The below mentioned essay provides an overview on Leaf. After reading this essay you will learn about: 1. Origin of Leaves 2. Origin of Branches 3. Origin of Reproductive Shoot Apex 4. Anatomy of the Leaf.
Contents
Essay on the Origin of Leaves:
A leaf initiates by periclinal divisions in a small group of cells at the side of an apical meristem. In angiosperms, the tunica and the corpus are responsible for leaf initiation. In the dicotyledonous plants the periclinal divisions initiating the leaves occur, not in the surface layer, but in one or more layers beneath it.
If the tunica is single- layered, such divisions take place within the corpus, otherwise they occur both in tunica and corpus or in the tunica only. In certain monocotyledonous plants the superficial tunica layer undergoes periclinal divisions and gives rise to some or most of the tissue.
In the case of gymnosperms the leaves initiate from the peripheral tissue zone. In the vascular cryptogams (pteridophytes) the leaves are initiated either from single superficial cells or from groups of such cells.
The periclinal divisions which initiate a leaf primordium are responsible for the formation of a lateral prominence on the side of the shoot apex. This prominence constitutes the leaf base which is also known as leaf buttress. Subsequently the leaf grows upwardly from the buttress. As shown in the figure, in Hypericum spp., the apical meristem is less prominently elevated above the youngest leaf buttress.
Before the initiation of a new leaf primordium the apical meristem appears as a rounded mound. It gradually widens, and, then leaf buttresses are initiated on its sides. While the new leaf primordia grow upward from the buttresses, the apical meristem again becomes like a small mound.
Essay on the Origin of Branches:
In angiosperms, branches commonly are initiated in close association with the leaves – They originate in the axils of the leaves, and in their nascent state they are known as axillary buds.
The axillary buds commonly initiate somewhat later than the leaves subtending them and therefore, it is not always clear whether the meristem of the axillary bud is derived directly from the apical meristem of the main shoot or whether it originates from partly differentiated tissue of the internode.
Both situations may occur because plants vary with regard to time of appearance of axillary buds. On the one hand, the axillary buds may be directly related to the apical meristem of the parent shoot; on the other hand they may intergrade, ontogenetically with the adventitious buds which arise in obviously differentiated tissue regions.
The initiation of the axillary bud in seed plants is characterized by a combination of anticlinal divisions, in one or more of the superficial layers of the young axis, and of various divisions, sometimes predominantly periclinal, in the deeper layers (see Fig. 41.2). This coordinated growth in surface of the peripheral region and growth in volume at greater depth cause the bud to protrude above the surface of the axis.
Depending on the quantitative relationships between the tunica and the corpus in the shoot apices of angiosperms. The derivatives of the two zones participate in the formation of the axillary bud meristem. If the axillary bud develops into a shoot, its apical meristem gradually organizes — commonly duplicating the pattern found in the parent shoot apex and proceeds with the formation of leaves.
Essay on the Origin of Reproductive Shoot Apex:
In the reproductive state in angiosperms, floral apices replace the vegetative apices either directly or through the development of an inflorescence. The flower, which may occur singly or as part of an inflorescence, is formed during the reproductive phase of growth.
It develops from a terminal or lateral vegetative shoot apex and results in the culmination of meristematic activity of that particular meristem. Thus, the floral apex, like the leaf primordium and unlike the vegetative shoot apex, shows determinate growth.
The change to the reproductive stage may be first detected by the modified growth habit of the shoot. When the flowers develop an axillary-branch inflorescence, there appears an acceleration in production of axillary buds, which shows one of the earliest indication of approaching flowering.
Simultaneously, the nature of foliar organs subtending the axillary buds also changes. They develop as bracts more or less distinct from the foliage leaves. Here during the reproductive stage the axillary buds appear earlier and grow more vigorously than the subtending bract primordia. The next feature that reveals the beginning of the reproductive state is the sudden increase in the elongation of internodes.
From the viewpoint of histology and cytology the reproductive meristem differs from the vegetative meristem in varying degrees. It may have the same quantitative relationship between the tunica and the corpus as was present in the vegetative apex, or the number of separate surface layers may be reduced or increased.
The most conspicuous change is exhibited in the distribution of the eumeristematic and the more highly vacuolated cells.
In many species the apex of the inflorescence or the flower shows a uniform, small-celled mantle-like zone of one or more layers enclosing a large celled core; this type of apex may be flatter and wider than the vegetative one.
It is not necessary that the mantle may coincide with the tunica; a part of corpus may be included in it. The cells of the central tissue enlarge and become vacuolate, and the meristematic activity remains restricted to the mantle zone. This activity is concerned only with the production of floral organs.
Essay on the Anatomy of the Leaf:
Commonly there are two types of leaves:
1. Dorsiventral Leaves (dicotyledonous) and
2. Isobilateral Leaves (monocotyledonous).
The dorsiventral leaves usually grow in a horizontal direction with distinct upper and lower surfaces, the upper being more strongly illuminated than the lower. There exists a difference in the internal structure between the upper and lower surfaces of the dorsiventral leaf due to its unequal illumination.
Most of the dicotyledonous leaves are dorsiventral. The isobilateral leaves hang vertically so that both surfaces of the leaf receive direct and equal amount of sunlight. The isobilateral leaves possess a uniform structure on both upper and lower surfaces. A very few dicotyledons and most monocotyledons have isobilateral leaves.
Usually the leaf is composed of various tissues, which furnish various functions. In discussions of the form and anatomy of the leaf, it is customary to designate the leaf surface that is continuous with the surface of the part of the stem located above the leaf insertion as upper, ventral, or adaxial side, the opposite side as the lower, dorsal or abaxial.
Anatomy of Dicotyledonous Leaf:
To study the anatomy of leaf, several vertical sections passing through the mid-rib are required.
The internal structure of the dicotyledonous leaf is as follows:
Epidermis:
The leaf is covered on both surfaces by a single-layered epidermis. The outer walls of the epidermis are usually thickened, and covered over with a waxy substance called cutin. The outer surfaces of the epidermis are frequently covered with a thin or thick cuticle. This cuticular layer is formed of cutin.
As the outer walls of the epidermis are thick and cutinized, water does not pass through them rapidly and the transpiration from the surface of the epidermis is greatly reduced, only small quantity of water is evaporated by transpiration. The epidermis checks the transpiration to a great extent.
The epidermis also prevents the entrance of pathogens into the interior of the leaf. Another function of the epidermis is the protection of the soft internal tissue of the leaf from the mechanical injuries. Sometimes in the xerophytic leaves the epidermis cells become radially elongated and somewhat lignified. In Nerium leaf, the epidermis is multi-layered.
Numerous small openings called stomata are found in the epidermal layers of the leaves. Stomata are found in most abundance in the lower epidermis of the dorsiventral leaf. They are very few in the upper epidermis and sometimes altogether absent.
In the floating leaves, stomata remain confined to the upper epidermis; in the submerged leaves the stomata are absent. In xerophytic leaves either stomata are sunken or situated inside the depressions.
Each stoma remains surrounded by two semilunar guard cells. The guard cells are living and contain chloroplasts; they regulate opening and closing of stomata. The guard cells may remain surrounded by two or more accessory cells in addition to epidermal cells. The stomata are found in scattered condition.
Usually the stomata are meant for exchange of gases in between the plant and the atmosphere. To facilitate the diffusion of gases properly, each stoma opens internally into a respiratory cavity or sub-stomatal chamber. The transpiration takes place through the stomata, and the surplus water is being evaporated.
Mesophyll Tissue:
The tissue of the leaf that lies between the upper and lower epidermis and between the veins consist of typically thin walled parenchyma is known as mesophyll. This tissue forms the major portion of the inner of leaf. Commonly the cells of mesophyll are of two types—the palisade parenchyma or palisade tissue, and the spongy parenchyma or spongy tissue. The mesophyll tissues always contain chloroplasts in them.
The palisade parenchyma is generally composed of elongated and more or less cylindrical cells which are close together with long axes of the cells perpendicular to the epidermis. In transverse section the cells appear to be arranged quite compact, are really separate from each other having intercellular spaces among them.
The palisade tissue may consist of a single or more layers. These cells are arranged near to the upper surface of the leaf, where they receive sunlight and facilitate to carry the function of photosynthesis. Sometimes the leaves hang vertically (e.g., Eucalyptus), so that both surfaces of leaf are equally illuminated.
In such leaves the palisade parenchyma may occur on both sides. The compactness of the palisade parenchyma depends upon light intensity. The leaves which receive direct sunlight develop more compact parenchyma in comparison to the leaves which develop in shady places.
The lower portion of the mesophyll in the leaf is known as spongy parenchyma or spongy tissue. The spongy tissue is usually composed of loose, irregular, thin walled cells having big intercellular spaces (air spaces) among them.
The cells of spongy parenchyma also contain chloroplasts and carry on photosynthesis, but in comparison of palisade parenchyma less chloroplasts are developed. Due to the presence of a large air space in the spongy tissue they are more adaptable to the exchange of gases between the cells and the atmosphere.
The large air spaces that surround the spongy parenchyma cells are near the stomata and directly connected with them. There is therefore a much more free circulation of gases around these cells than around the palisade parenchyma cells, with the result that they are better suited to the exchange of gases between the cells and the surrounding atmosphere.
The air spaces of the spongy chlorenchyma are not isolated chambers but a series of intercommunicating passages.
Both spongy and palisade parenchyma contain discoid chloroplasts arranged in parallel rows in the cells. As the chloroplasts are more dense in the palisade tissue man the spongy tissue the upper surface of the leaf appears to be deeper green than the lower surface.
Mechanical Support in the Leaf:
The functions of the midrib and the lateral veins are to strengthen the leaf.
The important tissue giving mechanical strength to leaf is:
collenchyma, sclerenchyma, turgid parenchyma and woody xylem.
Collenchyma:
In the centre of the upper portion of the midrib, just below the epidermis, there is usually a group of cells which give strength by having thickened walls and by being turgid A group of the same kind of cells usually occurs also just above the lower epidermis. These cells constitute the collenchyma.
Collenchyma is composed of living cells with walls which are thickened at the angles where three or more cells come in contact with one another. The thick places in the walls increase the strength of the cells, while the thin places allow for a more rapid transfer of materials from cell to cell than would take place if the cell walls were thickened throughout.
These cells are more or less turgid, and so give strength to the leaf in the way also. The weight of the leaf causes it to tend to bend downward, with the result that there is a tendency for the upper portion to be stretched and the lower portion compressed. The collenchyma occurs, therefore, in those parts of the midrib in which there is the greatest need for strengthening material.
Sclerenchyma:
Usually the sclerenchyma cells or the fibres are associated with the vascular tissues of the leaves. They occur usually as bundle caps adjacent to the phloem. Sometimes the fibres are found on both the sides of large vascular bundle of the leaves.
Usually these cells are thick walled, dead and lignified. Their position just exterior to thin-walled phloem affords mechanical protection to the latter. The fibres are greatly elongated in the longitudinal direction of the midrib.
Turgid parenchyma:
The regions between the collenchyma cells and the central portion of the midrib are occupied by parenchyma cells. In structure the parenchyma cells are not specially modified for any particular function, but they perform all the general functions of cells to a limited extent. Parenchyma cells have thin walls, but on account of their turgidity they strengthen the midrib.
Xylem:
Usually the vessels and tracheids of xylem conduct water, but due to their thick walled nature they also give mechanical support to the leaves. The xylem elements are composed of lignified and dead cells.
Orientation of Vascular Tissue:
In the leaf traces of flowering plants, before they have the stele, the phloem is always found towards the outside of the stem. The leaf traces after their entrance in the petiole and lamina, also maintain the relative position of the xylem and the phloem, i.e., the phloem is always found towards the lower side and the xylem towards the upper side in the leaf.
Sometimes the xylem ring remains surrounded by a ring of phloem. The phloem occurs only below the xylem or rarely both above and below it.
Conducting system:
The tissues which constitute the conducting system are situated near or at the centre of the midrib. This system may have various shapes, e.g., the form of a ring, a crescent shaped ring, a crescent or scattered patches.
In the ring shaped conducting system parenchyma cells are usually found in the centre of the ring. The inner part of the ring is composed of xylem (towards upper surface), and phloem (towards lower surface).
Xylem is composed of various kinds of vessels, trachieds wood fibres and wood parenchyma. Specially the vessels are annular and spiral. Xylem conducts water, raw food material and also gives mechanical support to the leaf. The phloem consists of sieve tubes, companion cells and phloem parenchyma. The phloem serves for the translocation of prepared food material from the mesophyll of the leaf.
Veins:
The structure of large veins is more or less similar to that of a midrib. As they pass from the base of a leaf blade towards the apex or margin of the leaf, they get reduced in size, and simple in structure. The small veins consist of only of few conducting cells.
The xylem is always found towards the upper surface and phloem towards lower even in very small veins. The cells of the mesophyll (chlorenchyma) are usually arranged so that the conduction of materials to and from the veins is facilitated.
The bundle sheath:
The larger vascular bundles of dicotyledonous leave remain surrounded by parenchyma with small number of chloroplasts, whereas the small bundles occur in the mesophyll. However, these small bundles do not remain in contact with intercellular spaces but are commonly enclosed with a layer of compactly arranged parenchyma, the bundle sheath.
In dicotyledons the bundle-sheath parenchyma is also called border parenchyma.
The bundle sheaths of dicotyledonous leaves usually consist of cells elongated parallel with the course of the bundle and having walls as thin as those of adjacent mesophyll. In some plants these cells have chloroplasts similar to those the mesophyll (e.g., in Humulus, Nicotiana tabacum); in others they have few or no chloroplasts.
The bundle sheath cells are in direct contact with the conducting cells of the vascular bundle of parenchyma and on the outer face with the mesophyll tissue. Individual sheath cells may contain crystals.
The parenchymatous bundle sheaths are more common, but in certain dicotyledons bundles of various sizes are enclosed in sclerenchyma, e.g. Winteraceae, Melastomaceae; (Bailey and Nast, 1944; Foster, 1947).
Vertical leaves:
The leaves of many species of Eucalyptus do not spread out horizontally but hang vertically, so that both surfaces of the leaf receive direct sunlight. In keeping with this fact, palisade chlorenchyma is developed on both sides.
Anatomy of Monocotyledonous Leaf:
The monocotyledons as a group show greater diversity of specialized leaf types. The leaves of this group are not made up of stipules, petiole and leaf blade. In general monocotyledonous leaves are parallel-veined.
Most of monocotyledonous leaves are nearly erect and more or less both surfaces usually receive direct and equal amount of sunlight. Such leaves are called isobilateral (isos = equal; bi = two; lateris = side). The internal structure of such leaves is more or less similar in both the upper and lower halves.
The epidermis on either side contains the stomata and the mesophyll is usually not differentiated into palisade and spongy parenchyma, but consists only of parenchyma cells, having chloroplasts and intercellular spaces among them.
Anatomy of Leaf of Zea Mays (Maize)—Monocot:
Epidermis:
The epidermis is found on both upper and lower surfaces of the leaf. The epidermal layers are uniseriate and composed of more or less oval cells. The outer wall of the epidermal cells is cuticularized. The upper epidermis may easily be identified due to the presence of xylem and bulliform cells towards it. Stomata are confined to both the epidermal layers.
Mesophyll:
As the leaf is isobilateral, the mesophyll is not differentiated into palisade and spongy tissues. It is composed of compactly arranged thin walled, isodiametric chlorophyllous cells having well developed intercellular spaces among them.
Vascular bundles:
The vascular bundles are collateral and closed as found in monocotyledonous stems. Most of the bundles are small in size but fairly large bundles, also occur at regular intervals. The xylem is found towards upper side and phloem towards lower side in the bundles. Usually each bundle remains surrounded by a bundle sheath consisting of thin walled parenchyma cells.
The cells of bundle sheath generally contain starch grains in them. Xylem consists of vessels and phloem of sieve tubes and companion cells. Sclerenchyma cells occur in patches on both ends of the large vascular bundles which give mechanical support to the leaf.
Anatomy of the Leaf of Triticum Aestivum (Wheat)—Monocot:
Epidermis:
As usual the epidermis layers are found on both upper and lower surfaces of the leaf. The epidermises are uniseriate and composed of more or less oval cells having no intercellular spaces among them. The outer walls of epidermal cells are cuticularized.
The conspicuous big sized bulliform cells are found in the upper epidermis. The stomata are confined to both epidermis layers. The sub-stomatal chambers are also seen in vertical section.
Mesophyll:
It is composed of more or less oval chlorenchyma cells having intercellular spaces among them. The mesophyll tissue is not clearly differentiated into palisade and spongy parenchyma; however, the cells towards epidermal layers are somewhat elongated and palisade-like. Sub-stomatal chambers are seen beneath the stomata.
Vascular bundles:
The vascular bundles are collateral and closed as found in monocotyledonous stems. The bundles are arranged in parallel series. Xylem occurs towards upper surface and phloem towards lower surface. Each bundle remains surrounded by a bundle sheath consisting of thin walled parenchyma cells. The sclerenchyma strands are found on both the ends of each big vascular bundle.
