Groktilar Chimaera, also spelt as chimera, refers a plant or plant organ having tissues of varied genetic origin lying adjacent to one another. The treatment temporarily blocks mitosis. It is thought that plasmodesma controls the gene theoyr that leads to the formation of protoderm, ground meristem and provascular tissue. Monocotyledons have one to four layers of cells in tunica; one and two layered tunica predominates in it. The following points highlight the top three theories of shoot apical meristem. Comments on histogen theory What made you want to look up histogen theory?
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The theories are: 1. Apical Cell Theory 2. Histogen Theory 3. Tunica-Corpus Theory. Apical Cell Theory: Nageli in advocated this theory. The cell is very large and is shaped like an inverted pyramid. The apical cell is tetrahedral in shape and has three or four cutting faces among which single face is directed upward and the rest points downward. The side of apical cell that is directed upward is triangular or square in shape and forms a part of the outer surface of the shoot apex Fig.
The cutting faces of apical cell divide in an orderly fashion that is in helical succession. The cell divides by an asymmetric division; as a result a narrow and flat cell is formed. The next division of the apical cell is also asymmetric. This type of asymmetric division is repeated in the downwardly pointed faces of the apical cell. As a result all cutting faces have their daughter cells.
The daughter cells also divide and form large packet of cells. The packet of cells differentiates and forms different segments of shoot. A single apical cell composing an apical meristem is present in vascular cryptogams. After the discovery of solitary apical cell in vascular cryptogams, it was supposed that such apical meristem might exist in higher plants as well. Later extensive investigations refuted the universal occurrence of solitary apical cell in a meristem.
In higher plants the apical cell theory was replaced by the concept that the different parts of a plant body have independent origin. So the apical cell theory was later superseded by histogen theory. According to this theory the tissues of a plant body originate from a mass of meristem where the following three histogens can be distinguished Fig.
It is the outermost layer of the meristem. It gives rise to epidermises of root and stem. This region occurs internal to dermatogen but peripheral to plerome. This histogen is destined to form cortex of root and shoot and inner tissues of leaves. It surrounds plerome. This region gives rise to vascular cylinder of stem and root including pith. It is the central core of stem and root and the cells composing this zone are very irregular.
This region is enveloped by a variable number of mantle-like layers which are represented by dermatogen and periblem. Later investigations reveal that the sub-divisions — dermatogen, periblem and plerome have no universal application due to the following two reasons: i In gymnosperm and angiosperm there exists no clear distinction between periblem and plerome. The histogens — dermatogen, periblem and plerome are committal and respectfully give rise to epidermis, cortex and stele.
Later this theory was superseded by tunica- corpus theory because the zones are noncommittal. It is to note that Hanstein visualized this long before when the concept of gene and DNA was established. The present-day concept is that the different zones in the shoot apex are fundamentally same and capable of producing all tissues. It is the position in the meristem that determines the destiny of derivatives.
This is due to the fact that in zones some genes are activated and others are repressed resulting in the production of different tissues. Tunica-Corpus Theory: Schmidt in postulated tunica- corpus theory on the basis of studies of shoot apices of angiosperm.
This theory is concerned with planes of cell division in the apex. In contrast to apical cell theory and histogen theory tunica-corpus theory is applicable only to shoot apex and not to root. Schmidt distinguishes two tissue zones in the shoot apex and termed them as tunica and corpus.
Researchers designate the layers as L1, L2 and L3 to denote respectively outer layer of tunica, inner layer of tunica and corpus. Plasmodesmata exist between the cells of tunica and corpus. It is thought that plasmodesma controls the gene expression that leads to the formation of protoderm, ground meristem and provascular tissue.
It consists of one or more peripheral layers of cells. Dicotyledons exhibit one to five layers of cells in tunica; two layers of cells are represented by largest number of species. Monocotyledons have one to four layers of cells in tunica; one and two layered tunica predominates in it.
One single layered tunica is termed as monostratose. Many layered tunica is termed as multistratose. Xanthorrhoea media shows eighteen layered tunica. Tunica is characterized in having anticlinal plane of cell division, i. This division reflects to the surface growth in the apex.
As a result tunica grows as a sheet but not in thickness. To increase in thickness a tunica cell will have to undergo cell division with periclinal wall, i. Normally, periclinal division does not occur in tunica where anticlinal division predominates except at the point of origin of leaf primordium and axillary bud. In multistratose tunica each layer arises from separate initials present in the shoot apex. The independence regarding origin among different layers of tunica and between tunica and corpus is strongly supported with the results of investigation by the use of periclinal chimeras.
Such chimera in a plant body exhibits layers with different chromosome numbers. Such differences can be traced from promeristem through continuous cell lineages to similar differences in the derivatives of promeristem. Promeristem may be induced to develop cytochimeras by treating the shoot apex with colchicine. Using this technique it was determined that outermost layer of tunica gives rise to epidermis whereas the inner layers donate cells to form other tissues.
In some plant the meristematic region consists of at least two populations of cells that differ in their chromosome numbers. The chimera of this plant is recognizable only at the cytological level and termed as cytochimera.
Usually two types of chimera are recognized: 1 Sectoral or mericlinal chimera — where the two populations of cells are arranged in sectors on the circumference around the axis of the meristem. L1, that is the outer layer of tunica, is commonly destined to form the epidermis of all the leaves and stem formed by the shoot and it is revealed on analyzing the periclinal chimera.
Shoot apices are induced to form cytochimeras to study the function of L1, L2 and L3 layers. Treating the shoot apex with colchicine does this.
The alkaloid colchicine, a chemical mutagen, is obtained from the plant Colchicum autumnale. Colchicine has the property to disrupt the spindle microtubules formed during mitosis. The drug colchicine is applied to the shoot tips by slow trickling or as paint.
The treatment temporarily blocks mitosis. The cells that are in metaphase stage cannot complete chromosome movement due to the failure of spindle mechanism but the other events of mitosis do continue.
As a result nuclei are formed with tetraploid chromosome numbers. Then colchicine was washed out and the meristems were allowed to continue cell division. Staining properties of nucleus can recognize the descendents of tetraploid cell.
Using this technique it becomes possible to find that the LI layer, i. In stems LI gives rise to epidermis and the rest forms the cortex and vascular tissue. In Datura the shoot apices were experimentally induced to develop cytochimera. The apices exhibited that the cells of LI, L2 and L3 layers had nuclei with chromosome number respectfully 8n, 2n and 2n; 4n, 2n and 4n, and 2n, 8n and 4n].
It is the anticlinal plane of cell division that characterizes tunica. In maize, certain grasses like Agropyron repens and Chlorogalum pomeridianum Liliaceae etc. One view regards that the tunica should include only those layer s that exhibit- anticlinal plane of division. If any inner layer s of tunica show periclinal division they are to be assigned to corpus.
Other workers treat tunica in a very loose sense and regard that the inner layer s may divide periclinally. So to avoid discrepancies the term mantle was proposed in a loose sense instead of tunica. Corpus was replaced by the term core.
Mantle overarches core. Popham and Chan advocated the mantle and core concept in the shoot apex and they did not take into account to the planes of cell division. Cytologically two zones are recognized in the tunica though all cells exhibit the same anticlinal cell division.
The first zone is central apical zone and the second occurs between the central apical zone and leaf primordium. The central apical zone consists of one or few initial cells that are larger and contain large nuclei and vacuoles than the cells of the second zone. The cells of the second zone are small and more darkly staining than the cells of the first zone. In contrast to first zone periclinal division may occur in the second zone close to the leaf primordium in addition to anticlinal division.
The main function of tunica is to give rise epidermis. Sometimes the inner layers of tunica form cortex and vascular tissue. Corpus: Corpus is the inner tissue zone of shoot apex. It consists of cells that are several cell layers deep. Tunica overarches corpus. Meristematic tissues composing this zone are larger than tunica.
The initial cells of corpus occur below the tunica.
Gardarg This is due to the fact that in zones some genes are activated and others are repressed resulting in the production of different tissues. So to avoid discrepancies the term mantle was proposed in a loose sense instead of tunica. The apical initial is histoven in histpgen and generates root cap from one side. Please tell us where you read or heard it including the quote, if possible. It consists of one or more peripheral layers of cells.
The theories are: 1. Apical Cell Theory 2. Histogen Theory 3. Tunica-Corpus Theory.