Let us learn about Anatomical Structure of Epiphytic Roots and Storage Roots.
Anatomy of Epiphytic Root:
The orchids (of family Orchidaceae-monocotyledons) are epiphytes. They possess the aerial roots hanging in the air. The anatomy of the root of Dendrobium (an orchid) is given here.
Velamen:
The velamen consists of several layers of dead cells often with spirally thickened and perforated walls, which act as sponge, soaking up water that runs over it. The velamen is multiple epidermis. The velamen is thought to function as a protective tissue, preventing undue water loss from the delicate cortical cells of the exposed aerial root.
Exodermis:
It is the outermost layer of the cortex. This layer is composed of alternating long and short cells; the long cells become thick-walled on their radial and outer tangential surface, but the small cells remain thin-walled and are called passage cells.
Cortex:
The main cortex consists of thin-walled parenchyma cells having intercellular spaces among them. The innermost layer of the cortex is endodermis that consists of compact barrel-shaped cells having starch grains in them. The endodermis completely encircles the stele.
Pericycle:
Immediately beneath the endodermis a single-layered pericycle is found. The cells are thin-walled.
Vascular system:
The vascular bundles are radial, i.e., the xylem and phloem strands are equal in number and arranged alternately. The bundles are more than six (i.e., polyarch condition). The xylem is exarch. The protoxylem poles are found towards periphery and metaxylem towards centre.
The vessels of the proto-xylem are narrow and possessing annular and spiral thickenings whereas the vessels of metaxylem are broad and possessing reticulate and pitted thickenings on their walls. The phloem bundles are made up of sieve tubes, companion cells and phloem parenchyma.
The conjunctive tissue is represented by the presence of parenchyma cells in between and around the vascular bundle.
Anatomy of Storage Roots:
The underground roots may become very much thickened and serve as organs for the storage of food. Such is the case in sweet potatoes, radishes, turnips, carrots and dahlias. In such roots the food may be stored largely in the cortex or xylem region or in both.
In turnips food is stored largely in the xylem, and the phloem and cortex are relatively narrow. In the radish and sweet potato the xylem is also the chief region of food storage, but food is also stored outside the xylem. In the carrot there is a more even distribution between xylem and bark.
In beets there are alternate layers of xylem and phloem owing to the formation of successive cambia. The secondary tissues of the root accumulate starch in the same kind of cells as those of the stem, that is various parenchymatous and some sclerenchymatous cells of the xylem and the phloem. In general, roots possess a higher proportion of parenchyma cells than do stems.
Daucus Carota. Carrot (Umbelliferae-Dicot):
In this case, the hypocotyl and base of taproot form jointly one fleshy structure. Here, the fleshy organ has large amount of storage parenchyma associated with ordinary arrangement of tissues.
In this type of development, where the hypocotyl and the upper part of the tap root, after sloughing off the cortex, become fleshy through a massive development of parenchyma in the phloem and the activity the massive parenchyma, that makes the storage tissue, adds to the thickness of the root.
Raphanus Sativus. Radish (Cruciferae- Dicot):
The fleshy roots of radish show a proliferation of parenchyma in the pith and in the secondary xylem, and a differentiation of concentric vascular bundles within this parenchyma. Here, the fleshy roots show a diarch primary xylem.
The normal cambium cuts off secondary phloem towards periphery and secondary xylem towards the centre. Several concentric vascular bundles are seen in the transverse section of the fleshy root. The concentric bundles are composed of secondary cambial rings with a few vascular elements in the centre.
Ipomoea Batatas. Sweet Potato (ConvoIvulaceae-Dicot):
It exhibits a complicated type of anomalous secondary thickening. In primary state the root is pentarch or hexarch. The cortex is delimited by a single-layered distinct endodermis from the stelar region.
In the normally developed but highly parenchymatous primary and secondary xylem, anomalous cambia arise around individual vessels or vessel groups and produce phloem rich in parenchyma and with some laticifers away from the vessels, and tracheary elements toward them. Massive amounts of storage parenchyma cells are developed in both the directions, thus forming the tuberous roots.
Beta Vulgaris. Beetroot (Chenopodiaceae-Dicot):
The anatomy of beet root has been described in detail by E.F. Artschwager (1924, 1926). The young beetroot possesses a diarch proto-xylem plate. The sugar beet forms its fleshy hypocotyl root organ by anomalous growth. It shows a useful type of primary and early secondary development.
The primary cambium that gives rise to the innermost vascular ring in the beet root develops in the interstitial parenchyma except opposite the two protoxylem poles where it is derived from the pericycle. The first secondary cambium in the root and lower part of the hypocotyl arises in the phloem parenchyma, whereas in the upper part of the hypocotyl is derived from the pericycle.
Later, however, a series of supernumerary cambia arise outside the normal vascular cylinder and produce several increments of vascular tissue, each consisting of a layer of parenchyma, parenchymatous xylem and parenchymatous phloem.
Practically all the supernumerary cambia that give rise to the vascular rings in the mature root have already been developed when the diameter of the young root is no greater than five millimetres or so. All the cambia are active at the same time.
