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The following points highlight the five main types of petioles found in plants. The types are: 1. Petiole of Cucurbita sp. 2. Petiole of Helianthus Annuus 3. Petiole of Piper Betle 4. Petiole of Nymphaea Stellata 5. Petiole of Eichhornia sp.
Type # 1. Petiole of Cucurbita sp. (Family: Cucurbitaceae):
Transverse section of the petiole is wavy in outline and shows the following internal tissue organization from periphery towards the centre (Fig. 31.57).
Epidermis:
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It is single layered and composed of horizontally flattened cells. The cells are compactly arranged and have well developed cuticle on their outer walls. Numerous multicellular epidermal outgrowths are present here and there. The epidermis is thrown into ridges and furrows.
Ground tissue:
The peripheral layer just below the ridges is composed of a few layers of collenchyma. The cells are thickened at corners. Collenchyma occurs as patches and not in a continuous band. The rest of ground tissue is parenchymatous.
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The cells are thin walled with conspicuous intercellular spaces. Chloroplast may be present in the parenchyma. There is no tissue at the centre and in this region there lies a large air cavity. Vascular bundles remain embedded in the ground tissue. Endodermis and pericycle are absent.
Vascular bundle:
The bundles are arranged in a ring in the ground tissue. The bundles are large and small, and many in number. The large vascular bundle lies below the ridges whereas the smaller ones occur below the furrows. Each vascular bundle is bicollateral and similar to those present in the stem of Cucurbita.
Comment:
Collenchyma patches and xylem are the mechanical cells. They form flanges of the composite I-girders. They provide strength against inflexibility. The central large air cavity is meant for aeration. The ring arrangement of bicollateral vascular bundle is the characteristic of Cucurbita petiole.
Type # 2. Petiole of Helianthus Annuus (Sunflower. Family: Compositae):
Transverse section of petiole shows a prominent groove on one side whereas the other side is round. It shows the following internal tissue organization from periphery towards the centre (Fig. 31.58).
Epidermis:
It is single layered and composed of horizontally flattened cells. Numerous epidermal outgrowths occur here and there.
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Ground tissue:
The peripheral layers are composed of collenchyma and chlorenchyma cells. These cells occur as bands and they alternate each other thus forming a continuous band next to epidermis. This forms the hypodermis. The remaining portion of the ground tissue is composed of parenchyma. The cells are isodiametric and enclose conspicuous intercellular spaces. There is no endodermis and pericycle.
Vascular bundle:
Numerous small and some quite large vascular bundles are present in the ground tissue. The vascular bundles are arranged in the form of a band. Each bundle is collateral. Xylem occurs towards the inner side and phloem lies towards the peripheral side.
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Comment:
Hypodermal collenchyma and xylem are the mechanical cells that provide strength against inflexibility. The presence of numerous small and some quite large bundles and their arrangement in the form of a band is the characteristic of the petiole.
Type # 3. Petiole of Piper Betle (Family: Piperaceae):
Transverse section of petiole is cordate in shape, i.e. there occurs a prominent groove on one side whereas the other side is round. The following internal tissue organization is observed from periphery towards the centre (Fig. 31.59).
Epidermis:
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It is single layered and composed of horizontally flattened parenchyma cells. The cells are compactly arranged and cutin deposits on the outer walls.
Ground tissue:
Peripheral layers are composed of collenchyma cells. It forms hypodermis and is not in a continuous band. The continuity of it is interrupted by the presence of chlorenchyma cells. These chlorophyll-containing cells occur as patches here and there.
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The remaining portion of the ground tissue is composed of parenchyma cells. At the centre there occurs a large mucilage canal. A few smaller canals are present here and there in the ground tissue. There is no endodermis and pericycle.
Vascular bundle:
Numerous bundles of different sizes remain embedded in the ground tissue. They are arranged in the form an arc. Each bundle is collateral. Xylem is towards the inner side and phloem lies on the peripheral side.
Comment:
Hypodermal collenchyma and xylem are mechanical cells that provide strength against inflexibility. The presence of a large central mucilage canal and a few small canals here and there, and the arc arrangement of the vascular bundle in the ground tissue are the characteristics of the petiole.
Type # 4. Petiole of Nymphaea Stellata (Family Nymphaeaceae):
Transverse section of petiole is more or less round and reveals the following tissue organization from periphery towards the centre (Figs. 31.60).
Epidermis:
It is single layered. The cells are more or less round, thin walled, compactly arranged with thin cuticle on their outer walls. Chloroplastids occur in the epidermal cells. Unbranched multicellular hairs are present on epidermis here and there.
Ground tissue:
The peripheral layer of it forms hypodermis. It is two or more layered. It is composed of collenchyma cells that are thickened at the corners. The inner layers are composed of parenchyma cells. The cells are thin walled. Many large air spaces occur in this region. Numerous trichoblasts remain attached to the cells that border the air spaces. It is of different shapes and deposited with crystals of calcium oxalate.
Vascular bundle:
Large number of vascular bundles are present and they are scattered over the ground tissue. They remain on the partitions of air chambers. Each vascular bundle is collateral and closed. Xylem is poorly developed. The elements of xylem break down to form lacunae below the phloem.
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The lacunae resemble air chambers. Phloem is well developed. The vascular bundles may be large or small. In the large bundles two patches of phloem occur on the peripheral side of xylem in contrast to small bundles that have a single patch of phloem.
Comment:
Petiole shows hydrophytic nature due to the presence of large air spaces in the ground tissue, poorly developed xylem and thin cuticle on the outer wall of epidermis.
Collenchyma at hypodermis is the only mechanical cell that provides strength to the petiole to hold the leaf in position. Anomaly in the petiole is due to the presence of scattered vascular bundles though it is a dicot. Presence of trichosclereids with calcium oxalate deposition is the characteristic of the petiole.
Type # 5. Petiole of Eichhornia sp. (Family: Pontederiaceae):
Transverse section of petiole is more or less oval in outline and shows the following internal tissue organization from periphery towards the centre (Fig. 31.61).
Epidermis:
It is uniseriate and composed of thin walled compactly arranged cells with thin cuticle on their outer walls.
Cortex:
The peripheral few layers are composed of thin walled parenchyma cells. It encloses little intercellular spaces. There is no endodermis and pericycle. The rest of the cortex has numerous air spaces. Parenchyma cells of one layer thick form the wall of the spaces. It also forms the partition walls between spaces.
This is lacunate cortex. There are many scattered vascular bundles. The bundles are present on the partition walls of lacunae. Each vascular bundle is composed of xylem and phloem and collateral in arrangement. Xylem is towards the centre and phloem occurs towards the peripheral side. The vascular elements are poorly developed.
Comment:
The petiole exhibits hydrophytic characters due to the presence of thin cuticle on epidermis, air spaces, reduced xylem and mechanical cells.
