The following points highlight the three main types of structures of root-apex according to plant groups. The types are: 1. Root Apex of Pteridophyte 2. Root Apex of Gymnosperm 3. Root Apex of Angiosperm.
Type # 1. Root Apex of Pteridophyte:
Most of the vascular cryptogams have a promeristem with a single apical cell. But some members of Marattiaceae exhibit a few initials in the root apex. The initials are arranged in one tier.
The root apex of pteridophyte is illustrated below taking the example of small aquatic fern Azolla (Fig. 7.17) because it has been studied in details due to the presence of a conspicuous apical cell, prominent cell wall of derivative(s) of apical cell and the lineage of any cell, except the root cap, can be traced to apical cell.
Moreover the meristem of Azolla root has genetically determined limit to its growth, i.e. root is determinate organ and this condition is in contrast to most roots where the activity of meristem is indeterminate —growth persisting as long as plant lives.
Azolla pinnata shows a large apical cell. The apical cell of the genus Azolla is four-sided. The side facing the root cap, i.e. the distal face is curved. The other three sides face the base of root and they are approximately triangular. The walls of basal three sides show numerous plasmodesmata. The apical cell is sparsely vacuolated in young roots.
With the increase of age of the root, the vacuolation also increases, but the size of the apical cell remains unchanged. The root cap is generated towards the distal curved face. The apical cell divides and the cell plate is formed parallel to the curved face.
The derivative cell again divides in the same plane resulting in two cell-layers. These two cell-layers then undergo numerous anticlinal divisions to produce the surface of the root cap. The root cap throughout its surface remains two cells thick only.
After the initial division to produce root cap, the apical cell divides sequentially from its three basal faces. The derivatives also divide and form a group of cells analogous to ‘packet’ of cells of shoot meristem. The initial divisions at the basal faces are parallel to surface.
The divisions in the derivatives are also parallel to surface. As a result a series of successive segments are formed. These segments are referred to as merophyte. A merophyte consists of derivative(s) of apical cell. The derivatives can be recognized as a group of cells by their thick walls and pattern of divisions.
The initial division to produce merophyte is parallel according to figure 7.17B. The divisions occur in precise sequences in a clockwise or counter clockwise manner. Later divisions may be at right angles to the original parallel divisions. The divisions to produce merophytes are distinct, species-specific and the patterns of cell divisions are predictable.
The figure 7.17C illustrates the production of merophytes and the formation of layers of root from them. The root cap is generated from the curved side of the four-sided apical cell and the three basal sides form epidermis, cortex and stele.
Type # 2. Root Apex of Gymnosperm:
Allen in 1947 illustrated the root apex of Pseudotsuga (Fig. 7.18B). The root apex consists of a central group of permanent initials with three groups of temporary initials. The temporary initials are generated by the central group of permanent initials on the peripheral side. The temporary initials are the mother cells that are destined to produce vascular cylinder, cortex and columella.
The first group of temporary initials is the meristem of vascular cylinder from which stele develops. The second group of temporary initials is meristem of cortex. As the name implies cortex is developed from this meristem. The peripheral layer of cortex is transformed into protoderm that gives rise to epidermis. The third group of temporary initials is the mother cells of columella (Fig. 7.18A) that produce columella.
In roots the term columella is used to designate the central part of a root cap where the cells are arranged in longitudinal files. Columella donates cells on the periphery of root cap by periclinal divisions. Wilcox in 1954 reports the presence of two groups of temporary initials in Abies procem. Among the two groups one group forms the central cylinder and the other forms the columella.
Type # 3. Root Apex of Angiosperm:
The root apex organization exhibits a central group of permanent initials with three groups of temporary initials. The temporary initials are the mother cells of protoderm, cortex, vascular cylinder and root cap, and they occur on the periphery of permanent initials. Guttenberg in 1960 (Fahn, 1997) regarded two types of root apex namely-closed and open (Fig. 7.19).
In closed type the temporary initials (i.e. the initial cells of stele, cortex, epidermis and root cap) are distinct and separate from each other. They remain close to central cells (=permanent initials). In closed type of root apex there are three groups of temporary initials and their derivatives differ between dicotyledon and monocotyledon.
In Brassica (dicotyledon) the first group of temporary initial forms meristem of vascular cylinder; other group of temporary initial produces meristem of cortex. Protoderm and root cap are generated from the third group of temporary initial. Protoderm develops into epidermis. The meristem (=temporary initial) that generates both epidermis and root cap is termed as dermatocalyptrogen.
Some members of Compositae, Solanaceae and Rosaceae etc. also exhibit dermatocalyptrogen. In Zea mays (monocotyledon) one group of temporary initial gives rise to meristem of vascular cylinder. The other group of temporary initial forms protoderm and meristem of cortex. Root cap is generated from the third group of temporary initial.
The meristem (temporary initial) that generates root cap only is termed as calyptrogen by Janczewski in 1874. Calyptrogen is also exhibited in some members of Zingiberaceae and Arecaceae etc. In open type of root apex the initials of epidermis, cortex and root cap are not distinctly delimited.
All tissues of root (e.g. Allium) or all except the central cylinder (e.g. Helianthus) appear to be generated from a common meristematic group of cells that are situated on the periphery of central group of cells. Later the meristems of different tissue systems become discrete and occur some distance away from the central cells.
Eames and MacDaniels illustrated the root apices of vascular plants (Fig. 7.20) in the following ways. They interpreted the root apex according to the terms of histogen theory (dermatogen, periblem and plerome) though the theory was abandoned in case of shoot apex.
Diagram illustrating the types of root apex of vascular plants.
Vascular cryptogams have a solitary apical cell in the root apex. Root cap, epidermis, cortex and vascular cylinder are generated by the single apical cell, e.g. some species of Selaginella, ferns and Equisetum etc. In the above species the apical cells are similar and large. The root cap is structurally distinct from the other tissues of root and its origin can be traced back to the apical cell.
The root apex of gymnosperm consists of two groups of initials. Plerome is developed from the inner initials. The outer initials give rise to periblem and root cap. There exists no clear line of demarcation between plerome and periblem. The distal region of periblem proliferates to produce root cap. Dermatogen originates from periblem behind the apex where the periblem and cap adjoin.
The root apex of angiosperm differs in dicotyledon and monocotyledon. In dicotyledon the root apex exhibits three groups of initials. The innermost initials are the plerome. The median group of initials develops into periblem. Dermatogen and root cap are generated from the distal group of initials.
It is to note that root cap and dermatogen have common origin. Like dicots, monocots also exhibit three groups of initials among which the innermost develops into plerome. The median initials give rise to periblem and dermatogen. The root cap is generated independently from the distal group of initials. This is in contrast to the dicots.
Newman (1965) illustrated the following five types of root apex (Fig. 7.21) based on the concept of Eames and MacDaniels (1947) and Esau (1965) with modifications. Newman did not propose any terminology and the types are numbered for convenience only without any significance.
Type 1:
It has one initiating layer consisting one solitary apical cell. Root cap and all tissues of root are generated from the single initiating layer. Ex. Ranales, Amentiferae, Pisum and Allium etc.
Type 2:
It has two initiating layers. The first or distal initiating layer forms epidermis, cortex and root cap. Vascular cylinder is generated by the second or inner initiating layer. Ex. gymnosperm, dicot and monocot.
Type 3:
It has two initiating layers. The first or distal initiating layer forms epidermis, outer cortex and root cap. Inner cortex and vascular cylinder are generated by the second or inner initiating layer. Ex. Tiliaceae, Umbellales etc.
Type 4:
It has three initiating layers. The first or distal initiating layer produces epidermis and root cap. The second or median initiating layer forms cortex. Vascular cylinder is generated from the third or innermost initiating layer. Ex. Rosaceae, Solanaceae and Compositae etc.
Type 5:
It has three initiating layers. The first or distal initiating layer-produces root cap only. The second or median initiating layer gives rise to cortex and epidermis. Vascular cylinder is generated from the third or innermost initiating layer. Example: Monocotyledon.
Type 1 represents the simplex apex and the types 2, 3, 4 and 5 exhibit duplex apexes.