Internal Structure of Root (With Diagrams)

The below mentioned article provides an outline of the internal structure of root.

The root develops from the radicle of the embryo. Due to the fact that the extreme tip of the root remains covered by a cap, the apical meristem here is subterminal, as opposed to the terminal apical meristem of the stem.

However, the primary body con­sists of three tissues systems; and, in fact, boundaries between the tissue systems are more precise here.

The outstanding characters by which root differs from the stem are the fol­lowing the epidermis in roots is usually uniseriate, composed of thin-walled cells. Cutinisation of outer walls and cuticle are absent. Hence the terms epiblema, piliferous layer and rhizodermis have been proposed for the epidermis of root.

The stomata, so characteristic in aerial organs, are absent. Formation of root hairs, which are prolongations of epidermal cells themselves, is confined to a particular zone of the root. The cortex is comparatively more simple and homogeneous. It is made of mainly parenchymatous cells and is often massive for the purpose of storage.

The endodermis, the last layer of cortex with charac­teristic Casparian thickenings, is of universal occurrence. Pericycle, normally uniseriate and rarely multiseriate, invariably occurs next to the endodermis. The vascular cylinder is more compact, firstly due to absence of gaps, and secondly, due to presence of endoder­mis and pericycle.

The complex tissues, xylem and phloem, occur as separate patches showing radial arrangement. Xylem is always exarch, due to centripetal mode of diff­erentiation from the procambium, so protoxylem occurs towards circumference and metaxylem towards the centre.

According to the number of protoxylem groups roots may be monarch, as in Trapa natans, diarch, as in Lycopersicon, Nicotiana; triarch, as in Pisum; tetrach, as in Vicia, Cicer; pentarch, as in Ranunculus. Polyarch condition with many xylem groups is characters of monocotyledons.

In dicotyledonous roots Xylem plates usually join at the centre forming a solid core. Hence the stele is regarded as a protostele (Fig. 600). Endogenous development of the branch roots from the pericycle, as opposed to exogen­ous formation of a branch from the growing point of a stem, is a marked feature.

A few typical roots are selected for the study of internal structures.

Dicotyledonous Roots:

1. Root of Gram:

A transverse section of the root of gram (Cicer arietinum of subfamily Papilionaceae) is taken and stained suitably for the study of internal structure.

It shows the following plan of arrangement of tissues from the epi­dermis to the centre of the organ (Fig. 601):

I. Epidermis:

Epidermis, also known as epiblema or piliferous layer, is typically uniseriate outermost zone consisting of tabular living cells. Cuti­cle on the outer walls and stomata are absent. Some epidermal cells prolong to form the typically unicellular root hairs, which occur at a particular zone of the root, referred to as root hair zone, located just above the region of active growth and elongation.

II. Cortex:

It is relatively more simple and homogeneous, forming a massive zone which consists of unspecialised parenchyma cells with cons­picuous intercellular spaces. The cells are living and possess abundant leucoplasts.

They are particularly concern­ed with storage of food, though at the early stage they are responsible for translocation of water and solutes to the conducting elements. The last layer of cortex is endodermis.

It is of uni­versal occurrence in roots and consists of compactly arranged barrel-shaped cells forming a distinct zone surround­ing the stele. The endodermal cells possess Casparian thickenings on the radial walls.

III. Stele:

The stele or central cylinder is precisely demarcated from the cortex due to presence of endodermis. It includes the vascular tissues and intrastelar ground tissues.

Next to endodermis there lies a layer of thin-walled parenchyma cells forming the pericycle. This non-vascular tissue occurring inside the stele is the seat of origin of branch roots.

The vascular bundles are radial. Xylem and phloem occur in separate patches arranged on alternate radii, intervened by small parenchyma cells. The latter form the conjunctive tissue.

The bundle is tetrarch here, because four patches of Xylem alternate with equal number of patches of phloem. Protoxylem vessels occur towards periphery and metaxylem towards centre, thus showing centripetal mode of differentiation from pro- cambium.

This is the typical exarch xylem of roots. The central portion is usually occu­pied by a metaxylem vessel, so that all the plates of Xylem are joined forming a solid core. Hence it is regarded as a protostele. Phloem patches are rather small and consists of sieve tubes, companion cells and phloem parenchyma.

The outer part of this tissue lying next to pericycle is protophloem and the inner is the metaphloem; of course they are not readily distinguishable like protoxylem and metaxylem. A few sclerenchyma cells occur against every phloem patch. Pith is normally absent in dicotyledonous roots. At early stages a few parenchyma cells may be located at the central portion, which is very soon obliterated by development of metaxylem.

2. Root of Pea:

A transverse section of the root of pea (Pisum sativum of subfamily Papilionaceae) is taken and stained suitably for the study of internal structure.

It shows the same plan of arrangement of tissues as found in gram root from the periphery to the centre (Fig. 602):

I. Epipermis:

It is the uniseriate outermost layer with typical root hairs.

II. Cortex:

Cortex is parenchymatous with intercellular spaces. The last layer is endodermis with distinct Casparian thickenings.

III. Stele:

It includes the vascular ele­ments and intrastelar ground tissues. The bundles are radial. Here it is triarch, three patches of xylem alternating with three patches of phloem. Xylem is typically exarch having metaxylem towards centre and pro­toxylem towards periphery.

Phloem is rather small with constituent elements— sieve tubes, companion cells and phloem parenchyma. A few fibres occur against every phloem group. Pericycle lying next to endodermis is single-layered and paren­chymatous. Small parenchymatous con­junctive tissues occur between Xylem and phloem groups. A very small pith is noticed at early stage which is obliterated later.

3. Root of Buttercup:

A transverse section through the root of buttercup (Ranunculus sp. of family Ranun- culaceae) would exhibit the following plan of arrangement of tissues (Fig. 603):

I. Epidermis:

It is as usual a unise­riate zone, composed of a row of living tabular cells. It remains more or less in a collapsed condition and disorganised proto­plast may be noticed in some cells.

II. Cortex:

It is quite massive, as in all roots, enveloping the stele. A narrow zone, called exodermis, corresponding to the hypodermis of stems, occurs next to the epider­mis. It is composed of comparatively -epidermis smaller cells rather compactly arranged with very scanty intercellular spaces. Exodermis may be called the outer zone of cortex.

So-called inner zone occurs internal to exodermis. It consists of quite a good number of larger parenchyma cells with conspi­cuous intercellular spaces. The corti­cal parenchyma cells contain abun­dant starch grains. The limiting layer of cortex is known as endodermis.

It is made of a row of barrel-shaped cells with Casparian thickenings. Secondary depositions on the endo-dermal walls are also noticed. In that case some thin-walled cells, in which secondary deposition has not taken place, often occur, particularly against the protoxylem groups. These are called the passage cells.

III. Stele:

The outermost por­tion of the stele is the uniseriate parenchymatous pericycle. Vascu­lar bundles are radially arranged. Four or five strands of Xylem alternate with equal number of patches of phloem. Xylem is typically exarch. Phloem forms small patches. Paren­chymatous conjunctive tissues occur between xylem and phloem. Pith is absent.

Monocotyledonous Roots:

1. Root of Arum:

A transverse section through the root of arum (Colocasia sp. of family Araceae) would reveal the following anatomical structure (Fig. 604):

I. Epidermis:

It is uniseriate, composed of a row of tabular cells attached end on end without having intercellular spaces.

II. Cortex:

The cortex is quite massive, as in other roots, and mainly consists of un-specialised parenchyma with profuse schizogenously formed spaces. In a slightly old root a few layers of cortex next to epiblema undergo chemical changes—suberisation, and thus give rise to a zone meant for protecting the internal tissues. This band is known as exo­dermis.

Formation of exodermis may be initiated before the epiblema loses its function, but once epiblema is decayed exodermis takes over the function of protection. The last
layer of cortex is the endodermis. It is composed of barrel-shaped compactly-set cells with conspicuous Gasparian strips.

Due Jo secondary thickening the endodermal cells may have considerably thick radial and inner walls. In that case some thin-walled cells usually occur against the protoxylem groups; obviously they are meant for ready diffu­sion of fluids. These are known as passage cells or transfusion cells.

III. Stele:

The central cylinder consists of radially, arranged vascular strands and intrastelar ground tissues. Uniseriate pericycle, made of thin-walled parenchyma cells, occurs next to endodermis. Xylem and phloem remain arranged alternately as separate patches, the xylem being typically exarch.

As a good number of vascular strands are pre­sent, as opposed to the limited number of dicotyledonous roots (triarch, tetrarch, etc.), this is referred to as polyarch. Phloem is composed of sieve tubes, companion cells and parenchyma. Though protophloem occurs on the outer side and metaphloem on the inner, the two can hardly be distinguished.

Small conjunctive parenchyma cells are pre­sent between xylem-and phloem patches. The central part of the stele is occupied by a fairly large pith.

2. Root of Maize:

The internal structure (Fig. 605) of the maize root (Zea mays of family Graminaceae) is more or less similar to that of arum, so far as epiblema and cortex are concerned. Formation of exodermis is common in almost all monocotyledonous roots. The endo­dermis is composed of thick-walled cells; in fact, secondary and tertiary layers are deposited, so that the Casparian strips are no longer recognisable.

The pericycle is uniseriate, but unlike that of the previous one, it is partly scle- renchymatous here. Vascular bundles are as usual polyarch with a pretty good number of Xylem and phloem strands.

Parenchyma cells associated with xylem undergo sclerosis and thus become thick-walled. The central portion is occupied by a large pith—made of loosely-arranged parenchyma cells containing abundant starch grains.

3. Root of Smilax:

It (Smilax sp. of family Liliaceae) is a herbaceous monocotyledon, the roots of which are quite suitable for the studies of internal structure. Though in general it (Fig. 606) shows the same plan as found in other monocotyledonous roots, but some distinctive features are to be noted.

Epidermis is as usual uniseriate—made of parenchyma cells with rounded outer walls. A single row of heavily thick-walled cells occur just internal to epidermis, forming the exodermis.

The rest of the cortex is composed of thin-walled parenchyma cells with distinct intercellular spaces. Starch grains are abundantly present in the cortical cells. Endodermis is of thick-walled type, where radial and inner walls in particular undergo considerable secondary thickening.

The stele consists of a large number of xylem and phloem strands arranged alter­nately, so it is also polyarch. The pericycle, unlike that of other roots, is multiseriate and consists of a few layers of thick-walled sclerenchyma cells. Protophloem occurring on the outerside are smaller than the metaphloem elements. Pith occupying the central portion of the stele is fairly large, and is made of thick-walled parenchyma.

4. Root of Orchid:

The roots of epiphytic orchids (Vanda spp. of family Orchidaceae) possess a spongy outer tissue for absorbing moisture from the atmosphere. This tissue is known as velamen (Fig. 607). It consists of a few layers of compactly-set dead cells, which often form a silvery outer coat.

The walls are usually porous, so that the cells work like a sponge. These empty cells have walls variously thickened by spirally or reticulately arranged fibres which take up the form of supporting ribs (Fig. 607B). During dry weather the cells remain filled
with air, and during rains they quickly absorb water.

Special structures, called pneumato­ses, consisting of groups of cells with dense spiral wall thickening are present. They are helpful in gaseous exchange when the roots are saturated with water. Velamen is deriva­tive of protoderm, and hence may be interpreted as a typical multiseriate epidermis, specially adapted to serve as an absorbing tissue. The outermost layer of velamen is known as limiting layer.

Cortex:

The outermost layer of cortex consists of a row of thick-walled cells forming what is known as exodermis. The thickenings due to deposition of suberin are more pronounced on the outer and lateral walls of the cells. Unthickened ones, called passage cells, occur here and there, which may serve as channels for flow of water absorbed by the velamen.

The main bulk of the cortex occurs internal to exodermis. It is composed of a few layers of parenchyma cells with intercellular spaces. Chloroplasts are present in these cells; this fact explains the greenness of the roots, particularly when wet. The last layer of cortex is the endodermis with suberised radial and inner walls. Passage cells occur in the endodermis, usually opposite the protoxylem vessels.

Stele:

Pericycle is uniseriate, made of thick-walled cells; only the cells just lying internal to passage cells of the endodermis are thin-walled. A good number of xylem and phloem groups occur alternating in the stele. Conjunctive tissues sin-rounding the phloem groups are sclerenchymatous. Central portion is occupied normally by the parenchymatous pith, but these cells may undergo sclerosis.

Breathing Roots (Pneumatophores):

These roots are found in the plants growing in situations with scanty oxygen. Unlike the normal roots, they come vertically upwards—thus becoming negatively geotropic and negatively hydrotropic. They absorb oxygen from the outer atmosphere through spe­cially located lenticels at the tips. Thus strictly primary condition can hardly be noted. The anatomical structure resembles that of a stem in the nature and disposition of the vascular system in particular.

A transverse section would show the following arrange­ment of tissues (Fig. 608):

I. Cork:

A few layers of cork cells occur at the outermost portion with small lenti­cels which are really instrumental for absorption of oxygen.

II. Cortex:

It is quite massive, composed of a few layers of more or less rounded parenchyma cells with well-developed intercellular space system. The last layer of cortex is as usual the endodermis—a uniseriate zone made of small barrel-shaped cells.

III. Stele:

Just internal to endodermis occurs pericycle made of parenchyma and sclerenchyma. Usually the outer portion is parenchymatous and the inner sclerenchyma­tous. The vascular bundles actually resemble those of the dicotyledonous stems.

They are collateral and open. In fact, the bundles form a continuous cylinder with xylem and college botany phloem, the cambium occurring between them. The Xylem is endarch. Large pith com­posed of parenchyma cells occurs at the central portion.

Origin of Lateral Roots:

The lateral roots are endogenous in origin. They develop from mature cells, some distance away from the apical meris­tem (Fig. 609) usually behind the root hair zone. The apical meristem of the root does not lay down any appendage. It is a marked point of contrast with the shoot where the primordia of the leaves and branches develop in the apical meristem.

In higher plants, angiosperms and gymnosperms, the development of lateral root is commonly initiated in the pericycle—an intrastelar ground tissue of the parent root, and the lateral root ultimately makes its way by piercing the cortex and epidermis. In lower vascular plants the branch roots originate in the endodermis.

The method of formation of a lateral root in a higher plant is as follows (Fig. 610). A few mature cells of pericy­cle, usually opposite a protoxylem group, become meristematic and go on dividing periclinally and anticlinally. Thus a number of cells are produced, which form something like a protrusion. This is really the primordium of the lateral root. It soon takes the shape of a growing point with its initial cells, the cap and other histogens.

With gradual develop­ment of the primordium other tissues surrounding it get stretched and ultimately ruptured. That is how it eventually comes out piercing all the tissues. As regards the mecha­nism of growth of the lateral root some workers have sug­gested that it partially digests the cortical tissue during its advance; whereas others are of opinion that it is entirely a matter of mechanical penetration.

It has been reported in some cases that the endodermis also undergoes anticlinal division and forms a layer surrounding the lateral root primordium; and by further periclinal division it may be even more than one layer in thickness. But at any rate those cells die and are shed when the lateral root comes out.

The lateral roots develop from the mature tissues in acropetal order, though there is no regularity in the order of develop­ment with reference to each other. In roots having more than two xylem strands the lateral roots commonly originate against the protoxylem groups, or less commonly opposite the phloem groups.

Thus they come out in vertical rows, the number being equal to that of the xylem strands present. But in diarch roots having two xylem strands, the primordia develop at each side of the phloem group, so that the number of lateral roots formed is double that of xylem strands.

They exhibit all the characters of the primary root, having four distinct regions. Thus an extensive root system is formed which ramifies through the soil particles. All the lateral roots do not grow equally vigorously. In fact, many of them continue normal growth and form the root system, whereas some remain undeveloped, or may even be lost. In some fleshy roots like those of carrot additional lateral roots may arise at the bases of original ones when the latter perish.

Formation of Adventitious Roots:

Adventitious roots are of diverse types. They may arise either in association with buds or independently; either in the young organs or in the comparatively mature tissues which have retained the potentialities of cell division. They usually originate endogenously from the primordium already formed and lying in dormant con­dition, or they may form new primordium. Exogenous development cannot be ruled out. Primordia of adventitious roots may be formed from epidermis with cortical tissues, different internal regions, even from the tissues of leaf margins and petioles, e.g. Begonia, Kalanchoe.

Adventitious roots from the stems are the most common ones (Fig. 611). It has been established that initiation of such roots takes place near about the differentiating vascular tissues of the organ by a group of cells forming the primordium near the periphery of vascular system in case of young stem, and near the vascular cambium in case of mature stem.

The cells forming the primordium are derived from inter- fascicular parenchyma, medullary rays, or from vascular rays. It is often stated that the pericycle is the seat of origin of adventitious roots, but the —stages. Very existence of pericycle in a stem has been rather doubtful.

Because of the origin of adventitious root from the tissues stated above, the deve­loping root lies close to the xylem and phloem of the stem; and thus vascular connection

between the two organs is easily established. It is similar to a lateral root as regards the organisation of the growing point, formation of cortex, etc., and mechanism of growth.

Root-Stem Transition:

It has been stated that the vascular skeleton in a plant is form­ed due to continuity of root-stem axis and the lateral appendages. The epidermal tissues and ground tissues are directly continuous in the two organs, stem and root.

But the arrangement of primary vascular tissues is distinctly different in the two organs, roots having radial vascular bundles with exarch xylem and stems usually having collateral bundles with endarch xylem.

A region actually exists where changes and adjustments take place, so that the two markedly different types of vascular tissues ultimately become con­tinuous. The change involving inversion or twisting of xylem strands from one type of structure to another is referred to as vascular transition, and the region of the axis where changes occur is called transition region.

This region is usually short, changes may take place gradually or rather abruptly at the top of the radicle and more commonly in hypocotyl—at its base, centre or upper part. The structure of the region often becomes more complex due to origin and departure of cotyledonary traces. The stele may enlarge in diameter in the transition region. The changes occur according to some plans. An account of a few types that have been studied is given here (Fig. 612).

I. In this type xylem strands fork by radial division and the two branches formed swing laterally, one to the right and one to the left by 180°, and join the phloem strands. The latter have remained unchanged all through, and run as straight strands from the root to the stem. That is how the radial bundles with exarch xylem become collateral ones with xylem endarch.

The number of primary bundles in the stem here is equal to the phloem groups present in the root. This type of vascular transition has been noted in Mirabilis of family Nyctaginaceae, Fumaria of family Fumariaceae, etc.

II. The second type, what is really more common than the first one, involves forking in both xylem and phloem strands. The phloem branches remain in same position, whereas the branches of xylem strands swing laterally, as in the first type, and ultimately join up with the phloem strands.

Thus the number of bundles in the stem is twice that of phloem groups present in the root. This type has been noticed in Cucurbita of family Cucurbitaceae, Acer and Phaseolus of family Leguminosae, Tropaeolum of family Geraniaceae, etc.

III. In this type xylem strands do not fork, but while passing upwards they swing laterally by 180°. The phloem strands divide; the branches swing and eventually join up with xylem strands.

So the number of bundles in the stem is equal to that of the phloem groups of the root. This type occurs in Lathyrus, Medicago of family Legumi­nosae and Phoenix of family Palmae.

IV. Here half of the Xylem strands fork and swing, whereas the other half do not divide, but become inverted. The phloem strands do not undergo any division, but simply fuse. In the meantime the triple xylem strands—the branches of one strand and an unbranched inverted one, join up with the phloem strands which have united in pairs.

Thus, the bundle in the stem is the product of fusion of five strands and the number of bundles is half that of the phloem groups present in the root. This type, though of rare occurrence, has been found in some monocotyledons like Anemarrhena of family Liliaceae.

Some authors are, however, of opinion that this approach towards interpretation of transition region between the stem and the root is not happy. According to them “transi­tion region represents connection not between two axial organs with somewhat different arrangement of tissues but between an organ with an axial vascular cylinder and one whose vascular system develops in relation to leaves”.—Esau.

In that case transition region should really explain the relations between the roots and the traces of the first- formed foliar organs of plants. It has been worked out in some cases that inversion of the xylem strands does not occur.

In carrot the cotyledon has three traces—of which the median one consists of exarch xylem flanked by two lateral phloem groups, whereas the two lateral traces are collateral with outer phloem and inner endarch xylem. That shows the continuity between the vascular system of the cotyledon and root without inversion of the xylem strand.