in origin.
4. Laticiferous Ducts:
Material:
Stem of Calotropis, Euphorbia hirta, Carica papaya, Argemone Mexicana.
Fix the materials in 70% ethyl alcohol for 24 hours. Cut L.S. of the stems mount in 50% glycerine and observe under microscope.
Laticiferous ducts or tubes are long tube-like bodies running longitudinally within the plant organs and contain a viscous fluid called latex.
Latex is usually milky but may also be watery or yellowish (Latex is an emulsion of various substances like proteins, sugars, enzymes, etc. in a watery matrix.). As latex readily oozes out of freshly cut plant organs, it is necessary to fix the tissues in alcohol before section cutting.
Laticiferous ducts may be:
(I) Non-articulate latex ducts or latex cells or simple laticifers and
(ii) Articulate latex ducts or latex vessels or compound laticifers.
Non-articulate latex ducts or latex cells are single cells and run lengthwisethrough other tissues. They never join with one another and thus never form a network. They are coenocytic, i.e. multinucleate. They may be branched or un-branched. These are found in members of Apocynaceae, Asclepiadaceae, Euphorbiaceae, Urticaceae, etc.
Articulate latex ducts or latex vessels are compound structures as they arise from a series of longitudinally placed cells with their end walls partially or completely dissolved. These are branched and form a complex system of network. They are also coenocytic.
They are found in Caricaceae, Papavaraceae, Musaceae and the rubber-yielding plant Hevea brasiliensis of Euphorbiaceae. One way of distinguishing members of Cactaceae from xerophytic members of Euphorbiaceae is that cactii usually do not contain laticifers while Euphorbias usually contain laticifers.