The first root of a seed plant evolves from the radicle and is called a tap root. In most dicotyledons it goes to the deeper layers of soil and branches to form a tap root system. In the monocotyledons these roots become short lived and are replaced by superficial adventitious roots.
Certain roots become specialized due to their peculiar function e.g., fleshy roots of beet, carrot, radish, etc. In certain mangroves there develop aerial prop roots while the parasites develop sucking roots. All these roots though vary in their internal structure, but still there are certain common features in them.
The internal organization of the root, though variable in various plants, is simpler than that of a stem. Since it is a leafless axis, there is little difference in the internal structure at different levels. The transverse section of a root visible a clear differentiation of the three kinds of tissue systems—epidermal, ground and vascular tissue systems.
i. Epidermis:
In young roots it is single layer of cells with root hairs. The root hairs are tubular extensions of the epidermal cells. These are more commonest at those places where the xylem tissue is mature. These increase the absorptive surface of roots. The absorption is done by other epidermal cells also. A thin cuticle can be present on the epidermal cells.
ii. Cortex:
It is made-up of only parenchyma. At later stages some sclerenchyma may develop. Intercellular spaces are conspicuous. The cortex in plants growing in moist places viz., rice forms aerenchyma. The aerenchyma in roots is taken as storage organ for oxygen and to serve in gas transport. Cortical cells are highly vacuolated and generally do not have chlorophyll in their plastids.
iii. Endodermis:
It is innermost layer of the cortex. In young roots the endodermal cells possess suberin in the walls in a band-like structure called as casparian strip, extending completely around the cells within the radial and transverse walls. If it remains even after the secondary growth, the wall evolves thickening (Fig. 9.1).
iv. Exodermis:
It develops beneath or outside the epidermis. It can be one or more layers of uniform or variable cells. The cells can be suberized very soon.
v. Vascular Cylinder:
The tissue lying within the endodermis forms the vascular part. It includes one or more non-vascular layers—the pericycle and the vascular tissue. The pericycle can be parenchymatous or partly sclerenchymatous. The xylem is a solid core with ridge-like projections extending towards the pericycle. In relation to the number of xylem ridges present in the roots these can be known as diarch, triarch, tetrarch, pentarch, hexarch or polyarch (Fig. 9.2). The phloem strands alternate with xylem ridges. In the center a pith is present. It can be parenchymatous or sclerenchymatous.
The roots can be differentiated from the stem based on the following characters:
(1) The stomata are lack of the epidermis. It is called as epiblema or pillferous layer.
(2) Unicellular hairs are present on the epiblema.
(3) The hypodermis is not differentiated. A well-evolved cortex is present.
(4) The endodermis is prominent with casparian thickenings.
(5) The vascular bundles are radial with exarch protoxylem.
Monocotyledonous Root:
(A) Root of Canna:
1. Epiblema:
This is the outermost layer. It has thin walled cells with unicellular root hairs. The cuticle and stomata are lack.
2. Cortex:
Inner to the epiblema there is present a large band of parenchyma with intercellular spaces. These cells often possess starch grains and leucoplast.
3. Endodermis:
This is the innermost layer of cortex. The cells are barrel-shaped with prominent casparian strips. The inner walls are thickened. Inner cellular spaces are lack. The passage cells are present outside the protoxylem.
4. Pericycle:
It is the outermost layer of the stele lying inner to the endodermis. It has a single layer of thin-walled cells.
5. Vascular Bundle:
These are radial and arranged in a ring. The xylem bundles are number of and alternate with phloem bundles. Protoxylem is exarch. Structurally they are similar to the bundles of dicotyledonous root.
6. Conjunctive Tissue:
The small parenchyma in between the xylem and phloem bundles is the conjunctive tissue.
7. Pith:
It is well differentiated and includes parenchymatous cells with intercellular spaces (Fig. 9.3)
(B) Other Monocotyledonous Roots:
The structure of most of the roots is similar to that of Canna root. They can differ in the number of bundles or other minor details viz., in Maize root there is present sclerenchyma outside the phloem bundles (Fig. 9.4).
(C) Root of Orchi:
1. Velamen:
This is a special tissue present outside the epiblema having few layers of thin- walled, elongated and dead cells. Such is meant for absorption of water from the atmosphere.
2. Epiblema:
It is a single layer of thick-walled cells, interrupted here and there by thin- walled passage cells.
3. Cortex:
It includes many layers of parenchymatous cells with intercellular spaces. The outer cells sometimes have chloroplasts.
4. Endodermis:
This is more or less similar to exodermis in being single-layered, thick- walled, barrel-shaped cells, interrupted by thin-walled passage cells.
5. Pericycle:
This is a single layer of thin-walled cells.
6. Vascular Bundles:
Typical of a monocotyledonous root.
7. Conjunctive Tissue:
It is the parenchyma located in between the xylem and phloem bundles. Some of the cells may be sclerenchymatous.
8. Pith:
It includes a mass of thin-walled cells occupying the center.
Dicotyledonous Root:
(A) Young Root of Gram:
It has the Following Structures:
1. Epiblema or Piliferous Layer:
This is formed of a single layer of thin-walled cells. Certain of the cells are enlarged towards to form tubular and unicellular root hairs.
2. Cortex:
Below the epiblema there are present several layers of parenchyma with intercellular spaces. The cells possess leucoplast and starch. Sometimes, the epiblema degenerates and the outer layer of cortex is cutinized to produce exodermis.
3. Endodermis:
It is the innermost layer of the cortex and forms a ring of compact, barrel- shaped cells around the stele. The radial and also the inner walls are thickened. The radial thickening is in the form of casparian strips. Intercellular spaces are lack. Just above the protoxylem and also at other places certain cells of the endodermis remain thin walled and are called passage cells.
4. Pericycle:
The outermost layer of the stele lying just below the endodermis is the pericycle. It includes thin walled, small cell with abundance of cytoplasm.
5. Conjunctive Tissue:
The tissue lying between the xylem and phloem bundles is parenchymatous and is known as conjunctive tissue.
6. Vascular Bundles:
The vascular bundles are radial. The numbers of xylem and phloem bundles are four each and the root is termed tetrarch. The two types of bundles alternate with each other and are arranged in a ring. The protoxylem is exarch and includes annular and spiral vessels.
The metaxylem has reticulate and pitted vessels. The phloem includes sieve tissue, companion cells and phloem parenchyma. Opposite to each phloem bundle towards endodermis a patch of sclerenchyma is present.
7. Pith:
This is the central parenchyma which is very small in size. It is soon changed to thick-walled tissue.
(B) Aerial Root of Ficus:
The aerial roots of Ficus are located in a special type of environment. The structure of the root, hence, shows some special features. The young aerial roots are brown, hard and brittle and branch profusely. Kapil and Rastogi (1966) studied the anatomy of these roots in details.
There are given below:
1. Exodermis:
A few layers of parenchyma remain outside the epiblema in young roots and can be known as exodermis. These layers dry up after the secondary growth has initiated in the root.
2. Epiblema:
It is a single layer of parenchyma and is located below the exodermis.
3. Cortex:
It is many-layered parenchyma with chlorophyll bearing cells. The outer few layers together with epiblema cells are filled with tannin like substances.
4. Endodermis and Pericycle:
The endodermis generally is a single layer of cells which do not have characteristic thickenings. The pericycle is represented by a few layers of parenchyma.
5. Xylem Bundles:
These are six or more in number (a monocot-like character). These alternate with the phloem bundles.
6. Pith:
The pith is well evolved and is parenchymatous (cf. monocots).
7. Secondary Growth:
It is a typical dicot type. The epiblema, cortex and endodermis are soon eliminated as bark after the formation of cork cambium from the pericycle. The cork layers are also removed regularly as bark. Therefore, in the final stage, one or two cork layers are followed by cork cambium. Below the cork cambium is present secondary cortex.
There develop some stone cells in the cortex. The secondary phloem is visible next below the cortex. Below the phloem, cambium is seen as one or a few layers at different places. Secondary xylem is well evolved.
The xylem parenchyma is paratracheal and exists in the form of alternating bands of thin and thick walled cells. Primary xylem patches remain distinctly embedded within secondary xylem. Pith remains well marked even after secondary growth.
Although the aerial roots of Ficus do not vary much from the terrestrial roots, still a few differences are there as given below:
(C) Other Dicotyledonous Roots:
The structures of most of the roots are similar to that of Gram except for certain minor differences. In Cucurbita root, the pith is well differentiated and the sclerenchyma outside the phloem is lack. Tinospora has typical tetrarch root with well evolved secondary growth.
i. Origin of Lateral Root:
The lateral roots arise at certain distance from the promeristem at the periphery of vascular cylinder and are of endogenous origin i.e., arising from an inner layer, the pericycle. Such is true for all the cases irrespective of the fact whether the lateral root is arising from main root or a branch or adventitious root. Sometimes endodermis may also participate in the formation of lateral root.
The cells of the pericycle generally in the region of the protoxylem divide tangentially to form a few layers, which again divide and push the endodermis and grow into the cortex to form root primordium. As the activity of the pericycle continue the newly- produced cells ultimately rupture the endodermis and also the epidermis to come out.
The cells of endodermis also divide anticlinally and for some time cover the primordium. The cells of cortex are pushed aside or crushed or dissolved by enzymatic activity. In the meantime, three regions of the root apex such as, dermatogen (or calyptrogen), periblem and plerome are differentiated. Certain part of endodermis and cortex form the root cap, which is soon replaced by another root cap formed by the dermatogen.
When xylem and phloem begin to differentiate in the lateral roots, these tissues become associated with the equivalent tissues in the parent root by differentiation of the intervening parenchyma cells into vascular elements. First parenchyma cells derived from the divided pericycle are used for the purpose. Later cells beneath the pericycle can also participate in forming vascular connection.
The position of lateral roots is variable with respect to the position of vascular tissue. In diarch roots, it develops between xylem and phloem, in triarch and tetrarch against protoxylem and in polyarch outside the phloem.
ii. Adventitious Roots:
The adventitious roots also arise endogenously in a way similar to the origin of lateral roots. They arise in close proximity of vascular tissue. In young stems these arise in the inter fascicular parenchyma while in the old stem in the vascular ray near the cambium.
This region is also known as pericyclic region. In case of injury, the parenchyma cells of the callus, near a wound divide to form the adventitious root primordium similar to lateral roots in the pericyclic region.
iii. Transition from Root of Stem:
If is an established fact that the vascular tissue is continuous throughout the root and stem. It is also understood that the orientation of vascular bundles is different in the two parts. The protoxylem elements in the roots exist in peripheral positions in the vascular cylinder and the metaxylem develop in the center.
Thus, the direction of maturation of xylem elements is centripetal and the xylem is exarch. In the epicotyl region the protoxylem is placed towards the centre and metaxylem towards the periphery. The evolution of xylem being centrifugal and the xylem is endarch.
The bundles in roots are radial while in stem they are conjoint. There is a region in the ‘plant body known as transition region where the change from radial type to conjoint type of bundles takes place. This is known as the hypocotyle region. In this transition the metaxylem strands of root rotate through 180° around its longitudinal axis. Protoxylem, however, does not take part in rotation.
The rotation can take place in a number of ways, of which four important methods are listed below:
i. Mirabilis Type:
In such type, each xylem strand is forked by a radial division. The branches, one to the left and other to the right, turn through 180° and join the phloem strand of the respective sides. Therefore, in the primary structure of the stem, the number of bundles may be similar to the number of phloem bundles of the root, viz., Mirabilis and Fumaria.
ii. Lathyrus Type:
In this type, the xylem as such rotates by 180° and the phloem bundles are forked. The halves of the phloem bundles swing laterally to take the position and to join with xylem strand. Therefore, the number of bundles in young stem remains unchanged, viz., Lathyrus, Medicago and Phoenix.
iii. Cucurbita Type:
In this type, both the xylem and phloem strands get forked. The xylem strands turn through 180° as general and join one phloem strand each. Thus in the young primary stem the numerous bundles become double the number of phloem bundles of the root, viz., Cucurbita and Tropaeolum.
iv. Anemarrhena Type:
In this type, the half of the, xylem strands are forked and both, the divided and undivided strands, get inverted. The branches of the forked bundles swing laterally and join the entire strands. The phloem bundles also migrate in pairs (one from either side) to the position of xylem and fuse with it. Therefore, the number of bundles in young, primary stem becomes half the number of phloem bundles in root, viz., Aneniarrnena and certain more monocotyledons.
Scheirer and Hilson (1973) described a special case of transition in Helianthus annus. It has 4 phloem bundles of which only two lying on one side divide into two each. Of the xylem bundles also only two divide and therefore, six conjoint collateral bundles are produced after the rotation of xylem. In some cases the phloem bundles situated opposite to each other divide. The bundles so produced by further bifurcations form more bundles.
Bisalputra (1961) has visible that in Chenopodiaceae the vascular connection is between root and cotyledon and not between root and stem. The vascular strands of first foliage also evolve before the formation of cauline strands. Similar condition is also found in Yucca. Sporne (1974) based on these studies concluded that various patterns of arrangement of bundles in stem evolve due to the difference in their phyllotaxy whose vascular supply finally form the cauline vascular system (Fig. 9.13).
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