After reading this article you will learn about: 1. Contents of the Plant Body 2. Fundamental Parts of the Plant Body 3. Development of the Plant Body 4. Internal Organization of Vascular Plant 5. Modular Growth of Plant Body.
Contents of the Plant Body:
The plant body consists of a number of organs, i.e., root, stem, leaf and flower. The flower consists of sepals, petals, stamens, carpels and sometimes also sterile members. Each organ is made up of a number of tissues. Each tissue consists of many cells of one kind.
The complex multicellular body of the seed plant is a result of evolutionary specialization of long duration. This specialization has given rise to the establishment of morphological and physiological differences between the various parts of the plant body and also caused the development of the concept of plant organs.
The organization of the plant body of the oldest known land plants, the Psilophytales, suggests that the differentiation of the vegetative plant into leaf, stem and root is a result of evolutionary development from an originally simple axial structure (Arnold, 1947; Eames, 1936).
As regards the morphologic nature of the flower it is thought that the flower is homologous with a shoot and the floral parts with leaves.
Fundamental Parts of the Plant Body:
The axis, consists of two parts—that portion which is normally aerial is known as the stem, and the portion which is subterranean is called the root.
There are three types of appendages arising from the axis:
The strands of vascular tissue pass through the leaves. The leaves are characteristic of the stem and do not occur on the root. The leaves are found to be arranged on the stem in a definite manner, and bear an intimate structural relation to the skeleton of the axis.
The leaf is looked upon as the lateral expansion of the stem, continuous with it. All fundamental parts of the stem are concerned with the formation of the leaf.
In the appendages of the second rank only the outermost layers of stem, the cortex and the epidermis, are usually present which are known as emergences. The prickles of the rose make a good example of it.
The appendages of the third rank are hairs. These are projections of the outermost layer of the cells. The emergences and hairs occur on both axis and leaves, usually without definite arrangement.
Development of the Plant Body:
A vascular plant begins its existence as a morphologically simple unicellular zygote (2n). The zygote develops into the embryo and thereafter into the mature sporophyte. The development of the sporophyte involves division and differentiation of cells, and an organization of cells into the tissues and tissue systems.
The embryo of the seed plant possesses relatively a simple structure as compared with the mature sporophyte. The embryo bears a limited number of parts— generally only an axis bearing one or more cotyledons. The cells and tissues of this structure are less differentiated.
However the embryo grows further, because of the presence of meristems, at two opposite ends of the axis, of future shoot and root. After the germination of the seed, during the development of shoot and root, the new apical meristems appear which cause a repetitive branching of these organs. After a certain period of vegetative growth, the reproductive stage of the plant is attained.
Primary and Secondary Growth:
The first-formed plant body is known as the primary plant body, since it is built up by means of first or primary growth. The tissues of this first-formed body are known as primary tissues; for example the first-formed xylem is called primary xylem. In most vascular cryptogams and monocotyledons, the entire life cycle of the sporophyte is completed in a primary plant body.
The gymnosperms, most dicotyledons, and some monocotyledons show an increase in thickness of stem and root by means of secondary growth. The tissues formed as the result of secondary growth are called secondary tissues. Generally the new types of cells are not formed by means of secondary growth. The bulk of the plant increases because of secondary growth.
Especially the vascular tissues are developed which provide new conducting cells and additional support and protection. The secondary growth does not fundamentally change the structure of the primary body. The primary growth increases the length of the axis, forms the branches and builds up the new or young parts of the plant body.
Thus, a secondary body composed of secondary tissues is added to the primary body composed of primary tissues.
A special meristem, the cambium, is concerned with the secondary thickening. The cambium arises between the primary xylem and the primary phloem, and lays down new xylem and phloem adjacent to these. Thus the secondary masses of xylem and phloem are found entirely within the central cylinder and between the primary xylem.
The newly formed secondary xylem cloaks and ultimately surrounds the primary xylem and the pith. During this process the primary structure is not changed but engulfed intact within secondary xylem. The primary phloem and all other tissues outside the cambium are forced outward by secondary growth and ultimately crushed or destroyed.
In addition, a cork cambium or phellogen commonly develops the peripheral region of the axis and produces a periderm, a secondary tissue system assuming a protective function when the primary epidermal layer is disrupted during the secondary growth in thickness.
Meristems Based on Position in Plant Body:
As regards their position in plant body, the meristems may be classified into three groups: apical meristem, intercalary meristem and lateral meristem.
The apical meristem lies at the apex of the stem and the root of vascular plants. Very often they are also found at the apices of the leaves. Due to the activity of these meristems, the organs increase in length. The initiation of growth takes place by one or more cells situated at the tip of the organ.
These cells always maintain their individuality and position and are called ‘apical cells’ or ‘apical initials’. Solitary apical cells occur in pteridophytes, whereas in higher vascular plants they occur in groups which may be terminal or terminal and sub-terminal in position.
The intercalary meristems are merely portions of apical meristems that have become separated from the apex during development by layers of more mature or permanent tissues and left behind as the apical meristem moves on in growth. The intercalary meristems are inter-nodal in their position.
In early stages, the internode is wholly or partially meristematic, but later on some of its part becomes mature more rapidly than the rest and in the internode a definite continuous sequence of development is maintained. The intercalary meristems are found lying in between masses of permanent tissues either at the leaf base or at the base of internode.
Such meristems are commonly found in the stems of grasses and other monocotyledonous plants and horsetails, where they are basal. Leaves of many monocotyledons (grasses) and some other plants, such as Pinus, have basal meristematic regions. These meristematic regions are short living and ultimately disappear; ultimately, they become permanent tissues.
The lateral meristems are composed of such initials which divide mainly in one plane (periclinally) and increase the diameter of an organ. They add to the bulk of existing tissues or give rise to new tissues. These tissues are responsible for growth in thickness of plant body. The cambium and the cork cambium are the examples of this type.
The cambium does not fall definitely in either group (primary and secondary). It arises from apical meristem of which it is late and specialized stage. However, the accessory cambia are secondary. The tissues formed by the cambium are secondary, whereas the primary meristems form only primary tissues.
The primary growth of an axis is completed in a relatively short period, whereas the secondary growth persists for a longer period and in a perennial axis the secondary growth continues indefinitely.
The stem apex like the root apex consists of a meristematic zone of cells that remain in a continuous and rapid state of division. This is called promeristem having cells with very thin walls. Immediately beneath the promeristem there is zone of determination which has no visible boundary with the promeristem. In the dicots, this zone has a group of conspicuous cells with dense cytoplasm.
These cells in a transverse section are arranged in a circle (Fig. 31.4.). It is a remnant of a primordial meristem, which remains behind in a maturing segment, and it retained its activity to divide. Due to its circular appearance it is also called the ring meristem. The cells in the centre are protopith and those that are external to the ring meristem are the protocortex.
The cells of protocortex and protopith divide and build up the mass of ground tissue. The cells of the ring meristem divide longitudinally and form elongated cells that later on develop in vascular bundles and are known as procambical strands. The first formed phloem elements and so the xylem elements differentiate from the pro-cambial strands.
Internal Organization of Vascular Plant:
The cells or the morphologic units of the plant body are associated in various ways with each other forming tissues. In the plant body the cells are of several kinds and their combinations into tissues are such that different parts of the same organ may differ from one another. The larger units of tissues may show topographic continuity or physiologic similarity, or both together. Such tissue units are called tissue systems.
Thus the complex structure of the plant body results from variation in the form and function of cells and also from differences in the type of combination of cells into tissues and tissue systems.
As pointed out by Sachs (1875), the plant body of a vascular plant is composed of three systems of tissues:
(1) The dermal,
(2) The vascular and
(3) The fundamental or ground system.
The three vegetative organs, i.e., stem, root and leaf, are distinguished by the relative distribution of the vascular and ground tissues. The vascular system of the stem is found between the epidermis and the centre of the axis. In such type of arrangement the cortex (ground tissue) is found between the epidermis and the vascular region and the pith in the centre of the stem (Fig. 31.6 B, C.).
In the root, the pith may be absent (Fig. 31.6 E), and the cortex is generally shed during secondary growth (Fig. 31.6 D).
The primary vascular tissues are commonly being arranged in the form of a ring of bundles as seen in transverse section of stem (Fig. 31.6 B). During secondary growth the original primary vascular system may be obscured by secondary vascular tissues between the primary xylem and the primary phloem (Fig. 31.6 C).
In the leaf the vascular system consists of many interconnected strands (bundles) found in the ground tissue. In the case of leaf the ground tissue consists of photosynthetic parenchyma, and is known as mesophyll (Fig. 31.6 G).
The above mentioned tissue systems of the primary plant body are derived from the apical meristems (Fig. 31.6 F, H). The partly differentiated derivatives from these meristems may be classified as—protoderm, procambium and ground meristem.
They make the meristematic precursors of the dermal, vascular and fundamental (ground) systems, respectively. The vascular tissue system enlarges by secondary growth which takes place in the vascular cambium. The periderm may be derived from a separate meristem, the phellogen or cork cambium (Fig. 31.6 D).
Modular Growth of Plant Body:
Variation in organisational form that arises from size dependent relationship among parts is a fundamental aspect of development and evolutionary change. Allometric analysis is one of the earliest morphometric tools that help in revealing ontogenetic or evolutionary changes in shape as a consequence of changes in organ or body size (Gayon, 2000).
In plant population the individuals change widely in size may be due to asymmetric competition for light (Weimer, et al, 2001) or meagre distribution of other resources. The variability is dependent on the way plants are constructed. Plants grow through the repeated addition of similar morphological sub-units called metamers and modules.
This mode of development contributes to size variation by creating the potential for both indeterminate and exponential growth.
Modularity presents special challenge and opportunities for allometric analysis. In biological system there are different levels of organisation, e.g., genetic families → individuals → population → communities
Modular organisms have additional levels of organisation due to their clear within individual substructure. The modular growth provides serial adjustment to the phenotype through the addition of new parts during development.
Module and metamer may be defined as follows:
Module is a product of an apical meristem; while metamers are serially homologous repeated units along an axis and are generally sub-units of modules. A vegetative metamer consists of a leaf, the segment of stem subtending it and its axillary meristem.
Types of Modularity:
On developmental genetics modules refer to largely autonomous developmentally and functionally integrated units (Carrol, 2001). According to comparative morphology the modular growth occurs through the retention of the same few types of structures, e.g., branches and flowers in plants.
In contrast to developmental genetic modules, the modular subunits comprising an organism are similar to each other, i.e., serially homologous and loosely analogous to individuals in population.
Modularity in Plants:
In plants, module is the product of apical meristem, e.g., branches, cones or flowers. Vegetative modules produce new meristem that may give rise to additional vegetative or reproductive modules, creating a modular hierarchy.
Usually modules themselves are composed of repeated units or metamers. For example, a typical vegetative metamer consists of a node and an internode, a leaf and an axillary meristem (Fig. 31.9).
In seed plants, e.g., gymnosperms or angiosperms, the plant body is essentially modularly organised. The smallest elementary unit of construction in the shoot system is called a metamer. Metamers make branches and branches construct branching system. Each shoot apex is a potential point of further development. Development is iterative or repetitive process. The finite bit of programme is repeated to add new metamers.
The number of iterations can be infinite in some species. In annual plants iterations certainly terminate, but their number is not strictly defined and may depend on environment (Schlichting and Smith, 2002). The number of leaves of a tree can vary with age and habitat conditions. It is due to capability of formation of organs continuously. Organs, metamers and branches’ are disposable without death of organism.
Shoot System Modules:
For the first time, the term metamer was used in animals. Metamers unite to form module. Modules divide due to outgrowths of axillary buds or adventitious buds. When the axillary bud grows continually without undergoing rest period is called sylleptic sub-module while the axillary bud that undergoes some dormancy period is called proleptic sub-module.
The activity of module is directly related with that of apical meristem. After getting a stimulus, the axillary bud activates and module branches. New modules may arise some time playing an important role in regeneration, e.g., in Eucalyptus, after fire new modules grow out rapidly from hidden buds produced by modification of the base of stem.
Any module or sub-module terminates in a flower or inflorescence depending upon the portion or orientation of metamers in shoot apex.
Root System Modules:
Primary and lateral roots are modules of root system. Lateral roots are well developed in herbs. Various types of roots, such as respiratory roots, storage roots, etc., are types of modules.