Organisation living body - Plant, Plant Tissue, Morphology of Plant, organisation of Animals
Organisation of Plant and Animals- Specially for students of America, UK, Canada Australia India and other countries
Organisation of the Living Body :
A multicellular organism is not a heap of cells but a highly organised body, the complexity of which increases as we proceed from simple primitive forms to the more advanced. Cells, the structural and functional units of the living body, are organ- ised into tissues which in turn are organised into organs and organ systems. The organisation is, in fact, for the division of labour as the number of cells increases because of the increase in body size. However, the organisation is different in plants and animals since their ways of life are different.
Organisation of Plants:
The organisation in lower plants (algae and bryophytes) is generally very simple-the cells may be organised into colonies or filaments (algae) or into flattened thallose forms (bryophytes). In large-sized marine algae (sea weeds) and some bryophytes, however, the plant body shows differentiation into root, stem, and leaflike forms although true root, stem, and leaf are not present. Internally also, there is not much differentiation of cells into different tissues. But in ferns, gymnosperms (cycads, conifers etc.) and flowering plants, the plant body is differentiated into true root, stem and leaves which in turn are made up of different kinds of tissues. These plants are called vascular plants since they have a well-developed vascular system (conducting tissues like xylem and phloem).
Plant Tissues
Tissues are organisations of cells of one or more types with a common origin performing a common function or a set of functions. They are variously classified. However, on the basis of their stage of development, the tissues are classified into two types: meristematic tissues and mature nonmeristematic tissues.
Meristematic Tissues
If the cells of a tissue retain the capacity to divide repeatedly adding new cells to the plant body, it is called a meristematic tissue. The meristematic tis- sues are variously classified based on: their posi- tion in the plant body, the type of derivatives pro- duced, and their origin.
On the basis of their position in the plant body, the meristematic tissues are classified into the following types:
(i) Apical Meristems: As the name suggests, they are found at the root and shoot apices, whether terminal or lateral, and are responsible for the growth in length of both the root system and the shoot system. In lower vascular plants (Pteridophytes), the apical meristems are simple consisting of one or more initials that give rise to all the cells of the apex. However, the apical meristems of spermatophytes (seed plants) are more complex showing distinct zonation.
(ii) Basal Meristems: They are found at the base of an organ, e.g. in prickly pear cactus, the growth of the spines is dependent on the activity of meristem at the base of the spine. The basal region, in this case, contains only meristematic cells and can be divided into a protoderm and a ground meristem. Cells out of the meristem flow upward, elongate, and then differentiate into fibres.
(iii) Intercalcary Meristems: This type of meristems are often found in grasses at the base of each internode or leaf lamina intercated between fully differentiated tissues and are responsible for the rapid growth of grass internodes or lamina at maturity. The meristem contributes cells on both sides. Other than grasses, intercalary meristems have been found in some species of Caryophyl- laceae, Polygonaceae, and Chenopodiaceae. Intercalary meristems are also found in peduncles of inflorescences in certain plants and the growth of the gynophore (a short stalk at the base of ovary) in the peanut plant (Arachis hypogaea) is a result of the activity of an intercalary meristem.
(iv) Lateral Meristems: Again, as the name suggests such meristems are laterally borne in the plant body and are thus responsible for growth in diameter. All the cambia, therefore, fall in this category, i.e. the vascular cambium and the cork cambium. The vascular cambium produces the wood (xylem) and the bark (phloem) and the cork cambium (phellogen) produces cork, a protective tissue.
(v) Axillary Meristems: These are actually the apical meristems of axillary buds but some anato- mists put them in separate category-the axillary meristems, because the buds are located in axils. On the basis of the types of derivatives produced, the meristems are classified into the following categories:
(i) Protoderm: The meristem that produces cells that differentiate into epidermis is referred to as the protoderm.
(ii) Procambium: The meristem producing pri- mary vascular tissues (primary xylem and primary phloem) is called procambium.
(iii) Ground Meristem: It is the meristem that produces relatively large amounts of more or less homogeneous tissues, e.g. cortex, pith, or masses of sclerenchyma fibres within a spine.
(iv) Promeristems: These are meristems that di- rectly give rise to other meristems or to other, dis- tinct parts of the same meristem.
Another classification of the meristems is based on their origin or the sequence of formation of the tissues. On this basis, the meristems are classified into two types: primary and secondary meristems.
(i) Primary Meristems: The meristematic tissues whose cells develop directly from the embroyonic cells constitute the primary meristems, e.g. the root and the shoot apical meristems.
(ii) Secondary Meristems: The tissues produced by the activity of primary meristems are called primary tissues. Any meristem that develops within these tissues constitutes the secondary meristem like the cork cambium and the vascular cambium of roots and many stems.
Mature Tissues
The products of the activity of all meristems differentiate into various kinds of tissues performing different functions. These tissues are called mature or permanent because it was believed that the tissues that undergo differentiation gradually lose the embryonic characteristics of the meristem and acquire mature state. The term permanent is, however, not preferred these days because many of these tissues may differentiate and become meristematic again.
If a mature tissue is made up of a single type of cells, it is called a simple tissue but if the tissue is composed of more than one type of cells perform ing various functions, it is referred to as a complex tissue.
(i) Simple Tissues
Parenchyma: This type of tissue is made simple thin-walled, generally polyhedral cells with living protoplasm and constitutes the major part of the plant body except the vascular tissues. Parenchyma is, however, highly diverse with regard to cell size, shape, metabolism, and functional role. Metabolically parenchyma cells are the most active performing various vital functions. Based on function, parenchyma is subdivided into five classes; synthetic parenchyma, structural parenchyma, boundary parenchyma, transport parenchyma, and storage parenchyma. up of
Collenchyma: A few outermost layers of cells in a young stem below the epidermis (stem skin) often consist of collenchyma. The cells are usually elongated but can be short and isodiametric or tapering fibre-like. They have living protoplasm and usually have wall thickenings in the corners. most striking and most important feature of The collenchyma cells is their wall which is considered to be primary, although it is much thicker. The thickening is, in fact, quite uneven with thin-walled areas alternating with regions that are much thick ened. On the basis of the pattern of thickening, the collenchyma is usually classified into four types:
angular collenchyma, lamellar collenchyma, lacunar collenchyma, and annular collenchyma. Sclerenchyma: This is the primary strengthen-
ing tissue found in mature plant organs as it allows no flexibility because of its highly thickened walls. The cells usually have no living protoplasm at maturity and may be highly elongated (then called fibres) or be isodiametric, irregular, or branched (called sclereids).
On the basis of their position, fibres are classified into xylary (in xylem) and extraxylary (out side xylem). Classification of sclereids is, on the other hand, based on their shape; brachysclereids (isodiametric), Macrosclereids (rod-shaped), astrosclereids (highly branched), osteosclereids (bone-shaped), and trichosclereids (long and hair- like, branched).
(ii) Complex Tissues
Xylem: The xylem is primarily a conducting tis- sue meant for the transport of water and mineral elements. Since the transport is under tension, the walls of its conducting elements are highly thickened and, therefore, provide strength to the plant body. Some of its cells are also involved in storage of water and nutrients. It is considered a complex tissue because it consists of four types of cells in the flowering plants-vessel elements, tracheids, fibres, and parenchyma. The vessel elements are connected end to end to form a vessel. Tracheids and vessels are the conducting elements of the xylem and together are generally called tracheary elements. Vessels are, however, not found in gymnosperms and ferns. Some primitive angiosperms also have no vessels.
Phloem: Phloem is meant for the transport of food material from the leaves, where it is synthesized, to the consumption/storage organs of the plant. It is considered a complex tissue because this too consists of four types of cells in flowering plants: sieve tube members, companion cells, fibres, and parenchyma. Like vessel members, the sieve tube members are joined end to end to form sieve tubes that are involved in transport of food. There are no companion cells is ferns and gymnosperms and their sieve elements too are simpler, as sieve cells. The end walls between two sieve tube members of a sieve tube have perforations in the form of sieve plates to facilitate food transport.
Epidermis: The epidermis is the outermost layer of the plant body and is primarily protective in function. It protects the plant against excessive loss of water through transpiration from the gen- eral surface of the shoot and also protects against microbial (fungal and bacterial) infection. Epidermis is considered a complex tissue because it con- sists of several types of cells: ordinary epidermal cells, guard cells, trichomes, subsidiary cells, and root hairs. The leaf and stem epidermis is always covered with a thick layer of cutin, a high-molecu lar weight lipid polyester. Because of its lipid nature, it is strongly hydrophobic and impervious to water. The layer of cutin over the surface is called cuticle. The epidermis of leaves and young twigs is interrupted by very fine pores called stomata which are guarded by a pair of guard cells. The guard cells together with the adjacent epidermal cells constitute the stomatal complex. The trichomes (hairs) that are part of the epidermis may be unicellular or multicellular, branched or simple. If the trichomes are secretory, they are called glan- dular hairs.
Secretory Tissues: The tissues secreting vari- ous types of substances-resin, mucilage, essen- tial oils, gums, nectar etc. have complex structures (several types of cells) and hence, in recent years, they are included among the complex tissues. They are classified variously based on: nature of secre- tory product, mechanism of secretion, purpose of secretion, and position and product of secretory structures.
### Morphology of the Plant
(i) Root System: The plant body is made up of root, stem, and leaves. The root may develop either from the radicle forming the tap root system or from the base of the stem forming the fibrous root system. The primary function of roots is fixation of the plant in the soil and absorption of water and minerals from it. However, in some cases the roots may get modified to perform special functions, e.g. food storage (radish, carrot, sweet potato, beet). support (maize, sugarcane, banyan), climbing (be- tel, pepper, ivy, Pothos), respiration (Jussiaea, mangroves) and absorption of food (Cuscuta. Loranthus, Orobanche).
(ii) Shoot System : The stem and leaves constitute the shoot system. The point of attachment of a leaf on the stem is called the node and the portion. between two nodes is the internode. The stem is meant to bear the leaves in such a way that they get maximum sunlight for photosynthesis and to transport water and minerals for the same. It may also get modified to take up some special functions, e.g. food storage (rhizome of ginger, turmeric, and arrowroot; tuber of potato; bulb of onion and garlic: corm of kachalu, saffron, Amorphophalus), climb- ing tendrils of grape vine, passion flower, balloon vine and Antigonon), protection (thorns of lemon, karonda, wood apple, Duranta), and photosynthesis (phylloclade of prickly pear and other cacti, cocoloba; cladode of Asparagus and Ruscus). The flower is also a shoot (stem) modified for reproductive purposes. Their arrangement on the axis (stem and its branches) is called inflorescence.
Leaves are the most important plant organs since their photosynthetic activity sustains all life, including man. The stalk with which the leaf is at- tached to the stem is called the petiole and its blade is referred to as lamina. It is said to be simple if its blade is not divided into segments, called leaflets, in contrast to the compound leaf in which it is.