Here is a list of sixteen types of roots found in plants.
1. Root of Cicer arietinum (Gram. Subfamily: Papilionaceae):
The transverse section of root is more or less circular in outline and shows the following internal tissues from periphery towards the centre (Figs. 31.29, 31.29A).
Epidermis:
It is also called epiblema or piliferous layer. It is uniseriate and composed of horizontally flattened, compactly arranged parenchyma cells. The cells are devoid of cuticle. Transverse section through the root hair region shows the presence of root hairs here and there. Root hairs are the tube like prolongation of some epidermal cells.
Cortex:
It is multiseriate and composed of parenchyma cells. The cells are of various shapes and enclose profuse intercellular spaces. The cells contain abundant leucoplasts. The cortex is large and the innermost layer of it is the endodermis. It is uniseriate and encloses all tissues present within the stele. The cells of endodermis are barrel shaped, compactly set and have casparian thickening on the radial walls.
Stele:
It includes pericycle, vascular tissues and ground tissue. The pericycle is uniseriate and composed of thin walled parenchyma. The vascular bundle is radial; xylem and phloem are separate and occur at alternate radii. The parenchyma situated between xylem and phloem is termed as conjunctive tissue. There are four groups of xylem and phloem, which are arranged alternately and so the bundle is tetrarch.
The protoxylem and metaxylem vessels occur towards the periphery and centre respectively and this is referred to as exarch xylem. A few sclerenchyma cells are present over each phloem patch. At a very early stage a few parenchyma cells are present at the centre and this is pith, which at later stage is obliterated after the formation of metaxylem. The four-metaxylem vessels are grouped at the centre and so in mature roots pith is completely absent.
Comment:
It is a root due to the presence of radial vascular bundle. It is a dicot root because the xylem is tetrarch and pith is absent. Epiblema with root hair is absorptive and protective in function. Cortical parenchyma helps in storage. In the stele the sclerenchyma patch and tetrach xylem are the mechanical cells only. This centralized mechanical cell provides mechanical strength against inextensibility.
2. Root of Pisum sativum (Pea. Subfamily: Papilionaceae):
A transverse section of root is more or less round and reveals the following anatomical structure from periphery towards the centre (Figs. 31.30 & 31.30A).
Epiblema:
It is uniseriate. The cells are more or less horizontally flattened and compactly set. Root hairs are present here and there. It is unicellular and prolongation of epidermal cells.
Cortex:
It is large and composed of parenchyma cells of various sizes and shapes with intercellular spaces. The innermost layer is the endodermis. It is uniseriate. The cells are barrel shaped, compactly set and have distinct casparian strips on the radial walls.
Stele:
Next to endodermis there lies the pericycle. It is uniseriate and parenchymatous. The vascular bundle is radial with triarch xylem. The xylem is .exarch, i.e. protoxylem is towards the peripheral side and metaxylem occurs towards centre. Three patches of phloem are alternately arranged with three xylem patches.
Between xylem and phloem patches there lies a small amount of thin walled parenchyma called conjunctive tissue. A small amount of sclerenchyma occurs between each phloem patch and the pericycle. Pith is absent and there occurs the metaxylem. At an early stage small pith is present, but during development it is obliterated and the metaxylem vessels occupy this region.
Comment:
It is a root due to the presence of radial vascular bundle. It is a dicot root because the xylem is triarch and pith is absent. Epidermis with root hair helps in absorption. Cortex is storage tissue. Sclerenchyma patches above phloem and xylem are the only mechanical cells that provide mechanical strength against inextensibility.
3. Root of Ranunculus sp. (Buttercup. Family: Ranunculaceae):
The transverse section of root is more or less circular in outline and shows the following internal tissues from periphery towards the centre (Figs. 31.31, 31.31A).
Epidermis:
It is single layered and composed of horizontally flattened cells.
Cortex:
The peripheral few layers that occur next to epidermis are composed of small thin walled parenchyma cells. The cells are compactly arranged and have little intercellular spaces. This narrow zone of tissue is called exodermis. The inner layers of cortex are many layered and composed of thin walled parenchyma of various shape and size.
It encloses prominent intercellular spaces. The innermost layer of cortex is endodermis. It is composed of barrel shaped cells that are compactly set with casparian thickening on radial walls. Some endodermal cells may undergo secondary thickening. The remaining un-thickened cells are known as passage cells and they occur against the protoxylem groups.
Stele:
Next to endodermis there lies the pericycle. It is single layered and made up of parenchyma cells. The vascular bundles are radial. The xylem strand is tetrarch or pentarch. It is alternately arranged with four or five small patches of phloem.
Protoxylem is exarch, i.e. it is towards the peripheral side and metaxylem is situated towards the centre. In between xylem and phloem there lies the conjunctive tissue made up of parenchyma cells with scanty intercellular spaces. The metaxylem vessels occupy the central position and so there is no pith.
Comment:
It is a dicotyledonous root due to the presence of radial vascular bundle with tetra- or pentarch xylem where pith is absent. The cortex is storage tissue. Xylem is the only mechanical cell that provides mechanical strength against inextensibility.
4. Root of Colocasia sp. (Arum. Family: Araceae):
The transverse section of root is circular in outline and shows the following arrangement of tissues from periphery towards the centre (Figs. 31.32, 31.32A).
Epiblema:
It is single layered. The cells are horizontally flattened and compactly arranged without any intercellular spaces. Unicellular root hairs are present here and there.
Cortex:
It is composed of parenchyma cells, many layered and large. Numerous intercellular spaces are present in it and they are formed schizogenously. The innermost layer is endodermis, the cells of which are
barrel shaped and compactly set. The cells have conspicuous casparian strips. Passage cells are present in the endodermis. Mature root shows exodermis. It is composed of few layers of cells whose cell walls have undergone suberization.
Stele:
It is radial where xylem and phloem lie at different radii. Xylem is polyarch, i.e. xylem strands are more than she in number. Protoxylem is exarch. In between the xylem phloem occurs. Conjunctive tissues are present in between xylem and phloem.
These are composed of small patches of parenchyma cells. Just below the endodermis there lies the pericycle. It is uniseriate and composed of parenchyma cells. At the centre of stele pith is present. It is large and composed of parenchyma cells.
Comment:
It is a monocot root due to presence of radial vascular bundle, polyarch and exarch xylem, and large pith. Epiblema is protective and absorptive in function. The exodermis protects the inner tissues when epiblema is decayed. Xylem is the mechanical cell. Centralized mechanical cells provide strength against inextensibility.
5. Root of Zea mays (Maize. Family: Poaceae):
The transverse section of root is circular in outline and shows the following internal organization of tissues from periphery towards centre (Fig. 31.33).
Epiblema:
It is uniseriate; the cells are thin walled and horizontally flattened; the cells are compactly set and devoid of cuticle. Unicellular root hairs are present here and there.
Cortex:
It is composed of parenchyma cells, many layered and large, and encloses air spaces. Numerous large air spaces are present and they are formed schizogenously. Mature roots show the exodermis. It is the peripheral layer of cortex and composed of cells whose walls have undergone suberization. They differ from cork cells, whose cell walls are also suberized, in possessing living cell contents.
The innermost layer of cortex is endodermis. It is composed of compactly set barrel shaped cells that have conspicuous casparian strips. Passage cells are present here and there in the endodermis. In mature roots the cells of endodermis may have additional layers on their walls when the casparian strips become inconspicuous.
Stele:
Next to endodermis there occurs the pericycle. It is uniseriate. It is composed of parenchyma and sclerenchyma cells. The vascular bundle is radial with protoxylem exarch. The bundles are polyarch with numerous xylem and phloem strands. Parenchyma cells those are in contact with xylem may undergo sclerosis.
Protoxylem and phloem occur towards the pericycle whereas the metaxylem lies towards the centre. The central portion of stele is the pith. It is large and composed of parenchyma cells with intercellular spaces. On the periphery of pith there lies a ring of metaxylem vessels. The pith cells contain abundant starch grains.
Comment:
The root shows the characteristics of monocot due to the presence of radial stele with polyarch xylem and phloem strands, exarch protoxylem and large pith. Epiblema is protective and absorptive in function.
Exodermis is protective layer and protects the inner tissues when epiblema is decayed. The xylem and sclerenchyma are the mechanical cells. The centralized mechanical cell reveals that it is an inextensible organ and provides mechanical strength against inextensibility.
6. Stilt Root of Zea Mays:
The internal tissue organization of nodal root is just like the root of Zea mays except that it has a continuous ring of many-layered sclerenchyma at the periphery of cortex just below the epiblema (Fig. 31.34).
Comment:
It is a root due to the presence of radial stele with exarch protoxylem. The monocot nature is revealed due to the presence of polyarch xylem and phloem strands, and large pith. Sclerenchyma at the periphery is the mechanical cell and it reveals that it is an inflexible organ.
The presence of centralized mechanical cells like xylem and sclerenchyma surrounding them reveal that the monocot root is an inextensible and incompressible organ. Radial stele, exarch protoxylem and polyarch vascular strands with peripheral and centralized mechanical cells (sclerenchyma) are the characteristic of stilt root of maize where inflexibility, incompressibility and inextensibility are operative.
7. Root of Can No sp. (Family: Cannaceae):
The transverse section of root is more or less circular in outline and shows the following internal arrangements of tissue from periphery towards the centre (Figs. 31.35, 31.35A).
Epidermis:
It is uniseriate. The cells are tabular, thin walled, compactly set and parenchymatous without any intercellular spaces. Cuticle is absent from the peripheral walls. Transverse section through the root hair region shows the presence of root hairs, which are the tube like prolongation of the epidermal cells. The epidermis with root hairs is also termed as epiblema or piliferous layer.
Cortex:
This zone occurs in between epidermis and stele. It consists of many layered, thin walled parenchyma cells with conspicuous intercellular spaces. This zone stores starch grains. In mature root, the peripheral cortex consists of compactly set parenchyma cells. The inner cortex is composed of parenchyma with conspicuous inter-cellular spaces.
The middle cortex shows many air spaces. The spaces are linearly elongated. They occur radially in all directions. The innermost layer of inner cortex is endodermis, which is uniseriate and composed of barrel shaped, compactly set parenchyma cells. The endodermal cells possess casparian thickening and its inner walls are thickened.
Stele:
It includes pericycle, vascular tissue and ground tissue. The pericycle is uniseriate and parenchymatous. The vascular bundle is radial, polyarch with exarch protoxylem. There are about 12 protoxylem and approximately seven metaxylem vessels of different diameter. Conjunctive tissues are present in between xylem and phloem. Pith is large and conspicuous. It is composed of sclerenchyma cells.
Comment:
It is a monocot root due to the presence of radial vascular bundle, polyarch xylem with exarch protoxylem and large pith. Linearly elongated air space at the middle cortex and sclerenchymatous pith is the characteristic of the root. Xylem and sclerenchymatous pith are the mechanical cells that provide strength against inextensibility.
8. Root of Smilax sp. (Family: Smilacaceae):
The transverse section of root is more or less circular in outline and reveals the following internal tissue organization from periphery towards the centre (Fig. 31.36).
Epidermis:
It is uniseriate. It is composed of parenchyma cells that are compactly arranged. The outer wall of the cells is round and cuticularized.
Cortex:
The outermost layer of the cortex is exodermis. This layer lies just internal to epidermis. It is uniseriate and the cells are very much thick walled. The inner cortex is composed of parenchyma cells.
The cells are more or less round and oval, and enclose intercellular spaces. The cells contain starch grains. The innermost layer of cortex is the endodermis. It is single layered and the cells are thickened on their radial and inner walls. It is discontinuous at the region of passage cells that are present here and there.
Stele:
The outermost layer is the pericycle. It occurs just internal to endodermis. It is multiseriate and composed of sclerenchyma cells. The stele is radial. The xylem is polyarch. Numerous xylem and phloem remain alternate to each other.
The protoxylem is exarch, i.e. it occurs towards the peripheral side. Protophloem and metaphloem can be differentiated. The former that occurs towards the outer side are smaller than the latter. Conspicuous pith is present. It is large and composed of parenchyma cells that are full of starch grains.
Comment:
It is a root due to the presence of radial stele. Polyarch xylem and large pith reveal that it is monocot root. In contrast to other roots it has multiseriate sclerenchymatous thick walled pericycle and the cells of pith are full of starch grains. Centralized mechanical cells like xylem and pericycle reveal that it is an inextensible organ and provide mechanical strength against inextensibility.
9. Root of Vanda sp. (Orchid root. Family: Orchidaceae):
The transverse section of root is circular in outline and shows the following tissue arrangements from periphery towards the centre (Figs. 31.37).
Velamen:
It consists of several layers of cells and forms a sheath around the cortex. It is multiple or multiseriate epidermis the outermost layer of which is the limiting layer. The cells of velamen are non-living and compactly set. The cells are devoid of any contents. In some cells strip like thickening can be seen on their walls.
The cells have perforated walls that act like a sponge. It soaks up water that wets the limiting layer. It usually occurs during rainy season. In dry weather air fills the cells. The cells are large, thick walled and horizontally flattened. Some cells are polygonal.
Cortex:
The peripheral and outermost layer of it is exodermis. It consists of long and short cells. At some regions the long and short cells alternate. The short cells are thin walled and called passage cell. The long cells are thick walled. The thickening occurs on outer tangential and radial surface. The exodermis occupies a limiting position between velamen on peripheral side and thin walled cells of the cortex on the inner side.
The thin walled cells are many layered, parenchymatous with intercellular spaces. The peripheral layers contain chloroplastids. Air chambers are present in the cortex. The innermost layer of cortex is the endodermis that completely encircles the stele. It is continuous and at some regions it is interrupted by the presence of passage cells.
The cells of endodermis are thick walled, compact and barrel shaped. The thickening occurs on inner tangential wall and radial wall. The cells contain starch grains and casparian strips. The passage cells are thin walled. They have a definite position in relation to protoxylem. They occur opposite to the protoxylem.
Stele:
The peripheral layer of it is the pericycle. It is continuous throughout. The cells are thick walled except the cells that are presented internal to the passage cell. The stele is radial and the vascular bundles are polyarch with protoxylem exarch. Conjunctive tissues are present. They are sclerenchymatous and remain surrounding the phloem. Pith is large, many layered and parenchymatous that may undergo sclerosis.
Comment:
Velamen forms a sheath of cortex and thus protects the inner tissues. It gives mechanical strength by the presence of strip like thickening on the walls of some cells. It has also absorptive function; the water thus absorbed is transported to cortex through passage cells of exodermis. The other cells of exodermis are thick walled and are mechanical cells. Parenchymatous cortex is storage in function.
Chlorophyllous parenchyma in the cortex is the photosynthetic tissue. The passage cells of endodermis allow radial diffusion of water. Radial stele with polyarch vascular bundle and exarch protoxylem is the characteristic of monocot root. The presence of spongy and absorptive velamen is the characteristic of epiphytic root.
The centralized mechanical cells like xylem and sclerenchymatous conjunctive tissue provide mechanical strength against inextensibility as the root hangs freely and bears its own weight. Presence of multiple epidermis (=velamen), exodermis with passage cells, radial vascular bundle with polyarch xylem strands, exarch protoxylem and sclerenchymatous conjunctive tissue are the characteristic of epiphytic orchid root.
10. Pneumatophore of Rhizophora sp. (Family: Rhizophoraceae):
The transverse section of breathing root or pneumatophore is more or less circular in outline and reveals the following internal organization of tissues from periphery towards the centre (Figs. 31.38, 31.38A).
Periderm:
It consists of phellem, phellogen and phelloderm cells. Phellem cells are cork cells and are few layered. It is outermost layer and may consist the uniseriate epidermal layer at an early stage. The epidermis and cork cells are interrupted by the presence of lenticel. It is composed of ruptured epidermis and cork cells.
In this region there lie the complementary cells that are parenchymatous and very loosely arranged. Phellogen is cork cambium that forms cork cells and loose complementary cells on the peripheral side. Phelloderm cells are the inner derivatives of phellogen and more or less like cortical cells.
Cortex:
It is few layered and composed of parenchyma cells. The cells are more or less round. They are arranged in such a way that several cells enclose a large air space. Abundant large air spaces are present in this region and they compose a well-developed intercellular space system. The innermost layer of cortex is endodermis. It is uniseriate and composed of barrel shaped cells that are small and compactly arranged.
Stele:
Next to endodermis there lies the pericycle. It is multiseriate. The uniseriate peripheral layer of it is parenchymatous and compactly arranged. The inner few layers are sclerenchymatous. The vascular bundles are collateral and open, i.e. cambium is present in between xylem and phloem. The protoxylem is endarch where it is towards the centre.
The vascular tissues form a continuous cylindrical band below the pericycle. In the band phloem occurs towards the peripheral side, xylem lies towards the centre and cambium is situated in between xylem and phloem. Pith is large and composed of parenchyma cells with intercellular space. It fills up the entire centre portion of root.
Comment:
Cork cells are impervious to air and water and so protective in function. Gaseous diffusion occurs through lenticels. Stele is like a dicotyledonous stem where collateral and open vascular bundles are present.
Cork cells interrupted by lenticels, large air spaces at the cortex and the stele like stem are the characteristic features of negatively geotropic and halophytic root, i.e. pneumatophore. Sclerenchyma at pericycle and xylem are the mechanical cells that provide strength against inflexibility.
11. Root (Aerial and Young) of Tinospora sp. (Family: Menispermaceae):
The transverse section of root is more or less circular in outline and shows the following tissue organization from periphery towards the centre (Fig. 31.39).
Epidermis:
It is uniseriate. The cells are thick walled, tabular in shape, compactly set and form a continuous uninterrupted layer.
Cortex:
It is many layered; the peripheral layers are thick walled and compactly set without any intercellular spaces. The cells are of various shapes. The peripheral layers of cortex in association with epidermis form a compact peripheral zone of thick walled cells. The inner cortex is parenchymatous, thin walled with intercellular spaces. The innermost layer is the endodermis and is composed of barrel-shaped compactly set cells.
Stele:
Internal to endodermis there occurs a few layered, thin walled parenchymatous pericycle. The vascular bundle is radial and the xylem is tri-, tetra- or pentarch. Protoxylem is exarch. In between the xylem, phloem occurs and it corresponds to the number of xylem present in the stele. Conjunctive tissues occur in between xylem and phloem. Pith is small and parenchymatous.
Comment:
It is a dicotyledonous root due to the presence of radial vascular bundle with exarch protoxylem and the number of xylem is less than six. The presence of peripheral thick-walled cell reveals that it is an inflexible organ and aerial in nature. Absence of root hair is the characteristic of aerial root.
The thick- walled cells provide strength against inflexibility. The centralized xylem is mechanical cell and provides strength against inextensibility. This dicotyledonous aerial root shows the remarkable combination of inextensibility and inflexibility.
12. Root (Aerial and Mature) of Tinospora sp.:
The transverse section of root is polyhedral circular in outline and shows the following tissue organization from periphery towards the centre (Figs. 31.40, 31.40A).
Periderm:
It is composed of cork cambium phellogen, phellem and phelloderm. It forms a continuous layer. The continuity at some region is interrupted by the presence of lenticels. It consists of ruptured phellem, loose complementary cell, phellogen and phelloderm. Phellem is situated on the peripheral side of phellogen.
The cells of phellem are more or less rectangular or polyhedral, thick-walled and many layered with little intercellular spaces. The cells of phellogen are thin-walled; two to three layered and are arranged in storied manner. Phelloderm lies below the phellogen. The cells of phelloderm are parenchymatous with conspicuous intercellular spaces.
Cortex:
It is composed of few layered, thin-walled, compactly set parenchyma cells. The innermost layer is endodermis, which is conspicuous.
Stele:
The stele shows primary and secondary vascular tissues. The primary vascular bundle is radial with tri-, tetra- or pentarch xylem. They are grouped together at the centre obliterating the pith. Internal to endodermis there occurs an inconspicuous pericycle. A continuous somewhat wavy cambium ring is present and on the peripheral side of it, there occurs the secondary phloem. Primary phloem is present as crushed patches over the secondary phloem.
A thick zone of secondary xylem is present at the inner side of cambium. The secondary xylem is composed of tracheids, fibres and tracheae with large lumen. The most characteristic of the root is the occurrence of broad vascular rays or medullary rays among xylem and phloem passing through the cambium. These rays are present corresponding to protoxylem. The secondary vascular tissues are arranged collaterally.
Comment:
Thick walled cork is protective and mechanical cell. Presence of thick walled cork at the periphery signifies that the root is aerial in nature and so inflexible organ. The dicotyledonous nature of root is revealed due to the presence of radial primary vascular bundle with exarch protoxylem, the number of primary xylem strand is less than six and the obliteration of pith as a result of secondary vascular tissue formation.
The primary and secondary xylem are mechanical cell that provides strength against inextensibility. The presence of mechanical cells at the periphery and centre side reveals that the aerial root is an inflexible and inextensible organ that exhibit the remarkable combination where both inflexibility and inextensibility are operative.
13. Root of Beta Vulgaris (Family: Chenopodiaceae):
The transverse section of root is more or less circular in outline and reveals the following internal organization of tissues from periphery towards the centre (Fig. 31.41).
Periderm:
It consists of phellem, phellogen and phelloderm. Phellem is the outermost layer and consists of a few layers of cells. Phellogen is the cork cambium that divides. The peripheral derivatives are the phellem. The inner derivatives are the phelloderm. Phelloderm is the innermost layer of periderm and the cells are more or less like cortical cells.
Cortex:
It is parenchymatous; few layered and is confluent with stelar tissues.
Stele:
It shows pericycle and numerous concentric rings of growth layers. Each growth layer consists of vascular bundles and parenchyma. The vascular bundles are collateral, i.e. xylem and phloem occur on the same radius. Phloem is situated on the peripheral side. The xylem contains very scanty lignified elements.
The vascular bundles are mainly composed of parenchyma. Bands of parenchyma are present between the vascular bundles. In between the growth rings parenchyma cells are also present. The stele exhibits anomalous secondary growth. The primary xylem is diarch. The first cambium ring originates from the parenchyma present in between xylem and phloem and pericycle.
The cambium though in a continuous ring produces separate vascular bundles separated by bands of radial parenchyma. Thus a ring of vascular bundle is produced. After a period of activity the first ring of cambium ceases its function. A second cambium ring originates from the phloem parenchyma produced by the first ring.
Like the first ring the cambium also produces a ring of vascular bundle that are separated by bands of radial parenchyma. After a period of activity its function also ceases. Then a third cambium ring originates from the pericycle. It also behaves like the first and second cambium ring. When its activity ceases another cambium arises from the pericycle.
This also has similar function. Thus concentric rings of cambia arise in succession and thus several rings of vascular bundles are produced. Narrow bands of radial parenchyma separate the vascular bundles produced by the first cambium ring whereas the vascular bundles produced by the later successive cambium ring are separated by wide bands of radial parenchyma.
Concentric rings of cambia that originate from pericycle enclose cells of pericycle that undergo repeated divisions thus forming more pericyclic layers of parenchyma. This is proliferated pericycle. It alternates with the rings of vascular bundles and parenchyma present between the vascular bundles.
In unstained cross section the ring of proliferated pericycle appears to be dark red and thus can be distinguished from the rings of vascular bundles, which are of lighter colour. The parenchyma cells present in proliferated pericycle, vascular bundles and in between vascular bundles are storage tissues. Pith is not clearly demarcated.
Comment:
Periderm protects the inner tissues from desiccation. Stele shows anomalous secondary growth. It is adaptive type of anomaly and occurs for storage purposes. The concentric rings of cambia are abnormal in origin and activity. Major bulk of tissues produced by cambia are storage parenchyma.
The proliferated pericycle is also storage tissue. The root swells when all the storage parenchyma cells store food materials. The increase in thickness is also due the proliferation of pericycle and the activity of successive rings of cambia. The diarch prirpary xylem and radial stele reveal that it is a dicotyledonous root.
14. Root of Lpomoea Batatas (Sweet Potato. Family: Convolvulaceae):
In the primary state the internal organization of root is more or less similar to that of other dicotyledonous root. The vascular bundle is pentarch or hexarch and surrounded by distinct endodermis. The stele remains encircled by wide cortex where intercellular spaces are present.
The transverse section of mature root is more or less circular in outline and shows the following internal tissue organization from periphery towards the centre (Fig. 31.42).
Periderm:
It is multiseriate. The cells are more or less rectangular in shape and compactly arranged. It is composed of phellem, phellogen and phelloderm.
The outermost few layers are phellem that develops from the phellogen or cork cambium. The phellogen originates from the pericycle and it forms phelloderm towards the inner side. The cells of phelloderm are parenchymatous. Below the periderm there lies a wide zone of parenchyma with conspicuous intercellular spaces.
Vascular tissue:
A cambium ring is present in between xylem and phloem. The secondary vascular tissues like secondary xylem and phloem are normally formed by the cambium. At a later stage, an anomalous cambium originates in the form of a ring surrounding a vessel or a group of vessels. These rings of cambia divide and the inner derivatives are differentiated into secondary xylem elements and the peripheral derivatives form the sieve tubes and laticiferous elements.
The anomalous cambia originate from the ground parenchyma present surrounding the vessel or a group of vessels. Tyloses are often present in the vessels with large lumen. The normal cambium and anomalous cambia produce considerable amounts of parenchyma on the peripheral and inner side. These are storage parenchyma.
Comment:
Periderm is protective tissue and it is pericyclic in origin. Anomaly in the stele is due to the formation of anomalous cambia. The normal cambium and anomalous cambia form large amount storage parenchyma that causes the root to swell.
15. Root (Aerial and Young) of Ficus sp. (Family: Moraceae):
The transverse section of root is more or less circular in outline and shows the following tissue organization from periphery towards the centre (Fig. 31.43).
Epidermis:
It is uniseriate. The cells are thick walled, tabular in shape, compactly set and form a continuous uninterrupted layer.
Cortex:
It is many layered; the peripheral layers are thick walled and compactly set without any intercellular spaces. The cells are of various shapes. The peripheral layers of cortex in association with epidermis form a compact peripheral zone of thick walled cells. The inner cortex is parenchymatous, thin walled with intercellular spaces. The innermost layer is endodermis and composed of barrel-shaped compactly set cells.
Stele:
Internal to endodermis there occurs a few layered, thin walled parenchymatous pericycle. The vascular bundle is radial and the xylem is polyarch and the number of primary xylem groups is more than six. Protoxylem is exarch. In between the xylem, phloem occurs and it corresponds to the number of xylem present in the stele. Conjunctive tissues occur in between xylem and phloem. Pith is large and parenchymatous.
Comment:
It is a root due to the presence of radial vascular bundle with exarch protoxylem. Anomaly in the stele is due to the presence of more than six numbers of primary xylem groups and large pith though it is a dicotyledonous root. The presence of peripheral thick walled cell reveals that it is an inflexible organ and aerial in nature.
Absence of root hair is the characteristic of aerial root. The peripheral thick-walled cells provide strength against inflexibility. The centralized xylem is mechanical cell and provides strength against inextensibility. This dicotyledonous aerial root shows the remarkable combination of inextensibility and inflexibility.
16. Root (Aerial Arid Mature) of Ficus sp.:
The transverse section is more or less circular in outline and shows the following internal tissue organization from periphery towards the centre (Figs. 31.44, 31.44A).
Epidermis and Periderm:
The outermost layer is epidermis. It is uniseriate. The cells are horizontally flattened, compactly arranged with thick cuticle on their outer wall. Next to epidermis there lies the periderm. It consists of phellem, phellogen and phelloderm. The peripheral layer is the phellem or cork cell. It is many layered and consists of thick walled somewhat rounded cells.
The cells enclose little intercellular spaces and are radially arranged. Phellogen, also called cork cambium, lies below the cork cells. The cells are thin walled and compactly arranged. It produces phellem on the peripheral side and phelloderm on the inner side. Phelloderm is few layered and composed of more or less isodiametric parenchyma cells. The cells enclose intercellular spaces and are radially arranged.
Cortex:
It is few layered and composed of parenchyma cells with intercellular spaces. It differs from phelloderm, as the cells are not radially arranged. The innermost layer is endodermis. It is uniseriate, compactly arranged and composed of barrel shaped cells.
Stele:
Next to endodermis there lies the pericycle. It is few layered and parenchymatous. The vascular bundle is radial in arrangement with protoxylem exarch. A complete cambium ring is present in the stele. Below the ring there lies a complete cylinder of secondary xylem produced by the cambium. It is many layered and composed of vessels, fibres and tracheids. On the peripheral side cambium produced secondary phloem.
It is few layered and consists of sieve tube, companion cell, and phloem fibre and phloem parenchyma. In between secondary phloem and pericycle the patches of primary phloem remain appressed. Below the secondary xylem there occurs the primary xylem. The centre is occupied by pith. It is small, parenchymatous with few intercellular spaces.
Comment:
Thick walled periderm is protective in function. The cylinder of secondary xylem and primary xylem are mechanical cells. It is a root due to the presence of radial vascular bundle. Its aerial nature is revealed due to the presence of peripheral cork cells and centralized mechanical cells. The latter provides mechanical strength against inextensibility as the root hangs freely and bears its own weight.