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The following points highlight the thirty-six major cell types found in plant tissues. The cell types are: 1. Antipodal Cells 2. Bulliform Cell 3. Cambium (Vascular) 4. Collenchyma 5. Companion Cell 6. Cork 7. Cork Cambium 8. Egg Cell 9. Endodermis 10. Endosperm Cell 11. Epidermis 12. Fibre (Sclerenchyma) 13. Gland Cell 14. Hair 15. Idioblast 16. Laticifers 17. Meristematic Apical Cells and a few other types.
Type # 1. Antipodal Cells:
These cells are present in the normal embryo sac, generally three in number and lie at the opposite end of micropyle. They have a nucleus and cytoplasm; in several plants no cell wall is formed and the antipodal nuclei remain free. It is haploid.
Type # 2. Bulliform Cell:
It is an enlarged group of cells present in the upper epidermis of grass leaves. The cell wall is thick and the cell is hyaline, vacuolated and lacks chlorophyll that distinguishes it from other epidermal cells. It is suggested that this cell expands or contracts to cause opening and closer of leaves.
Type # 3. Cambium (Vascular):
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This is a layer, strip or cylinder of meristematic cells that normally occur in the dicotyledonous plants. They are present in open vascular bundles as fascicular cambium and between the vascular bundles termed interfascicular cambium.
They divide to form secondary vascular tissues, i.e. secondary xylem and phloem towards interior and exterior respectively. This lateral meristem causes secondary thickening in most dicots.
Type # 4. Collenchyma:
This mechanical cell is present at the hypodermis of many herbs and petioles of leaves. The cells may be round or oval in cross section and their walls are thickened by pectin, hemicellulose and cellulose, which deposit mainly at the comers or at the tangential walls. The cells are living and can elongate.
Type # 5. Companion Cell:
This cell is found only in phloem of angiosperms, where it remains associated with sieve tube with which it has common origin. It is a nucleated cell with dense cytoplasm and apparently plays some role in the functioning of sieve tube.
Type # 6. Cork:
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Also known as phellem and is derived from the cambium called phellogen on the outside of roots and stems of dicotyledonous plants. A cork consists of suberized and dead cells and is protective in function. It replaces the epidermis and is impervious to gas and water vapour.
Type # 7. Cork Cambium:
It is also called phellogen, and composed of a layer of meristematic cells, present on the peripheral side of roots and stems of angiosperms. It cuts off phelloderm on the inside and phellem or cork on the outside. The three layers — phellem, phellogen and phelloderm are collectively termed as periderm.
Type # 8. Egg Cell:
The functional female reproductive unit is egg cell. It and the other two cells termed synergids compose the egg apparatus in the embryo sacs of angiosperm. The cell remains towards the micropyle and contains prominent nucleus, cytoplasm and vacuole. It is haploid.
Type # 9. Endodermis:
It consists of a layer of cells that delimits the cortex from the stele. It is considered as the innermost layer of cortex and in root it surrounds the radial vascular bundles. The endodermal cells possess the characteristic casparian strips on their radial walls.
Type # 10. Endosperm Cell:
It is nutritive and food storage tissue of seeds. It is developed in the embryo sac of angiosperm as a result of fusion between two polar nuclei and one male nucleus. It is usually triploid in angiosperm and provides nutrition to the developing embryo.
Type # 11. Epidermis:
It is outermost layer of any plant parts. The outer wall of epidermal cell may be thickened by the formation of cuticle. Epidermis may bear hairs, stomata etc. Usually it is uniseriate; multiple epidermis is noted in orchid root and Ficus leaf. It is the protective layer of primary tissues.
Type # 12. Fibre (Sclerenchyma):
It is elongated, narrow cell, tapering at both ends with lignified cell wall, mostly without living protoplast. Usually they lie parallel to the long axis of the organ in which they occur and add mechanical strength.
Type # 13. Gland Cell:
This cell is thin walled, densely protoplasmic, and has secretory function. Sometimes gland cells surround a cavity where the secretions are collected. They usually secrete a specific substance.
Type # 14. Hair:
It is uni- or multicellular epidermal outgrowth. Hairs are present on all parts of a plant including stamens (ex. Tradescantia) and seeds (ex. Gossypium). They may be absorptive (e.g. root hairs), protective or secretory (e.g. glandular hair).
Type # 15. Idioblast:
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These cells markedly vary in size, wall thickness, cell content and function from their neighbours, e.g. lithocyst, raphides, guard cells etc.
Type # 16. Laticifers:
They are simple or branched, broad, elongated and aseptate cell where latex is secreted.
Type # 17. Meristematic Apical Cells:
These cells occur at root tips, shoot tips and leaf tips. These are undifferentiated and actively dividing cells, which give rise to permanent tissues. These cells are generally isodiametric, contain large prominent nuclei and less intercellular spaces.
Type # 18. Mesophyll:
These are parenchyma cells containing large number of chloroplasts; occur between the upper- and lower epidermis of leaves. In isobilateral leaves the mesophyll cells are all alike in contrast to dorsiventral leaves where the cells towards the upper epidermis are cylindrical, arranged at right angles to epidermis termed palisade cells; the mesophyll cells towards the lower epidermis are spongy mesophyll cells, which are loosely packed with large air spaces and contains fewer chloroplasts.
Type # 19. Palisade:
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These are elongated parenchyma cells containing large number of chloroplasts, set at right angles to and toward upper epidermis of dorsiventral leaves. They are concerned with photosynthesis.
Type # 20. Parenchyma:
It is thin walled, vacuolated cell with living protoplast; it is of various shapes and occurs in all parts of plant organs, i.e. pith and cortex of roots and stems, in leaves as mesophyll, in the flesh of fruits, in the endosperm of seeds, in the conducting tissue as xylem and phloem parenchyma etc. It forms the basic and ground tissue of plants where other tissues remain embedded.
Type # 21. Pericycle:
It usually consists of parenchyma cells, which form a cylinder surrounding the vascular tissues in root. It lies immediately inside the endodermis and is usually single layered.
Type # 22. Phloem Sieve Tube:
It is one of the phloem elements of angiosperms. The cell is living but contains no nucleus at maturity. Sieve tubes are set in longitudinal files and there is continuity between them through sieve pores, present on their vertical end walls. Translocation of food materials occurs through it.
Type # 23. Phelloderm:
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These cells are more or less like the neighbouring cortical cells and present around the cortex. These cells are formed from the cork cambium – phellogen on the inner side. It is the inner part of a periderm. Phelloderm may be considered as secondary cortex and sometimes contains starch and chloroplast.
Type # 24. Pollen Generative Cell:
This cell is present in the pollen grains of gymnosperms like Cycas. It originates as a result of subsequent divisions of pollen nucleus and is destined to produce stalk- and body cell.
Type # 25. Pollen Generative Nucleus:
It is present in the pollen grains of angiosperm, originates from pollen nucleus and gives rise to two sperm nuclei —the male gametes.
Type # 26. Ray Cells:
These are parenchyma cells, sometimes called vascular rays that are developed in the secondary vascular tissues by the division of ray initial of vascular cambium.
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They are set horizontally and may be uniseriate (one cell wide) or multiseriate (several cells wide). In the Multilingual glossary of terms used in wood anatomy compiled by International Association of Wood Anatomists (IAWA), 1964 describes ray as a “ribbon like aggregate of cells extending radially in the xylem and phloem”.
Type # 27. Root Cap Cells:
These are parenchymatous cells, situated outside the root apical meristem. They are derived from root apical meristem (e.g. Dicots) and calyptrogen (e.g. Monocot). Because of their terminal position they protect the inner meristem of root apex.
Type # 28. Root Hairs:
These are tubular outgrowths of some single cells present on the epidermis of root. They occur in greatest abundance at a little distance away from the apical meristem. They have thin cell walls and function in absorption of water and solutes, and in anchorage.
Type # 29. Secretory Cells:
They have secretory function in the secretion of oils, nectars, terpenes etc. They occur in different parts of the plant and secrete particular substances. The secretory structure sometimes characterizes certain families.
Type # 30. Stomatal Guard Cells:
These are reniform epidermal cells, a pair of which forms a stoma by union at their concave ends (e.g. Dicot leaves). In grasses the guard cells are dumb-bell shaped. The cell walls are differentially thickened and the cells contain prominent nucleus and chloroplastids.
Type # 31. Stomatal Subsidiary Cells:
Also known as accessory cells which occur in association to stomatal guard cells; these cells are morphologically distinct from neighbouring epidermal cells and guard cells.
Type # 32. Synergids:
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These cells occur in the embryo sacs of angiosperms, a pair of which in association with the egg forms the egg apparatus.
Type # 33. Tracheid:
It is one of the nonliving elements of xylem. It is an elongated cell with tapering, obtuse or oblique ends. The cell wall is thick, lignified with pits on its lateral wall. The cells lie parallel to the long axis of the plant and conduct water and solutes. These cells are formed from a single cell. They are mechanical cells, i.e. give strength and support to the organ in which they occur.
Type # 34. Transfer Cells (Fig. 2.1):
These are specialized parenchyma cells characterized by wall ingrowths. The ingrowths are formed by the deposition of wall materials on the inner side of primary wall. The plasma membrane of the cell gets invaginated along with the outline of wall ingrowths and thus the surface area of plasma membrane is increased by many folds.
These cells are concerned with absorption and secretion where the invaginated plasma membrane facilitates the process. They occur in association with xylem and phloem, glandular hair, hydathodes, nectaries, salt glands, synergids etc.
Fig. 2.1. Xylem parenchyma transfer cell showing wall ingrowths
Type # 35. Tapetum:
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It is usually composed of a single layer of cells present within the anther of angiosperm. It is food rich layer, which surrounds the sporogenous tissues. The tapetal cells disintegrate to liberate the contents and thus they supply metabolites for the developing sporogenous cells.
Type # 36. Xylem Vessel:
It is also known as trachea, which is one of the non-living elements of xylem. Vessels are elongated tube like thick walled lignified cells with perforated ends and pits on the side walls. They are set end to end in longitudinal series and lie parallel to the long axis of the plant. Conduction of water and solutes occur through it, which also gives mechanical support.
