Tissues in Botany

Types of Tissue Systems

Flowering plants— like other multicellular organisms —are highly organized with intricate structures, into parts with varying specializations.

These structures may be referred to as the morphological¹ parts of a plant. The morphologyof a plant can be used to determine the specialization of that specific part.

The most recognizable morphological unit is a cell², which can group and unite together in the form of tissues³.

Tissues are a groups of cells united together to perform a specific role and specialization.

For example: Vascular tissues⁴ are mainly there for the conduction and transportation of water and nutrients throughout the plant.

Plant tissues are generally classified into two categories:

• Permanent tissues⁵
• Merismetic tissues⁶

Merismetic Tissue

Meristmetic tissues are composed of young cells which have the ability to divide and multiply.

These cells are present in points of growth like the tips of roots, shoots, and epidermis. They are tightly packed with no intercellular space.

Permanent Tissue

Permanent Tissues are composed of living or dead cells that have reached maturity and cannot grow or change in size.

They are usually present in the ground tissues and comprise fundamental tissue systems. They arrangement has intercellular space.

Dermal Tissue System

Dermal tissues⁷ are on the outer most part of a plant and is responsible for creating a protective covering.

Epidermis

The epidermal tissue system⁸ come from the dermatogen⁹ of the apical meristem¹⁰.

This outermost layer extends over the entire surface of the plant body; this system is composed of a single later of flattened cell, with straight, wavy, or beaded walls covered with a layer of cutin¹¹.

The functions of the epidermis is the following:

• protection of the internal tissue from external factors such as:
  • intense temperatures,
  • mechanical injury,
  • aggressive organisms, and
  • weather.
• prevention of excess water evaporation through the use of cuticles¹², wax, and other depositions.
• protection agains intense illumination and heat through the use of cuticles and cutinized hairs¹³.
• acts as a storage for water.
• minor functions such as:
  • photosynthesis
  • secretion, etc.

Stomata

Stomatas¹⁴ are small minute openings in the epidermis surrounded by two kidney shaped cells called guard cells¹⁵.

The term stoma¹⁶ is often referring to a stomatal arrangement, which is a slit like opening with guard cells.

The cells surrounding the stomata are called neighboring cells or subsidiary cells¹⁷, they often resemble other epidermal cells but varies on the species plant.

The function of the stomata¹⁴ is to facilitate gaseous exchange and transpiration in the plant body.

The distribution of stomatas differ on the species and varieties of plants.

• The stomatas are most abundant in lower epidermis⁸ of dorsiventral leaf¹⁸.
• In isobilateral leaves¹⁹, the stomatas are only present in the upper epidermis.
• In submerged leaves²⁰, no stomatas are present.
• In Buchu and Neem²¹, stomatas are only present in the lower surface.
• In Belladonna, Datura, and Senna²², stomatas are present on both surfaces.
• In desert plants²³, the distribution of stoma show great variation.
• In plants with xerophytic adaptations²⁴, the stomatas are situated in grooves or pits in the stem or leaf.

Trichomes

Trichomes²⁵ are elongated outgrowths of one or more epidermal cells.

Trichomes usually occur on the leafs but are also found else wear.

The functions of the trichomes is:

• to prevent any damage from insects,
• to prevent materials form clogging the stomatas,
• to aid in the dispersion of seeds,
• secretion of volatile oils.

Trichomes can be classified into two distinct types: Covering trichomes²⁶ and Glandular trichomes²⁷

Periderm

The periderm[^peridem] is the layers directly beneath the epidermis or phellogen²⁸ that divide and redivide.

The cells that divide towards the epidermis form brick-shaped or polygonal cork²⁹, and the cells that divide away from the epidermis form perenchymatous phelloderm³⁰.

Lenticels³¹ are present in the periderm and function similar to the stomata.

Vascular Tissue System

Vascular tissues are concerned with the transportation of valuable resources within the plant system. The vascular tissue system consist of vascular bundles[^vasclularbundles] that are distributed in the stele³².

The stele is the central cylinder of the stem and root systems that is surrounded by the endodermis[^endodermis].

The vascular bundles of a dicotylonous stem³³ consist of three well defined tissues:

• Xylen or wood³⁴,
• Phloem or bast³⁵, and
• Cambium³⁶

Xylem

The xylem or wood is the conducting tissue of the vascular bundles and is responsible for providing mechanical strength to the plant body.

The xylem is composed of four types of elements:

• tracheid³⁷,
• vessels or tracheae[^trachaea],
• wood fibers³⁸, and
• wood parenchyma[^woodparenchyma]

Tracheid

Tracheids are long elongated, tube-like cells with hard, thick, lignified walls and large cell cavities.

The tracheids have tapered ends with a rounded, chisel-like or rarely as a pointed form.

Tracheae

The vessels or tracheae are cylindrical, tube-like structures. It is formed as a row of cells placed end to end, that form a pipeline structure.

The tracheae comprised the majority of the xylem. They serve to conduct water and mineral salts from the roots to the leaves.

Like the tracheids, they are composed of dead cells.

Xylem Fibers

Xylem or wood fibers are sclerenchymatous cells that are abundant in woody dicotyledons and add mechanical strength to the plant body as a whole

Xylem Perenchyma

Xylem or wood parenchyma occur frequently in the xylem. They are generally living cells that assist directly or indirectly to the conduction of water upwards.

Phloem

The phloem or bast is the vascular tissue that conducts nutrients to different parts of the plant such as the storage organs and growth points.

The phloem is composed of four different type of element:

• sieve tubes³⁹,
• companion cells⁴⁰,
• phloem parenchyam⁴¹,
• bast fibers⁴²

Sieve Tubes

The sieve tubes are slender, tube-like structures composed of elongated cells placed end to end.

Their ends are filled with holes that resembles sieves. The walls of the sieve cells are thin and made of cellulose,.

Companion Cells

Companion cells are associated with sieve tubes. The companion cells are living cells that contain protoplasm and an elongated nucleus. This assists in the conduction of food.

Phloem Parenchyma

The phloen parenchyme are present in most forms of plants except for monocotyledons. These are composed of living cells in a cylindrical shape.

These store and conduct food material.

Bast Fibers

The bast fibers contain sclerenchymatous cells; but are generally only in the secondary phloem.

Cambium

The cambium is a thin strip of meristematic tissue between the xylem and phloem. These are composed of thin walled and rectangular cells.

Types of Vascular Bundles

Vascular bundles may be arranged in the following types

• Radial vascular bundles⁴³
  • Phloem and xylem are separated on different bundles which alternate each other; the most primitive vascular bundle.
• Conjoint vascular bundles⁴⁴
  • Phloem and xylem are in a singular bundle; comes in two forms
  • Collateral⁴⁵
    • When the xylem and phloem lie on the same radius; the phloem being on the exterior and the xylem on the interior
  • Bicollateral⁴⁶
    • Both phloem and cambium occur twice in a collateral bundle; with the xylem in between.
• Concentric vascular bundle⁴⁷
  • When vascular tissue is surrounded by another; comes in two forms.
  • Amphicribral[^aphicibralvb]
    • The phloem surrounds the xylem; Hadrocentric.
  • Amphivasal⁴⁸
    • The xylem surrounds the phloem; Leptocentric.

Ground Tissue System

Ground tissues⁴⁹ consist of simple cells strengthened by thickened cells.

Parenchyma

Parenchyma⁴¹ comprises the ground tissue system; they are comprised of living cells that serves as food storage.

Parenchyma cells that contain chloroplasts are called chlorenchyma⁵⁰, with the function to manufacture food.

Collenchyma

The collenchyma⁵¹ tissue consists of somewhat elongates, parenchymatous cells with oblique, slightly rounded, or tapered ends. They have thickened corners to prevent intercellular space, because of the deposits of cellulose, hemicellulose, and protopectine.

The collenchyma produced starches and sugars due to the presence of chloroplasts.

Sclerenchyma

The scerenchyma⁵² is a tissue of dead cells that serve only to provide strength in the plant body to withstand excess external stresses.

Sclereids

Scelereids⁵³ are a type sclerenchyma that is deposited in various parts to meet local mechanical needs. They are also called stone cells.

Footnotes

1. Morphology refers to the form of living things; in this context: the form of plants. ↩

2. Cells are the most easy to recognize morphological unit. ↩

3. Tissues are a unit of life that is comprised of cells; they have a specific purpose and role. ↩

4. Vascular Tissues are used for the conduction and transportation of water and nutrients in a plant. ↩

5. Permanent Tissues are tissues that cannot change in size or multiply; comprises fundamental tissue systems. ↩

6. Merismetic Tissues cells responsible for growth; capable of dividing and multiplying. ↩

7. Dermal Tissues comprise the outer visible layer of plants; responsible for protection. ↩

8. Epidermal Tissues make up the outer layer of the plant body; provide protection to the plant body. ↩ ↩²

9. Dermatogens are the outer layers of meristematic tissues that forms epidermal tissues. ↩

10. Apical meristems are the points of growth on the tips of plants. ↩

11. Cutin is the layer that covers the walls of epidermal tissue cells. ↩

12. Cuticles are thin layers composed of cutin that covers the epidermis. ↩

13. Cutinized Hairs are hair-like protrusions that are coated in cutin. ↩

14. Stomatas are small openings in the epidermis surrounded by two guard cells; responsible for gaseous exchange and transpiration. ↩ ↩²

15. Guard cells are kidney shaped cells that appear in pairs around the opening of a stomata. ↩

16. Stoma is a term that refers to the arrangement of a stomata. ↩

17. Subsidiary cells or neighboring cells are the epidermal cells that surround the stomata. ↩

18. Dorsiventral leaved plants have more stomas in their lower epidermis. ↩

19. Isobilateral leaved plants have stomas only in the upper epidermis. ↩

20. Submerged leaves have no stomas. ↩

21. In Buchu and Neem, stomas are only present in the lower surface. ↩

22. In Belladona, Datura, and Senna, stomas are present on both surfaces. ↩

23. In Desert plants, stomas distributions are greatly varied. ↩

24. In plants with Xerophytic adaptations, the stoma are in grooves or pits. ↩

25. Trichomes are elongated epidermal cells; prevents damage from insects and clogging of the stomata; aids in the dispersion of seeds and secretion of volatile oils. ↩

26. Covering trichomes or clothing trichomes cover the plant body. ↩

27. Glandular trichomes secrete volatile oils on their tips. ↩

28. The Phellogen is the layer directly beneath the epidermis. ↩

29. Cork is a box-shaped or polygonal cell that form towards the epidermis. ↩

30. Pheloderm is the parenchymatous cells that form away from the epidermis. ↩

31. Lenticels are the opening in periderm that function similar to stomatas. ↩

32. The Stele is the central cylinder of a plant from the roots to the stem. ↩

33. Dicotylonous stems are complex and well structured stems that form well defined vacsular tissues. ↩

34. The Xylem is the tissue responsible for conducting water and providing strength to the plant body. ↩

35. The Phloem or bast is responsible for the conduction of nutrients to storage organs and growth points. ↩

36. The Cambium is a thin strip of meristematic tissue between the xylem and phloem; composed of rectangular cells. ↩

37. Tracheids are tube-like cells with hard and lignified walls; it is composed of dead cells; it has tapered ends. ↩

38. Xylem fibers or wood fibers are sclerenchymatous cells that are abundant in woody dicot plants; adds strength to the plant body. ↩

39. Sieve tubes are tube-like structure composed of elongated cells placed end to end; they have thin walls made of cellulose. ↩

40. Companion Cells are associated with sieve tubs; living cells with protoplasm and an elongated nucleus; assists in food conduction. ↩

41. Parenchyma serves as a storage for food. ↩ ↩²

42. Bast fibers are vascular tissues that contain sclerenchymatous cells in the secondary phloem. ↩

43. Radial vascular bundles have the phloem and xylem in separate bundles and alternate each other. ↩

44. Coinjoint vascular bundles have the phloem and xylem in one singular bundle. ↩

45. Collateral vascular bundles have the xylem in the interior side and the phloem on the exterior. ↩

46. Bicollateral vascular bundles have the phloem and cambium occur twice on both sides, sandwiching the xylem. ↩

47. Conventic vascular bundles have a vascular tissue surround another vascular tissue. ↩

48. Amphivasal vascular bundles have the xylem surround the phloem. ↩

49. Ground tissues consist of simple cells strengthened by thickened cells. ↩

50. Chlorenchyma is a parenchyma with chloroplasts; responsible for manufacturing food. ↩

51. The Collenchyma is a tissue composed of elongated parenchymatous cells; produces chloroplasts. ↩

52. Sclerenchyma is composed of dead cells that purely strengthen the plant body. ↩

53. Sclereids are a type of schlerenchyma that are deposited in areas to strengthen them; also known as stone cells. ↩