The leaf

leaf

1. LEAF

2. LEAF Leaves are the thin and flat lateral outgrowths from stem nodes. They are the main vegetative organs of a plant body, concerned with photosynthesis, transpiration and gas exchange. Leaves are produced from a superficial meristematic tissue, called leaf primordia and hence they are exogenous in origin. The part of the stem that bears leaf is called the node. Each leaf possesses an axillary bud at its axil.

3. In many cases, the same plant bears different kinds of leaves. This condition is known as heterophylly. It is quite common among hydrophytes (e.g.. Limnophila heterophylla). Here, the submerged part of the shoot bears dissected leaves and the aerial part bears entire leaves. Some terrestrial plants may also have two types of leaves (e.g., coriander) Leaves show great variations in size, form, outline, surface texture, etc. In size, they vary from minute scales to the exceptionally large leaves of certain palms, which are sometimes 40 feet long. In form, some leaves are more or less circular, some others are long and narrow, but most leaves are of intermediate shape.

4. Parts of a Dicot leaf

5. Parts of a Dicot leaf A typical dicot leaf has three parts, namely Leaf base or Hypopodium Leaf stalk or Mesopodium Leaf lamina or Epipodium.

6. (a) Leaf base Leaf base is the initial part of a leaf that attaches to the stem. The upper angle between the leaf base and the stem is called axil. In some plants (e.g., Leguminosae), leaf base is swollen and is called pulvinus. Leaves with pulvinus are called pulvinate leaves (e.g., Mimosa pudica, Mangifera indica). Pulvinus is responsible for the sleep movement of leaves (nyctinasty) in Mimosa. In monocot leaves, leaf base has a sheath-like expansion, called sheathing leaf base (e.g., banana, grasses). It partially or completely encircles the stem.

7. In many dicots, a pair of leafy lateral outgrowths are given out from the leaf base. They are called stipules. Stipules usually protect the developing buds. Leaves with stipules are called stipulate leaves (e.g., Hibiscus, Ixora), and those without stipules are called exstipulate leaves (e.g., Mangifera). Stipules are believed to protect the developing embryonic leaf. Occasionally, they may get modified to enclose the leaf bud (e.g., Ficus).

8. Musa - sheathing Leaf base

9. (b) Leaf stalk Leaf stalk is the slender middle part that connects the lamina with the base. The stalk of a simple leaf is called petiole, and that of a compound leaf is called rachis. In some cases, leaves are without stalk and they are called sessile leaves (e.g., Calotropis, Zinnia). A leaf having petiole is called petiolate leaf (e.g., Hibiscus, Ficus). In some plants, petiole is wing-shaped (e.g., Citrus).

10. (e) Leaf lamina Lamina, or leaf blade, is the green, thin and expanded part of the leaf. It is actively involved in photosynthesis, respiration and transpiration. Its tip is called leaf apex, and the edge or border is called leaf margin. Petiole extends throughout the length of the lamina as a strong vein, called mid-vein or mid-rib. Mid-rib gives our bilateral branches to both sides. They are called veins. Veins, in turn, irregularly branch and re-branch into veinlets. Mid-rib, veins and veinlets form a network which constitutes the structural frame work of the leaf lamina

11. Also serves as the vascular strands or channels for the transport of organic food, water and minerals. Apex and margin of the lamina are variously shaped. Leaf margin is entire and smooth in some cases. (e.g. banana, grasses). In some others, it is toothed or serrated. (e.g. Hibiscus.) In yet others, it is lobed (e.g. tapioca), or wavy and uneven (e.g. Polyalthia) Leaf surface is smooth or curved, with a waxy coating. In some cases, it may bear hairs.

12. Monocot leaf

13. Monocot leaf Monocot leaf is similar to dicot leaf in its basic morphology. It also has the three parts leaf base, petiole and lamina. Leaf base is sheathing in monocots. It partially on completely encircles the stem. Petiole is absent in grasses. In some palms, it is modified into a rachis. In some monocots, there is a pair of small outgrowths from the junction between the leaf base and petiole. They are called ligules (e.g., grasses) The lamina of monocot leaves is isobilateral (lower and upper sides similar ) Venation is parallel in monocot leaves.

14. Monocot leaf

15. Venation Venation is the pattern of arrangement of veins in the leaf blade. There are two kinds of venation, namely reticulate venation and parallel venation . In reticulate venation the veins and veinlets form an anastomosing network In parallel venation veins are parallel to one another. Parallel venation is horizontal in some cases ( e.g. banana ), and longitudinal in others ( e.g. bamboo and other grasses, lilies ). Reticulate venation is characteristic of dicot leaves, and parallel venation is characteristic of monocot leaves. However, there are exceptions. For example, dicots, such Calophyllum, Alstonia and Eryngium, show parallel venation, and monocots, such as aroids, Dioscorea ( climbing yam ), etc. show reticulate venation.

16. Kinds of leaves Based on the number of lamina, or the nature of the incision of lamina, two kinds of leaves can be recognized, Simple leaves Compound leaves

17. (a) Simple leaves These are the leaves with only a single lamina (e.g., Hibiscus). In this case, the lamina is entire or incised, but the incision does not touch the mid-rib nor does it reach the tip of the petiole.

18. (b) Compound leaves These are the leaves whose lamina is divided into segments, called leaflets or pinnae. Unlike leaves, leaflets are devoid of buds or stipules in their axils. Individual leaflets may be stalked or sessile, and free from each other. At the same time, all of them are connected to a common stalk, called rachis. A compound leaf resembles a branch, and each leaflet of it resembles a simple leaf. Yet, a compound leaf differs from a branch in the absence of terminal and axillary buds and in its development from a node; a branch develops from the axillary bud of a leaf.

19. Types of compound leaves There are two types of compound leaves, namely Pinnately compound Palmately compound.

20. 1. Pinnately compound leaves These are the compound leaves in which leaflets are arranged biserially in two ranks on both the sides of the rachis. This is similar to the arrangement of the pinna or, of a bird's feather. e.g., Neem (Azadirachta indica), Moringa. There are 4 type of pinnately compound leaves. They are unipinnate, bipinnate, tripinnate and decompound leaves.

21. (1) Unipinnate or simple pinnate leaves These are the pinnately compound leaves in which the rachis bears leaflets in opposite or sub-opposite pairs. There are two types of unipinnate leaves They are paripinnate and imparipinnate. In paripinnate leaves, leaflets are paired and hence even-numbered and the rachis ends in a pair of them e.g. Tamarindus, Vicia In imparipinnate leaves, leaflets are odd-numbered and the rachis ends in a single leaflet. e.g., rose, neem, Murraya.

22. (ii) Bipinnate leaves These are the pinnately compound leaves in which the primary rachis beans secondary axes, called rachillae , which bear leaflets on both the sides. So, the leaves pinnate twice. e.g. Acacia, Mimosa pudica.

23. (iii) Tripinnate leaves These are the pinnately compound leaves in which the primary rachis gives out secondary rachi which, in turn, give out tertiary rachi to which leaflets are attached, So, the leaves pinnate thrice e.g. Moringa.

24. (iv) Decompound leaves These are the 2 types Pinnately compound leaves and Palmately compound leaves 1. Pinnately compound leaves Pinnately compound leaves in which the tertiary rachi undergo further branching so that the leaves pinnate more than thrice. e.g. Coriander.

25. 2. Palmately compound leaves Palmately compound leaves are the leaves in which leaflets radiate from the tip of the rachis, just like the fingers on palm. They are of five kinds, namely Unifoliate Bifoliate Trifoliate Quadrifoliate Multifoliate.

26. Unifoliate leaves are the palmately compound leaves with only a single leaflet e.g. Citrus.

27. Bifoliate leaves have two terminal leaflets e.g. Balanites, Bignonia.

28. Tri- foliate leaves have three terminal leaflets e.g. Trigonella, Tribolium, Rubber.

29. Quadrifoliate leaves have four terminal leaflets. e.g. Marsilea .

30. Multifoliate or digitate leaves have more than four terminal leaflets. e.g. Bombax (silk cotton), tapioca.

31. Differences between compound leaves and branches Compound leaf Branch Not divided into nodes and internodes Divided into nodes and internodes Axillary buds are absent in the axils of leaflets Axillary buds are present in the axils of leaves Terminal bud is absent Terminal bud is present Bears leaflets on a branching rachis Bears leaves at nodes Will not bear flowers and fruits Will bear flowers and fruits

32. Shape of leaf lamina Leaf lamina exhibits incredible variations in its shape. Some of the common types of lamina or leaves are the following. 1. Linear: Lamina is long and narrow - e.g., grasses. 2. Lanceolate: Lamina is lance-shaped, long and tapering at the ends, but broadest near the stalk - e.g., Bamboosa, Nerium, Gloriosa. 3. Oblanceolate: Lamina is lanceolate, but its broadest part is near the apex - e.g.,Calamus (cane). 4. Elliptical: Lamina is tapering at ends, but broad in the middle, just in the form of an ellipse - e.g., Vinca, Guava.

33. 5.Oblong: Lamina is two or three times as long as it is broad, with its side or less parallel and the ends rounded - e.g., plantain, Ixora. 6.Ovate: Lamina is egg-shaped, i.e., slightly broader at the base than at the apex - e.g., banyan. 7.Obovate : Lamina is inversely ovate, with the broader part near the apex – e.g., Tectona, Terminalia. 8.Orbicular or rotund: Lamina is more or less circular – e.g., Lotus. 9.Cordate: Lamina is heart-shaped - e.g., Thespesia. 10.Obcordate: Lamina is inversely heart-shaped - e.g., the leaflets of Oxalis 11.Reniform: Lamina is kidney-shaped, with a broad shallow depression at the base - e.g., Centella. 12.Cuneate or wedge-shaped: Blade is broadest beyond the middle and tapen straight down - e.g., Oxalis.

34. 13. Deltoid: Lamina is broadly cuneate, - e.g., Abutilon. 14. Sagittate: Lamina is shaped like an arrow head, with two sharp and straight lobes at the base - e.g., Sagittaria and Arum. 15.Falcate or sickle-shaped: Lamina is curved sideways - e.g., adult leaves of Eucalyptus. 16.Oblique: Right and the left halves of the blade are not symmetrical- e.g., Begonia and Guazuma. 17. Pinnatifid or pinnatisect : Lamina is cut about half-way towards the mid-rib he into a number of pinnately arranged lobes. 18. Lyrate: Lyre-shaped and pinnatifid lamina, with a much enlarger terminal lobe and smaller lateral lobes , e.g., Mustard. 19. Acicular: Lamina is needle-shaped, long, narrow and cylindrical, e. g., Allium 20. Lorate or loriform: Lamina is strap-shaped, e.g., Vallisnaria .

35. Margin of the leaf Leaf margin is variously modified, ornamented, or sculptured in different species. Some common types of leaf margin are the following. 1. Entire: Margin is even and smooth - e.g., Mangifera. 2. Serrate: Toothed and sow-like margin, with pointed and upwardly projecting teeth. e.g., Balsam and Acalypha. 3.Dentate: Toothed margin, armed with numerous sharply pointed, triangular and outwardly pointing teeth - e.g., Hibiscus . This differs from denticulate margin only in having less fine teeth. 4.Cenate: Margin with rounded and forwardly printing teeth. - e.g., Bryophyllum, Stachytarpheta. 5. Undulate or sinuate : Wavy margin- e.g., Polyalthia .

36. 6. Spiny: Margin provided with spines - e.g., pine-apple, Aloe. 7. Glandular: Margin with sticky glandular hairs - e.g., Jatropha glandulifera. 8. Ciliate: Margin bearing fine cilia-like projecting hairs - e.g., Cleome viscosa . 9. Fid or fidus: Broken margin with numerous clefts which extend half-way from the margin to the midrib, dividing the lamina into a number of segments or lobes. Pinnately fid - e.g., radish, mustard. Palmately fid - e.g., castor, papaya. 10. Sectile: Margin with cuts which extend nearly to the midrib. (a) Pinnatisect - e.g., Ipomoea quamoclit (b) Palmatisect - e.g., tapioca, rubber.

37. Apex of the leaf Some of the common types of leaf apices are the following: 1. Acute : Leaf tip is sharply ending and abruptly pointed. - e.g., Mangifera 2. Acuminate (caudate): Leaf apex progressively narrows to a long and slender tail and ultimately ends in a point. - e.g., Ficus religiosa 3. Obtuse: Blunt or rounded leaf apex - e.g., banyan and jack. 4. Cuspidate : Leaf tip ends in a long, rigid and spiny point, e.g., pine apple . 5. Mucronate: Rounded apex, terminating abruptly in a short and sharp point e.g., Caesalpinia, Crotalaria. 6. Retuse: Bluntly rounded apex, with a shallow central notch -e.g.. Calophyllun 7. Emarginate: Leaf tip with a deep notch or depression at the tip -e.g., Bauhenia biloba.

38. Surface of the leaf Some of the common types of leaf surfaces are the following: Glabrous: Smooth and shiny surface – e.g., Ficus religiosa, Vinca 2. Pubescent: Surface is covered with short and soft hairs, e.g., Hibiscus 3. Villous: Surface is thickly covered with soft and long hairs. 4. Hispid: Surface is covered with stiff hairs, e.g., Spermacoce. 5. Scabrous: Rough surface with short and stiff hairs- e.g., Lantana, Leucas 6. Tomentose: Surface is completely covered with matted hairs- e.g., Guazuma

39. 7. Glandular: Surface is covered with sticky and glandular hairs, e.g., Jatropha glandulifera. 8. Gland dotted : Surface with a number of translucent spots which become very clear when the lamina is held against light- e.g., Citrus . Each spot represents a place where oil is stored up. . 9. Srigose: Leaf surface bears sharp, stiff, straight and often basally swollen hairs, all pointing in the same direction. 10. Sericeous : Leaf surface with a silken sheen and provided with silky hairs, e.g., gingelly.

40. PHYLLOTAXY Phyllotaxy is the pattern of arrangement of leaves on stem and branches. The main purpose of phyllotaxy is to provide sufficient light to leaves. The form and arrangement of foliage leaves in buds is called prefoliation , the form in which young leaves are folded or rolled on themselves in the bud is called ptyxis, and the arrangeent of leaves within a bud is called vernation. There are two main kinds of phyllotaxy, namely radical phyllotaxy and cauline phyllotaxy In the former, leaves are arranged in a cluster on a very short and stumpy stem on the top of the root. e.g., radish. In cauline phyllotaxy, leaves are arranged on tem which has distinct internodes. There are three main types of cauline phyllotaxy, alternate, opposite and whorled.

41. (i) Alternate phyllotaxy This is the leaf arrangement in which leaves are arranged on alternate nodes, with only a single leaf on each node. Leaf arrangement may be spiral, distichous, tristichous, or pentastichous. In spiral phyllotaxy, alternate leaves are arranged spi rally around the stem (e.g.. Hibiscus). In distichous phyllotaxy, alternate leaves are arranged in two rows vertically on the two sides of the stem (e.g. Annona, grasses) In tristichous phyllotaxy, leaves are arranged in three rows (e.g., Cyperus), In pentastichous phyllotaxy, leaves are arranged in five rows (e.g., Hibiscus).

42. (ii) Opposite phyllotaxy Opposite phyllotaxy is the leaf arrangement in which a pair of leaves are arranged oppositely on each node. Opposite phyllotaxy is of two kinds, namely decussate and superposed. In decussate phyllotaxy, adjacent pairs of leaves are arranged at right angles to each other (e.g. Ixora, Calotropis Calotropis). In superposed phyllotaxy, adjacent pairs of leaves are arranged exactly one above the other in the same plane . (e.g. Quisqualis).

43. (iii) Whorled phyllotaxy This is the leaf arrangement in which three or more leaves arise from each node in the form of a whorl or circle around the stem. e.g. Nerium (Oleander), Alstonia, Hamelia. Heterophylly is the occurrence of more than one type leaf on the same plant. e.g. Limnophila heterophylla, Eucalyptus, Broussoneita, jack tree, etc.

44. Modifications of leaves Leaves are modified in different ways for climbing, protection and nutrition. These modifications include : tendrils, hooks, spines, scale leaves, pitchers, phyllodes, etc.

45. (i) Leaf tendrils These are long, spirally coiled and spring-like leaves. They help weak-stemmed plants to climb on a support. e.g. Pisum, Gloriosa, Lathyrus (sweet pea), Clematis. In Pisum, leaf tendrils are modified terminal leaflets, in Gloriosa they are modified leaf tips, in Lathyrus they are entire leaves, in Nepenthes they are modified petioles, and in Clematis they are modified rachillae. Leaf tendrils are sensitive to contact

46. (ii) Leaf hooks Hook Leaf hooks are the hook-like modifications of the terminal leaflets of the compound leaves of some weak-stemmed plants for clinging and climbing on a solid support. In Bignonia unguis cati , three terminal leaflets of compound leaves are modified into cat-claw-like hooks.

47. (iii) Leaf spines Leaf spines are sharply pointed structures. They are characteristic of xerophytic plants, such as cacti (e.g. Opuntia, Ananas, Asparagus,etc) . In Opuntia, they are formed from minute leaves of axillary buds. In Ananas and Agave, they are modified leaf margins. In Opuntia, ordinary leaves are modified into scale leaves, whereas the minute leaves of axillary buds become spines. Scale leaves fall off in a short time. Leaf spines are modifications for defence, protection and control of transpiration.

48. (iv) Leaf scales Leaf scales are thin, stalkless and membranous structures which protect the axillary buds located in their axils. e.g. onion. In onion, they are fleshy due to the storage of water, mucilage and food.

49. Leaf scales-Onion

50. (v) Pitcher Pitcher is a flask-shaped modification of the leaf lamina for trapping insects. It is found in some insectivorous plants. e.g. Nepenthes,Sarracenia. In Nepenthes leaf lamina is modified into a pitcher. The mouth of the pitcher is covered by a lid, called operculum. It is a modified part of the leaf tip. The basal portion of the petiole is flat, leaf-like and photosynthetic. The upper portion is coiled like a tendril to hold the pitcher in vertical position. The whole pitcher is a beautifully coloured to attract insects. A sugary secretion makes the rim of the mouth slippery. Insects, resting on the edge of the mouth, get accidentally slipped into the pitcher. Soon, operculum closes the mouth to prevent their escape. The insects are digested by the enzymes secreted by the pitcher wall. The digested food is soon absorbed to the pitcher wall.

51. (vi) Phyllode Phyllode is a flat, leaf-like and photosynthetic modification of the petiole or rachis. It is a xerophytic adaptation to reduce transpiration. In this case, normal leaflets fall at an early stage. Then, petide or rachis develops to a phyllode. Phyllodes are common among Parkinsonia, Acacia, etc. In Acacia auriculiformis (Australian Acacia), leaflets are altogether absent. But, in A.melanoxylon , they fall off at an early stage. In both the cases, rachis flattens out into a leaf-like phyllode.

52. Differences between phyllode and phylloclade Phyllode Phylloclade (i) is a leaf modification (a modification of petiole or rachis) Is a stem modification (ii) Usually unbranched Usually branched (iii) Nodes and internodes absent Nodes and internodes present (iv) Scale leaves absent Scale leaves present (v) Axillary buds and spines absent Axillary buds are modified as spine (vi) Never succulent Always succulent

53. Functions of leaves (i) Manufacture of food by photosynthesis. (ii) Gas exchange and transpiration through stomata. (iii) Storage of food and water by fleshy leaves. (iv) Vegetative propagation by adventitious leaf buds. (v) Defence and protection by leaf spines. (vi) Help in climbing by the formation leaf tendrils and leaf hooks. (vii) Food capture in insectivorous plants. (viii) Help in pollination by attractive colouration.