Read this essay to learn about the anatomy of selected roots:- 1. Gram Root 2. Pea Root 3. Ranunculus Root.
Essay # 1. Anatomical Study of Gram Root:
A transverse section of Cicer arietinum of subfamily Papilionaceae shows the following arrangement of tissues (Fig. 5.103):
(i) Epidermis:
Epidermis or epiblema or pili- ferous layer is uniseriate outer boundary layer consisting of tubular cells with cuticle on the outer walls. Some epiblema cells at a particular zone of the root protrude to form long unicellular root hairs. This zone is called root hair zone and located next to the root tip.
(ii) Cortex:
Next to epidermis there is a homogeneous massive parenchymatous zone called the cortex consisting of parenchyma cells with conspicuous intercellular spaces. The cells contain large amount of leucoplasts. Their main function is to translocate water and minerals absorbed by the root hairs to the conducting elements.
The innermost layer of cortex is the endodermis consisting of compactly arranged barrel-shaped cells forming a distinct cell layer surrounding the stele. The cells of this layer possess Casparian strips on their radial walls.
(iii) Stele:
The stele or the central core of tissue is precisely demarcated from the cortex by the endodermis. The stele is composed of vascular tissues and intrastelar ground tissues. Internal to the endodermis a layer of thin-walled parenchyma cells form the pericycle which is the zone of origin of branch roots.
The vascular bundles are radial with xylem and phloem being arranged on alternate radii. In-between the xylem and phloem small parenchymatous cells form the conjunctive tissue. The vascular bundle is tetrarch as four patches of xylem alternate with four patches of phloem.
Protoxylem vessels remain at the periphery and metaxylem at the centre, showing centripetal mode of differentiation from pro-cambium. This protoxylem arrangement is called exarch xylem of roots. This type of stele is called a protostele. The smaller phloem patches consist of sieve tubes, companion cells and phloem parenchyma. The outer part of the phloem is called protophloem and the inner part is the metaphloem.
A few sclerenchyma cells occur against every phloem patch. Pith is normally absent in dicotyledonous roots. During the early stages of development a few parenchyma cells remain located at the centre, which is later obliterated by development of metaxylem.
Essay # 2. Anatomical Study of Pea Root:
A transverse section of the root of Pisum sativum of subfamily Papilionaceae shows the following arrangement of tissues (Fig. 5.104):
(i) Epidermis:
The epidermis is the uniseriate outermost layer with typical unicellular root hairs.
(ii) Cortex:
The parenchymatous cortex with intercellular spaces. The innermost layer of the cortex is the endodermis with distinct Casparian strips.
(iii) Stele:
The stele is composed of the radial vascular bundles and intrastelar ground tissues. The vascular bundles in pea root are triarch with three patches of xylem alternating with three patches of phloem. Xylem is typically exarch with protoxylem towards the periphery and metaxylem towards the centre. The small phloem consists of sieve tubes, companion cells and phloem parenchyma.
A strip of sclerenchymatous fibres occur against every phloem group. A uniseriate parenchymatous pericycle layer exists next to endodermis. The xylem and phloem groups remain separated by small parenchymatous conjunctive tissues. Very small pith is found during the early stages of development and ultimately obliterated.
Essay # 3. Anatomical Study of Ranunculus Root:
A transverse section through the root of Ranunculus sp. of family Ranunculaceae exhibits the following arrangement of tissues (Fig. 5.105):
(i) Epidermis:
The single layered epidermis is composed of a row of living comparatively small tubular cells, but disorganised protoplast may be found in some cells of the layer.
(ii) Cortex:
The cortex is very massive encircling the stele. It consists of (i) a narrow outer zone next to epidermis, called exodermis, resembling the hypodermis of stems, and (ii) a broad inner zone occurring internal to exodermis. The exodermis is composed of compactly arranged smaller cells with scanty intercellular spaces.
The broad inner zone consists of larger starch containing parenchyma cells with conspicuous intercellular spaces. The innermost layer of cortex forms the endodermis. It is made of a row of barrel-shaped cells with Casparian strips and further secondary depositions. Only in certain cells of the layer facing against the protoxylem secondary deposition does not take place and their walls remain thin. These cells are called the passage cells.
(iii) Stele:
The central core of tissues forms the stele. The outermost boundary of the stele is the uniseriate parenchymatous pericycle. Vascular bundles are radially arranged. Four to five strands exarch xylem alternate with the same number of small patches of phloem strands. Parenchymatous conjunctive tissue separates the xylem and phloem.