Internal Structure of Leaf (With Diagram)

In this article, we propose to discuss about the internal structure of leaf.

The foliage leaves are characterised by green colour, thinness and flatness. They develop as protrusions from the shoot apex and are organs of limited growth. Leaves are very important vegetative organs, as they are chiefly concerned with the physiological process, photosynthesis and transpiration. Like other organs they also exhibit three tissue systems (Fig. 613).

Epidermal tissue system consists of the epidermal layers occurring on the adaxial (upper) and abaxial (lower) sides. Occurrence of stomata and outgrowths are distinctive features. The ground tissue system, as already reported in a preceding chapter, is known as mesophyll tissue.

It is often differentiated into columnar palisade parenchyma on the adaxial side and irregular or isodiametric spongy parenchyma on this differentiation in mesophyll is referred to as dorsi­ventral, what is very common in dicotyledons. One with undifferentiated mesophyll, as commonly found in the monocotyledons, is known as an isobilateral leaf.

Presence of conspicuous air spaces in the mesophyll is another marked feature. The gaseous exchange between the internal photosynthetic tissues and outside atmosphere thus becomes easy. The vascular tissue system is composed of vascular bundles which are usually collateral and closed.

But the bundles entering the leaf occupy such a position that xylem occurs on the upper side and phloem on the lower. The vascular tissues, in fact, form the skele­ton of the leaf, on which other tissues—the ground tissues, remain inserted. The petiole may continue into the midrib which bears branches and sub-branches ultimately rami­fying in the leaf lamina in both reticulate and parallel type of venation.

The ultimate branches are very small and terminate in what are known as bundle ends. Often these ends bend into minute specialised photosynthetic areas known as vein islets or they may just extend into the mesophyll. Bundle-ends vary considerably in the leaves, but com­monly it consists of a single tracheid with a single sieve element or specialised paren­chyma representing xylem and phloem respectively, surrounded by a parenchymatous bundle sheath (Fig. 613A).

In extreme cases the phloem may be absent and the veinlet may be made of a single spiral tracheid. Thus the size of the bundle depends on the posi­tion one prefers to take while making a section. Very commonly vascular bundles remain surrounded by a row of cells, which may or may not contain chloroplasts.

This row made of parenchy­ma cells is referred to as bundle sheath or border parenchyma. The leaves of monocotyledons often have two bundle sheaths—outer parenchymatous one usually with chloroplasts and an inner thick-walled one with­out chloroplasts.

In some dicotyledons the bundle sheath extends up to the epidermis, either on one or on both sides of the leaf, and is termed bundle sheath extensions.

Their contact with conducting elements on one side and mesophyll on the other and often the extension up to epidermis are suggestive of positive physiological functions. The morphology of bundle sheath was considered to be uncertain, but it is now regarded as an endodermis (Fann), where Casparian strips in -some cases have been observed.

Often parenchyma cells of the sheath contain starch, then it may be called a ‘starch sheath’. The leaves of mesophytes possess highly- thickened epidermis and patches of mechanical tissues, either as isolated patches or in association with vascular tissues, as they have to withstand shearing stresses in particular.

Ontogeny of the Leaf:

Leaves originate from the promeristem of the shoot apex. The outer layers of apical meristem just below the apex undergo anticlinal and periclinal divisions and give rise to a small rounded protrusion, what is the leaf primordium. The meristematic cells occurring at the tip of the primordium constitutes the apical meristem of the leaf. The apical growth takes place at the early stage.

It stops soon, and further expansion giving the leaf proper form is due to rapid enlargement by intercalary growth. During the development of the blade the basal part of the protrusion remains unexpanded which ultimately forms a meristem that gives rise to the petiole.

At the early stage of the development of the blade two strips of meristems, referred to as marginal meristem, occur along the two surfaces of the leaf axis. These are composed of files of initials known as marginal initials. They form the protoderm by anticlinal division.

The cells occurring beneath the marginal initials, known as submarginal initials, divide in all planes and produce the internal tissues of the leaf. They are often differentiated into three layers—adaxial, abaxial and middle layers. A median procambium develops from the procambial strands of the shoot apex.

After cessation of marginal growth further growth of the lamina is due to anticlinal division of the cells, so that surface area of the leaf is increased but in depth number of cell layers remain unchanged. The two protoderm layers produce upper and lower epidermis; the adaxial layer of submarginal initials gives rise to palisade parenchyma, the abaxial layer to lower spongy parenchyma and the middle layer to middle spongy parenchyma; and the procambium would form the vascular bundles.

Though there may be considerable variations, the above is the general account of the development of a dicotyledonous leaf from the primordium, which may be represent­ed by the following scheme:

Dorsiventral Leaves:

Internal structures of a few common dorsiventral leaves are being described below:

1. Leaf of Mango:

A thin section through the lamina of a leaf of mango (Mangifera indica of family Anacardiaceae) will show the following plan of arrangement of tissues (Fig. 614):

I. Epidermis:

There are two epidermal layers on adaxial and abaxial surfaces of the leaf. Each is uniseriate, composed of a row of compactly-set tabular cells. The outer walls are cutinised and possess thin cuticle, the thickness being more pronounced in the cells of the upper epidermis than those on the lower side. Stomata occur on the lower epidermis.

II. Mesophyll:

The ground tissue forming the mesophyll is differentiated into palisade and spongy cells. The palisade cells occur towards upper epidermis. They are columnar cells with scanty intercellular spaces and remain arranged more or less at right angles to the upper epidermis. Chloroplasts are abundantly present, which particularly occur along the radial walls of the cells.

There are two layers of palisade cells. The spongy cells occur towards the lower epidermis. They are quite loosely arranged with conspicuous intercellular spaces. The number of chloroplasts is naturally much smaller here, which explains the pale green colour of the lower surface of the leaf.

III. Vascular bundles:

Bundles are collateral and closed. They are located in the mesophyll. The size of the bundle depends on the position one chooses to take in making a section.

A bigger bundle is composed of xylem and phloem, the former occurring to­wards upper epidermis and the latter towards the lower side. The xylem is made of tracheary elements, and the phloem of sieve tubes and companion cells. The bundle remains surrounded by a row of colourless parenchyma cells.

This band is referred to as bundle sheath or border parenchyma. Thus the bundle is not in direct contact with the meso­phyll cells. Parenchyma and often collenchyma cells are present on the outer and inner sides of the bundle which may reach up to the two epidermal layers. These cells consti­tute what is known as bundle sheath extension.

2. Leaf of Banyan:

A transverse section through the leaf of Banyan (Ficus benghalensis of family Moraceae) would reveal the anatomical characters (Fig. 615) more or less similar to the pre­vious one.

I. Epidermis:

The upper epidermis is multiseriate, being made of a few layer of cells. Lithocysts are frequently present and well-developed calcium carbonate crystals, the cystoliths, occur here and there. The lower epidermis is uniseriate.

The outer layer of upper multiple epidermis and the lower epidermis as a whole are made of compactly-arranged tabular cells with cutinised outer walls having cuticle. The degree of cutinisation is more pronounced on the upper side. Stomata occur on the lower epidermis.

II. Mesophyll:

It is differentiated into palisade and spongy cells. Two or three layers of columnar cells with abundant chloroplasts remain arranged more or less at right angles to the upper epidermis. These are palisade cells.

This is the principal photo­synthetic tissue. The spongy cells occurring towards lower epidermis are isodiametric, and often irregular in shape, and have profuse intercellular spaces. The number of chlo­roplasts is naturally much smaller here in comparison to palisade cells.

III. Vascular bundles:

The bundles are as usual collateral and closed ones, with xylem lying on the upper and phloem on the lower sides. They remain surrounded by parenchymatous bundle sheath. In case of bigger bundles bundle sheath extensions are present.

3. Leaf of Banana:

Though a monocotyledon, the leaf of banana (Musa sapientum of family Musaceae) is dorsiventral.

It exhibits the following internal structure (Fig. 616):

I. Epidermis:

The upper and lower epidermal layers are composed of compactly- arranged roundish cells with cuticularised outer walls. Slightly sunken stomata are present on the lower side.

One or more layers of much larger thin-walled parenchyma cells occur next to epi­dermal layers—both adaxial and abaxial. Those on the upper side are larger in size. These cells markedly differ from the mesophyll.

The subepidermal cells have been desig­nated as hypodermis by older anatomists. Whether they belong to epidermis or to the ground tissues from ontogenetic point of view can be determined by developmental studies. They may, however, be regarded as water-storage tissues which do not possess chlorophyll.

II. Mesophyll:

It is differentiated into palisade and spongy cells. Palisade cells occur towards upper epidermis in two or three layers. These are long columnar cells rather tightly arranged with scanty intercellular spaces. The spongy cells are compara­tively much smaller and more or less isodiametric in shape.

A number of large air chambers occur at regular intervals towards abaxial side. Stellate cells were present in the regions of the cavities in young condition which ultimately have disintegrated. Thus these cavities, unlike the usual intercellular spaces, are formed lysigenously.

III. Vascular bundles:

These are collateral and closed, but bundles are poorly developed with scanty Xylem. Patches of sclerenchyma occur on the upper and lower sides of the bundles. Obviously they are meant for providing mechanical strength against shearing stresses.

4. Leaf of Oleander:

The leaf of oleander (Nerium oleander of family Apocynaceae) shows some distinct departures so far as the arrangement of tissues is concerned, from the ones described previously, exhibiting clear xerophytic adaptations.

A transverse section would reveal the following structure (Fig. 617):

I. Epidermis:

Both the adaxial and abaxial epidermal layers are multiseriate. They are composed of a few (usually three) layers of compactly-arranged isodiametric cells. The outer walls of the cells forming outermost layers are very strongly cuticula­rised.

Epidermal outgrowths are present. The stomata are present on the lower epidermis, but they are sunken stomata located in a depression known as stomatal pit. A good number of trichomes develop from the cells bordering the depression. It is an effective mechanism for reducing transpiration.

II. Mesophyll:

It consists of palisade and spongy cells. But unlike other dorsiventral leaves, palisade cells occur here both towards upper and lower epidermis and spongy cells are located in between them. On the adaxial side palisade cells occur in three or more layers forming a compact zone of columnar cells with little intercellular spaces.

Chloroplasts are abundantly pre­sent, usually along the radial walls of the cells. Inner palisade, i.e., palisade occurring on the abaxial side, is usually one- layered and consists of rather small cells. The spongy cells lo­cated between the two palisade layers are isodiametric, much smaller in size and much loosely arranged with profuse intercellu­lar spaces. Calcium oxalate crys­tals are present here and there.

III. Vascular bundles:

The bundles are collateral and closed ones with xylem on the upper and phloem on the lower sides. They remain surrounded by parenchymatous bundle sheaths.

Isobilateral Leaves:

These leaves are common in the monocotyledons. Here stomata occur on both the epidermal layers, though they are more abundant on the abaxial side. Mesophyll hardly shows differentiation between palisade and spongy cells. A few common isobilateral leaves have been selected for the study of internal structures.

1. Leaf of Tuberose:

A section through the leaf of tuberose (Polianthes tuberosa of family Amaryllidaceae), would show the following anatomical structure (Fig. 618):

I. Epidermis:

Both the epidermal layers are uniseriate, composed of compactly- arranged rectangular cells with rounded cuticularised outer walls. Stomata are present on both the epidermal layers.

II. Mesophyll:

It as usual forms the main bulk of the leaf, and is composed of iso­diametric cells with intercellular spaces. Chloroplasts are abundantly present. Thus the differentiation of mesophyll into palisade and spongy cells is absent; all the cells are of spongy type.

III. Vascular bundles:

These are present at regular intervals. The bundles are collateral and closed ones with xylem and phloem. A few sclerenchyma cells are present at the two ends of the bundles. The bundles remain surrounded by a row of parenchyma cells devoid of chloroplasts, which forms the bundle sheath.

2. Leaf of Maize:

A section through a leaf of maize (Zea mays of family Graminaceae) shows the follow­ing structure (Fig. 619):

I. Epidermis:

Both upper and lower epidermal layers are uniseriate and com­posed of more or less oval cells with cuticularised outer walls. Upper epidermis may be easily identified due to presence of large and empty bulliform cells. Stomata occur on both the epidermal layers.

II. Mesophyll:

The mesophyll does not show differentiation into palisade and spongy cells, but is made of rather compactly-arranged isodiametric cells.

III. Vascular bundles:

The bundles are collateral and closed ones which remain arranged in parallel series. Majority of the bundles are small, but fairly large bundles occur at regular intervals. Small bundles have xylem on the upper and phloem on the lower sides surrounded by large parenchyma cells forming the bundle sheath.

The cells of the sheath contain plastids, often with starch grains. It is assumed that this layer serves as a temporary storage tissue, apart from-conducting the products of photosynthesis to the phloem. Xylem, as usual, consists of tracheary elements, and phloem of sieve tubes and companion cells.

A large vascular bundle practically resembles that of a stem. Sclerenchyma cells occur in patches on both edges of the bundles, obviously for giving mechanical strength.

3. Leaf of Bamboo:

A section through the leaf of bamboo (Bambusa spp. of family Graminaceae) would reveal the following internal structure (Fig. 620):

I. Epidermis:

As usual there are two epidermal layers. The upper epidermis posse­sses a number of conspicuous bulliform cells. The lower one bears stomata and remain covered with strong cuticle. Considerable deposition of silicon is a distinctive character. Stiff sharply pointed hairs are also present.

II. Mesophyll:

It is composed of compactly-arranged cells, without showing any differentiation into palisade and spongy cells. The cells are of rather palisade type, though not much elongate. Some distinct cavities are present here and there.

III. Vascular bundles:

These are collateral and closed ones which remain arranged in parallel series. Xylem occurs on the adaxial and phloem on the abaxial sides. Larger bundles have more distinct xylem and phloem surrounded by a bundle sheath, and has patches of sclerenchyma cells on the two sides.

4. Leaf of Datepalm:

A section through a leaf of datepalm (Phoenix sylvestris of family Palmae) would show the following structure (Fig. 621):

I. Epidermis:

The two epidermal layers are composed of compactly-arranged tabular cells with cuticularised outer walls. Stomata are present here and there.

A layer of parenchyma cells with scanty chlorophyll occurs just internal to both the epidermal layers. These distinctly differ from the mesophyll. These subepidermal layers may be called special hypodermis.

II. Mesophyll:

It is composed of more or less isodiametric cells with small inter­cellular spaces, thus the differentiation into palisade and spongy cells is absent. Patches of sclerenchyma occur more or less in parallel series towards both the upper and lower epidermis, as I-girders for withstanding shearing stress.

III. Vascular bundles:

The bundles are collateral and closed ones. They remain arranged in parallel series. Each bundle has Xylem on the upper and phloem on the lower side. The bundles are of two types, viz., large and small ones. The large bundles have patches of heavily thick-walled sclerenchyma on the two edges, whereas the small bun­dles remain surrounded by sheath of parenchyma cells which have no chlorophyll.

Floating Leaf of Water-lily:

A section through the leaf of waterlily (Nymphaea stellata of family Nyphaeaceae) would reveal the following anatomical structure (Fig. 622). As an aquatic plant it has extremely reduced vascular and supporting tissues and well-formed air chambers.

I. Epidermis:

Epidermal layers are uniseriate both on the adaxial and abaxial sides. They are composed of closely-set cells. Stomata occur on the upper side. Moreover, there is deposition of waxy matters which prevents wetting and clogging of the stomata.

II. Mesophyll:

It is differentiated into palisade and spongy cells. A few layers of columnar cells occur towards the adaxial side forming the palisade. The spongy cells present towards lower epidermis and irregular in outline. Large air chambers are present in the mesophyll. Elongated sclerotic cells—the trichosclereids commonly called ‘internal hairs’, often with branched ends are frequently present.

III. Vascular Bundles:

These are very much reduced. As usual they are composed of xylem and phloem, and remain surrounded by parenchymatous bundle sheath.

Structure of a Succulent Leaf:

A section through a succulent leaf with halophytic adaptations reveals the following structure (Fig. 622A):

The epidermis is uniseriate made of compactly-arranged tabular cells with cuti­cularised outer walls. A row of palisade parenchyma occurs next to epidermis forming more or less a compact band.

The central part of the leaf is made of well-developed water-storage tissue, consisting of large parenchymatous cells with conspicuous vacuoles and intercellular spaces. Vascular bundles are very much reduced with very scanty mechanical elements and occur next to the palisade layer.

Centric Leaf of Onion:

These leaves are acicular in shape and are more or less rounded in cross-section. Thus they do not possess two epidermal layers. The differentiation of tissues into epider­mal, ground and vascular is clear.

A transverse section through a leaf of onion (Alliumcepa of family Liliaceae) would show the following internal structure (Fig. 623):

I. Epidermis:

It is uniseriate and continuous one, made of small round cells with strongly cuticularised outer walls. Slightly-depressed stomata are present.

II. Mesophyll:

The mesophyll consists of two types of cells. Just internal to epider­mis there are usually two layers of columnar cells with abundant chloroplasts. These are palisade cells. They actually form something like a green belt.

Next to this band occur a few layers of isodiametric parenchyma—spongy cells, with scanty chloroplasts. Conspi­cuous air spaces are present in the mesophyll. The central part of the leaf is hollow. The cells bordering the cavity are devoid of chloroplasts.

III. Vascular Bundles:

The bundles remain scattered in the lower part of the mesophyll. They are collateral and closed ones with xylem and phloem. Each bun­dle remains surrounded by parenchymatous bundle sheath.

The Petiole:

The petiole or the leaf stalk connects the lamina to the base, and thus supports it on the axis and exposes it to proper amount of light and air. As already stated, it is formed from an intercalary meristem located at the base of the finger-like protrusion of the shoot apex. The arrangement of tissues in the petiole more or less resembles that of the stem.

1. Petiole of Cucurbita:

A transverse section through the petiole of Cucurbita would show the following structure (Fig. 624). It is wavy in outline having distinct ridges and furrows and a large hollow cavity in the central regions.

I. Epidermis:

It is as usual uniseriate, made of a row of tabular cells with cuticularised outer walls. A large number of multicellular outgrowths are present.

II. Ground tissues:

Patches of collenchyma cells with thickened corners occur at the ridges. The rest of the ground tissue is composed of thin-walled parenchyma with distinct intercellular spaces. A big cavity is present in the central region.

III. Vascular bundles:

The bundles remain arranged in a ring. Individual bundles are bicollateral ones.

2. Petiole of Sunflower:

Here (Helianthus annus of family Compositae) it is grooved at one side and rounded at the other (fig.625).

A transverse section would show the same plan of arrangement of tissues:

I. Epidermis:

It is uniseriate with tabular cells and a large number of epidermal outgrowths.

II. Ground tissues:

A band of collenchyma, intercepted by chlorenchyma here and there, occurs next to epidermis forming the hypodermis. The rest of ground tissues is made of isodiametric parenchyma cells with distinct intercellular spaces.

III. Vascular bundles:

A number of vascular bundles occur in the form of a band; some bundles are small and some of them are quite large. Every bundle is collateral with Xylem on the inner and phloem on the outer side.

3. Petiole of Piper Betle:

The section (Fig. 626) of the petiole of Piper betle of family Piperaceae is more or less heart-shaped with a distinct groove at one side and rounded at the other.

I. Epidermis:

It is as usual uniseriate with cutinised outer walls.

II. Ground tissues:

Collenchyma cells occur next forming hypodermis. The conti­nuity of collenchyma is broken here and there by small patches of chlorophyll-containing parenchyma cells.

The rest of the ground tissue is distinctly parenchymatous. Occurrence of a large mucilage canal at the centre and a few smaller ones here and there is a dis­tinctive character.

III. Vascular bundles:

A good number of bundles of different sizes occur in the form of an arc. They are collateral ones with xylem on the inner and phloem on the outer side.

4. Petiole of Water-lily:

It (Nymphaea stellata of family Nymphaeaceae) is characterised by the presence of large air chambers, peculiarly branched trichosclerieds or ‘internal hairs’ with calcium oxalate deposition, and irregularly scattered vascular bundles with extremely reduced xylem elements.

A transverse section would show the following structure (Fig. 627):

I. Epidermis:

It is uniseriate, made of roundish cells with very scanty cuticularisation on the outer walls. Chloroplasts are present in the epidermal cells. Unbranched hairs are of frequent occurrence.

II. Ground tissues:

Next to the epidermis occur a few layers, usually three, of collenchyma cells with thickened corners, forming the hypodermis. The rest of the ground tissue is made of thin-walled parenchyma with pretty large number of air chambers.

Trichosclereids of peculiar shapes, often with deposition of calcium oxalate crystals are abundantly present.

III. Vascular bundles:

The bundles are very poorly developed and remain scat­tered in the ground tissues. The xylem vessels usually break down and form lacunae which resemble the air chambers.

The phloem elements are normally developed. There are two types of bundles—small ones with one patch of phloem, and comparatively larger ones with two patches of phloem.