The below mentioned article provides an overview on Vascular Tissue System. After reading this article you will learn about: 1. Introduction to Vascular Tissue System 2. Primary Vascular Tissue.
Introduction to Vascular Tissue System:
The vascular tissue system consists of a number of vascular bundles which are found to be distributed in the stele. The stele is the central cylindrical portion of the stem and the root, commonly surrounded by the endodermis, and consists of vascular bundles, peri-cycle, pith and medullary rays.
Each vascular bundle consists of xylem and phloem tissues with or without cambium. In roots separate xylem and phloem strands are found. The function of this system is to conduct water and other nutrients from roots to leaves through the xylem and translocation of prepared carbohydrates from leaves to other storage organs and growing regions of plant body through the phloem.
The vascular bundle elements are derived from the pro-cambial strands of the primary meristem. The vascular bundles may be arranged in circular ring as in the dicotyledonous stems and the roots, on the other hand, they are found to be scattered throughout the axis in the monocotyledonous stems.
In certain plants the vascular bundles remain scattered within the well defined pith; such bundles are called medullary vascular bundles (e.g., Mirabilis, Roerhaavia Bougainvillaea, Achyranthes, Amaranthus, etc.). Vascular bundles may also occur in the cortical region of the stem, such bundles are known as cortical bundles (e.g., Casuanna. Nyctanthes).
Primary Vascular Tissue:
The Procambium:
The first cells to mature, in either leaf or stem, belong to the vascular tissue. In the promeristem, where all cells are isodiametric and alike, continuing longitudinal divisions set apart in some areas strands of elongate, slender cells with dense cytoplasm. Thus, meristematic tissue forms the primary phloem and xylem and is known as the procambium.
Here, the term procambium is used to indicate the meristematic tissue that gives rise to the morphological vascular units. The first procambium appears as isolated strands very close to the apex in stem and root. It is continuous backward in the older tissues with older promeristem strands and mature vascular tissue.
The slender procambium strands increase in diameter by longitudinal cell division within themselves and by the addition of new cells on their borders by means of promeristem cells The increase in size of the strands is so great that a few or all of the strands fuse to form a hollow cylinder or a solid central core. Ultimately the procambium forms the vascular cylinder of the region.
As procambium develops, the diameter of the organ increases and simultaneously the promeristem cells multiply and enlarge in bulk. As the growth continues, the first phloem and first xylem cells are formed which mature on the inner and outer margins of slender strands.
They are separated more and more widely from each other as the strands enlarge in size. The first maturing cells in a young strand are the phloem cells, which are followed by first xylem cells thereafter.
The first mature xylem and phloem cells are separated radially by procambium in stems and leaves and tangentially by promeristem in roots.
Constituents of a Vascular Bundle:
A vascular bundle of dicotyledonous stem consists of three major zones:
(a) Xylem or wood,
(b) Phloem or bast and
(c) Cambium.
(a) Xylem or wood:
The xylem of a vascular bundle lies towards the centre and is composed of:
(i) Vessels or tracheae,
(ii) Tracheids,
(iii) Wood fibres and
(iv) A patch of xylem or wood parenchyma.
The vesslels may possess various kinds of thickenings such as—annular, spiral, scalariform, reticulate and pitted. The tracheids are also found to be associated with the vessels.
In the similar way wood fibres and wood parenchyma are also found. The xylem or wood parenchyma of secondary wood usually becomes thick-walled and lignified. The xylem elements, i.e., vessels and tracheids, aid in the conduction of water and mineral salts from the roots of the leaves, whereas wood or xylem parenchyma are living tissues, and aid in the storage.
The wood fibres give mechanical support to the plant body.
When the development of xylem takes place towards the centre of the axis, or in other words, the protoxylem develops towards the periphery, it is called centripetal xylem, and the xylem strand is said to be exarch. If the development of xylem is towards periphery of the axis, or in other words, protoxylem elements develop towards the centre, it is called centrifugal xylem, and the xylem unit is said to be end-arch.
When development is such that both centripetal and centrifugal xylem are formed, the xylem is mesarch. For example, the stem of seed plants is end-arch; the root is always exarch; the stem of club mosses (Lycopodium spp.) is exarch; mesarch xylem commonly found in the ferns and in the hypocotyl region of angiospermic seeding’s.
The first cells of the xylem to mature are collectively called the protoxylem. The protoxylem is complex tissue made up of tracheids vessels and parenchyma cells. The protoxylem consists of annular spiral and scalariform vessels which may stretch in length very easily. In the stems it lies towards the centre of the axis whereas in the root it lies towards periphery.
The vessles of protoxylem have smaller cavities. The xylem which develops afterwards and possesses reticulate and pitted vessels and some tracheids is called metaxylem. In the roots towards the centre. The vessels of metaxylem have bigger and wider cavities.
(b) Phloem or bast:
Usually in stems, phloem is found away from the centre of the axis towards the periphery and consists of sieve tubes or sieve cells only, or sieve tubes and companion cells only, or sieve tubes, companion cells and phloem parenchyma. In gymnosperms it is represented by sieve cells only.
In most of monocotyledons it consists of sieve tubes and companion cells only, whereas in dicotyledons, sieve tubes, companion cells and phloem parenchyma possess simple pits in their walls, particularly which lie against the sieve tubes.
Phloem serves for translocation of prepared carbohydrates from leaves to the storage tissue and other growing regions. Sieve tubes translocate proteins and some other carbohydrates, phloem parenchyma conducts amines, amino acids and soluble carbohydrates and companion cells also translocate many soluble food materials.
All phloem elements are living and formed of cellulose. In certain cases, the primary phloem is capped by a patch of sclerenchyma called hard bast as in the Helianthus stem.
The first cells of the phloem to mature are known as protophloem. The protophloem consists of narrow sieve tubes, and is found towards periphery. The inner portion of the phloem consists of bigger sieve tubes called meta-phloem. The metaphloem is complex tissue and consists of well developed cells of all types such as—sieve tubes, companion cells, phloem parenchyma and sometimes phloem fibres and sclereids.
(c) Cambium:
In between xylem and phloem, a thin strip of primary meristem is found in dicotyledonous stems, called the cambium. The cells of cambium are rectangular and thin-walled. The cambium strip may be uniseriate or multi-layered. The cambial cells are living, sufficiently elongated and possess oblique ends, but as they become flattened tangentially they look rectangular in cross-section.
Types of Vascular Bundles:
A vascular bundle consists of a strand like portion having xylem and phloem of the primary vascular system.
According to the arrangement of xylem and phloem in the vascular bundles, they are being arranged in the following main types:
(1) Radial,
(2) Conjoint and
(3) Concentric.
(1) Radial:
Those in which the xylem and the phloem lie radially side by side (e.g., in roots of seed plants). This is most primitive type.
(2) Conjoint:
Those in which the two types of tissues are separated from one another. Here, xylem and phloem together form a bundle.
The two subtypes are:
(a) Collateral and
(b) Bicol-lateral.
(a) Collateral:
The xylem and phloem lie together on the same radius in the position that xylem lies inwards and the phloem outwards. Here the phloem occurs on one side of the xylem strand. In dicotyledonous stem, the cambium is found to be present in between xylem and phloem, such bundles are called open (e.g., in Helianthus), and when the cambium is absent it is called closed (e.g., in monocotyledonous stems).
(b) Bicollateral:
In such bundles the phloem is found to be present on both sides of xylem. Simultaneously two cambium strips also occur. Various elements are arranged in the following sequence—outer phloem, outer cambium, xylem, inner cambium and inner phloem. Such bundles are commonly found in the members of Cucurbitaceae. Such bundles are always open.
(3) Concentric:
Those in which one type of tissue surrounds, or en-sheaths, the other. The concentric bundles may be of two subtypes, amphivasal and amphicribral. If the xylem surrounds the phloem it is called amphivasal bundle as found in Dracaena, Yucca and other monocots and some dicots. If the phloem surrounds the xylem, it is amphicribral as found in many ferns. Such bundles are always closed.
