Primary Structure: Dicotyledonous, Monocotyledonous Root & Leaf

Root is the underground part of a vascular plant and Leaf is the flattened outgrowth from stem of a vascular plant. In this article, the Primary structure of Root and Leaf will be discussed in detail.

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Primary Structure of Dicot Root

The root of Dicot plants usually consists of Epidermis, Cortex, Endodermis, Pericycle, Vascular bundle, etc. These are described as follows:

The important anatomical characteristics of dicotyledonous roots are as follows:

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• The xylem bundles vary from 2-6 in number, i.e., They may be diarch, triarch, tetrarch, pentarch or hexarch.
• Pericycle gives rise to lateral roots and secondary meristem (e.g., cambium and phellogen).
• The cambium appears later as a secondary meristem.
• Pith is scanty or altogether absent.

Epidermis

• Consists of closely packed elongated cells with thin walls that usually lack a cuticle and stomata.
• In some dicotyledons thickened outer walls occur in root parts growing in air.
• The root epidermis (also known as Piliferours layer, rhizodermis, or epiblema) is typically uniseriate.
• Most of the epidermal cells extend out in the form of tubular unicellular root hairs.
• Normally the root hairs are confined to a region between one and several centimetres in length near the tip.
• They are absent in the nearest proximity of the apical meristem, and they die off in the older root parts.
• Some roots also develop a specialized layer of exodermis beneath the epidermis. The exodermis arises from one or several of the sub-epidermal layers of the cortex.
• The cell walls of exodermis become suberized. The exodermis is present in few dicots.

Cortex

• The cortex is massive and thin-walled rounded or polygonal parenchyma cells having sufficiently developed intercellular spaces among them.
• The parenchyma cells of cortex contain abundant starch grains in them.
• In the roots of dicotyledons which possess secondary growth and shed their cortex, the cortex mainly consists of parenchyma.
• As seen in transverse sections, the cortical cells may be arranged in radial rows, or they may be alternate with one another in the successive concentric layers.
• The presence of schizogenous spaces are large and form distinct air spaces.
• The cortex of roots is generally devoid of chlorophyll. Exceptions are the roots of some water plants and aerial roots of many epiphytes (e.g., Tinospora spp). 
• Various secretory structures are found in the root cortex.
• Some dicotyledons (e.g., Brassica, Pyrus, Prunus, Spiraea, etc.) may develop prominent reticulate or band-like thickenings in cortical cells outside the endodermis.

Endodermis

The innermost distinct layer of the cortex is endodermis.

• The endodermis is uniseriate and almost universally present in the roots.
• Cells of endodermis are living.
• The endodermis is characterized by the presence of Casparian strips or Casparian bands on their anticlinal walls. The strip is formed during the early ontogeny of the cell and is a part of primary wall. The strip is typically located close to the inner tangential wall.
• Gutenberg (1943) says, that the suberin-like materials are found in the strips. The cytoplasm of an endodermal cell remains firmly attached to Casparian strip. This firm attachment controls the movement of the materials in the root and their passage into xylem cells.
• The thin-walled passage cells are also found in the endodermal layer which lies against the protoxylem poles.
• The passage cells either remain unmodified as long as the root lives or develop thick walls like the rest of the endodermis.

Pericycle

The layer next to the endodermis is commonly known as pericycle.

• The pericycle of relatively young roots consists of thin-walled parenchyma.
• It makes the outer boundary of the primary vascular cylinder of the dicotyledonous roots.
• It may be uniseriate or multiseriate (e.g.; Morus, Salix, Ficus benghalensis, etc.)
• The lateral roots in dicots arise in this tissue.
• The phellogen and part of the vascular cambium originate in the pericycle.
• Roots without pericycle are rare but may be found in water plants and parasites.

The vascular System

• The phloem of the root occurs in the form of strands distributed near the periphery of the vascular cylinder, beneath the pericycle.
• The xylem forms discrete strands, alternating with phloem.
• Sometimes the xylem occupies the centre with strand-like parts projecting from the center core-like ridges.
• If xylem is not differentiated in the centre, the centre is occupied by a pith.
• The roots typically show exarch xylem, i.eThe protoxylem is located near the periphery of the vascular cylinder, the metaxylem farther inward.
• The phloem is also centripetally differentiated, i.e., the protophloem occurring closer to the periphery than the metaphloem. 
• Most dicotyledons have few xylem strands. The taproot is frequently di-, tri-, or tetrarch, but it may have five to six and even more poles, (e.g., many Amentiferae, Castanea). 
• Only one xylem strand occurs in the slender root of the hydrophyte Trapa natans.
• In Raphanus, Daucus, Linum, Lycopersicon, and Nicotiana the roots are diarch.
• In Pisum the root is triarch.
• In Cicer, Vicia, Helianthus, Gossypium and Ranunculus the roots are tetrarch.
• In certain dicots the roots of the same plant may show di-, tri-, and tetrarch xylem. For example, tetrarch and polyyarch roots have been reported from Nymphaea chilensis.

Primary Structure of Monocot Root

The distinctive anatomical characters of monocotyledonous roots are as follows:

1. The xylem groups are numerous and generally vary from twelve to twenty.
2. The pericycle gives rise to lateral roots only.
3. The cambium is altogether absent even in later stages, as there is no secondary thickening in such roots.
4. The pith is large and well developed. In certain cases (e.g., in Canna), the pith becomes sclerenchyma

In the structure of Monocot Root, the following parts are found:

Epidermis

• The epidermis or outermost layer of the root is commonly known as rhizodermis, epiblema, or piliferous layer. 
• It is uniseriate and composed of compact tabular cells having no intercellular spaces and stomata.
• A well-known example of a multiseriate epidermis is the velamen of aerial roots of Orchids and epiphytic Aroids. 
• The velamen is a parchment-like sheath consisting of compactly arranged nonliving cells with thickened walls.
• The cells of velamen are quite big in size and contain air and water in them.
• The cell walls develop fibrous thickenings. 

Exodermis

• Generally, beneath the epidermis, there are present one or more layers of exodermis. 
• Usually, exodermis consists of a single row of cells with thickened outer and lateral walls except for certain passage cells which remain thin-walled.

Root hair

• A typical characteristic of the root epidermis is the development of root hair.
• The tubular unicellular root hairs are present on this layer.

Cortex

• Cortex consists of thin-walled parenchyma cells having sufficiently developed intercellular spaces among them.
• Usually in an old root of Zea mays a few layers of cortex immediately beneath the epidermis undergo suberization and give rise to a simple or multilayered zone – the exodermis. This is protective layer that protects internal tissues from injurious agencies.
• The starch grains are abundantly present in cortical cells.
• The sclerenchyma cells are commonly found in the cortex of monocotyledons.

Endodermis

• The endodermal cells possess Casparian strips in their anticlinal walls.
• The strip is typically located close to the inner tangential wall.
• Thick-walled passage cells are formed opposite the protoxylem poles.

Pericycle

• It is usually uniseriate and composed of thin-walled parenchymatous cells.
• In the monocotyledons, the pericycle often undergoes scarification in older roots, partly or entirely.
• In many monocotyledons (e.g., some Gramineae, Smilax, Agave, Dracaena, palms) the pericycle consists of several layers.
• The pericycle gives rise to lateral roots only.

Vascular tissue

• The phloem of root occurs in the form of strand near the periphery of the vascular cylinder, beneath the pericycle.
• Xylem forms discrete strands, alternating with the phloem strands.
• The xylem is exarch, i.e., the protoxylem lies towards the periphery and the metaxylem towards the centre.
• The phloem is also exarch.

  

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Internal Structure of Leaf

Leaf, in botany, is any usually flattened green outgrowth from the stem of a vascular plant. Commonly there are two types of leaves.

1. Dorsiventral leaves (dicotyledonous) and 
2. Isobilateral leaves (monocotyledons)

Dorsiventral leaf

 

• Dorsiventral leaves usually grow in a horizontal direction with distinct lower and upper surfaces, the upper being more strongly illuminated than the lower.
• There exists a difference in the internal structure between upper and lower surfaces of the dorsiventral leaf due to its unequal illumination. Most of the dicotyledonous leaves are dorsiventral.  
  Dorsiventral Leaf.

Epidermis

• The leaf is covered on both surfaces by a single-layered epidermis present on both surfaces.
• Outer wall of epidermis is usually thickened and covered with a waxy substance called cutin.
• The epidermis checks transpiration to a great extent. The outer surface of the epidermis is frequently covered with a thin or thick cuticle. This cuticular layer is formed of cutin.
• In xerophytic leaves, the epidermis cells become radially elongated and lignified.
• In Nerium leaf, the epidermis is multilayered.
• Stomata are found in most abundance in the lower epidermis of the dorsiventral leaf.
• 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 Stomata remains surrounded by two seminular guard cells.

Mesophyll tissue

• The tissue of leaf lies between the upper and lower epidermis and between the
  veins consist of thin-walled parenchyma in mesophyll. 
• The tissue forms a major portion of the inner leaf. 
• Mesophyll tissues always contain chloroplast.
• Commonly the cells of mesophyll are of two types: the palisade parenchyma and the spongy parenchyma.

Palisade Parenchyma

• The palisade parenchyma is composed of elongated and more or less cylindrical cells which are close together with long axis of the cells perpendicular to the epidermis.
• In the transverse section, the cells appear to be arranged quite compact, are really separated from each other having intercellular spaces among them.
• The palisade tissue may consist of a single or more layer.
• The compactness of palisade parenchyma depends on light intensity.

Function

• The cells are arranged near the upper surface of the leaf, where they receive sunlight and facilitate the function of photosynthesis.

Spongy Parenchyma

• 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 among them.
• In comparison to palisade parenchyma, fewer chloroplasts are developed.

Function

• The cells of spongy parenchyma contain chloroplasts which carry on photosynthesis.
• Due to the presence of large air space in the spongy tissue, they are more adaptable to the exchange of gases between the cells and the atmosphere.

Conducting system

• The tissues which constitute the conducting system are situated near or at the center of the midrib. The system has various shapes, e.g., the form of a ring, crescent-shaped ring, a crescent, or scattered patches.
• The inner part of the ring is composed of xylem (towards upper surface) and
  phloem (towards lower surface).

Bundle sheath

• The larger vascular bundles of dicotyledonous leaves remain surrounded by parenchyma cells 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, bundle sheath.

Isobilateral Leaf

• 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 few dicotyledons and most monocotyledons have isobilateral leaves.

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 be easily identified due to the presence of xylem and buliform cells towards it.

Mesophyll tissue

• The mesophyll is usually not differentiated into palisade and spongy parenchyma, but consists only of parenchyma cells, having chloroplasts and intercellular spaces among them.

Vascular bundle

• Most of the bundles are small in size but fairly large bundles also occur.
• Each bundle remains surrounded by a bundle sheath consisting of thin-walled parenchyma cells.
• The xylem is found towards the upper side and phloem towards the lower side in the bundles.
• Sclerenchyma cells occur in patches on both ends of the large vascular bundles which give mechanical support to the leaf.

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References

• Plant Anatomy by B.P. Pandey.

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Revised By

• Shajneen Jahan Shoily on 10th September, 2021.