Read this essay to learn about:- 1. Meaning of Shoot Apex 2. Angiosperm Shoot Apex 3. Gymnosperm Shoot Apex 4. Shoot Apex of Pteridophytes.
Essay # 1. Meaning of Shoot Apex:
The shoot apex proper is considered as the region of initiation of the primary organisation of the shoot in which the processes of unlimited growth take place. This region is situated immediately above the uppermost leaf primordium. In size and shape it varies enormously, but usually the shoot apex is more or less convex in longitudinal section.
The apical meristem widens considerably before the initiation of the leaf and it again becomes narrow after the appearance of the leaf primordium, thus exhibiting a rhythmic change in shape. It is the region where initiation of the primary organisation of the shoot begins.
The term plastochron has been used for the period between the initiation of two successive leaves. There are great variations in the shape and size of the shoot apices among spermatophytes. The apex usually appears more or less convex in a median longitudinal section.
The shoot apex size also varies among the seed plants. In some grasses the diameter is found to be 90 µ and in dicots it varies between 130-200 µ. In Musa it is 280 µ, but in Nympbaea and in some members of Palmae the width of the apex varies between 500-700 µ. In gymnosperms these variations are still greater.
In Pteridophyta a single or more apical initials are distinguishable from others and they give rise to all the cells of the apex. The shoot apices of conifers are relatively narrow and conical in Ginkgo and in the cycads they are little bit broader and flat. The shoot apices of some monocotyledons (e.g. Elodea) and dicotyledons (Hippuris, Myriophyllum) are narrow and elongated with round tips.
Again, in many dicotyledons (Drimys, Hibiscus syriacus etc.) the shoot apex is concave. The apical cell is usually pyramidal in shape in Psilotales, Equisetum and leptosporangiate ferns. It divides to produce the cells on all sides. The eusporangiate ferns, on the other hand, possess two to four apical initials.
Essay # 2. Angiosperm Shoot Apex:
A number of theories have been enunciated regarding the growth and differentiation of the angiosperm shoot apex. Initially the histogen theory of Hanstein was put forward, which distinguishes three zones in the shoot apex of angiosperms: the dermatogen, a plerome, and the periblem. He stated that the dermatogen, periblem and plerome develop from independent groups of initials, which act as direct histogens.
The epidermis develops from the dermatogen, the cortex, and the leaf internal tissues from the periblem, and the central cylinder from the plerome. The theory was accepted for a long time, but later on it was disproved by the facts that (a) in most seed plants the periblem and plerome are not distinguishable, and (b) no predetermination of the mature tissues could be traced in the various initials.
In 1924, Schmidt postulated the tunica- corpus theory for the angiosperm shoot apex development, which divides the apex into two regions — the tunica and the corpus. The theory states that there is no constant relationship between the particular initials of the promeristem and the inner tissues of the shoot.
The two regions are usually distinguished by the planes of cell divisions. The tunica consists of the outermost layer(s) of cells which surround the inner cell mass — the corpus.
The plane of cell division in the tunica is mainly anticlinal. In the corpus, on the other hand, the cells divide in all directions. Electron microscopic studies reveal that the ultrastructural differences between tunica and corpus cells are mainly quantitative. The tunica enlarges in surface area and the corpus in volume. The tunica- corpus theory is very adaptable and is generally accepted.
As discussed earlier, the number of layers in tunica is not always constant in a given genus or family, and not even in a species, and in some cases the number of layers varies in a single plant during different stages of the development of the shoot apex. The number usually varies between one and nine.
Sometimes, especially in certain monocots, a few periclinal divisions occur in the tunica, which is contradictory to the original concept of tunica. For that reason Popham and Chan (1950) coined the term mantle for all the outer layers of apex which is histologically distinguishable from the inner cell mass, the core — without taking into account the planes or division in these layers. They used the term tunica for those layers in which only anticlinal cell divisions take place.
In the entire tunica there are two cytologically recognisable zones. One of them is the central apical zone consisting of one or few initials which are usually larger and have larger nuclei and vacuoles than the other tunica cells and which, therefore, are also more lightly staining.
The second zone is the region on the sides of the apex between the initials and the leaf primordia. It consists of smaller, more darkly staining cells which divide more frequently and among which periclinal division may occur close to the primordia. The tunica is far more homogeneous than the corpus.
In angiosperms there are two principally recognisable types of corpus based on internal arrangement (Popham, 1952):
1. Usual Angiosperm Type:
In this type (Fig. 5.45H) 3 main zones can be distinguished in the corpus:
(a) The zone of central mother cells, which represent the corpus initials, located below the apical portion of the tunica, i.e., below the tunica initials;
(b) The rib meristem, and
(c) The peripheral meristem.
The latter two zones appear as continuation of the central mother cells.
2. Opuntia Type:
In this type (Fig. 5.45G) an additional cambium-like transitional zone can be distinguished. This cup-shaped zone is found between the central mother cells and the rib and peripheral meristems.
This zone differs from the other zones of the apical meristem in that its height and diameter vary considerably during the plastochron. This zone is, of course, only a temporary feature in many of the plants in which it occurs, as it disappears towards the end of the plastochron.
The examples of Opuntia type of shoot apex are Phoenix dactylifera, Chrysanthemum mori- folium, Opuntia cylindrica, Bellis perennis, Xanthium pennsylvanicum, Liriodendron tulipifera and Bougainvillea spectabilis.
In some of these examples the cambium-like transitional zone is seen during the plastochron only, whereas in others it is always found. So the presence or absence of this zone should not be the criterion of classification.
Usually the rib meristem contains rows of cells narrowing towards the apex. The cells of this zone usually divide horizontally, but diagonal divisions may also occur. The series of cells can be distinguished. With the increased cell size the vacuoles also enlarge.
Johnson and Tolbert (1960) coined the term metrameristem to include the initials of the tunica and corpus. It can be concluded that the epidermis and the derived structures originate from the outermost layer of the tunica. The leaf primordia, the cortex, all or part of the procambium, and sometimes also the outer region of the pith are developed from the peripheral meristem.
Functional Role of the Apical Meristematic Zones:
In the shoot apex two parallel cytohistological zones are generally distinguished:
(a) A central apical zone which includes the initials of the tunica and of the corpus and in which division is considered to occur rarely; and
(b) A peripheral zone to which much mitotic activity is observed.
The central apical zone is active only when the vegetative apex becomes reproductive (Buvat, 1952). This view is based on Plantefol’s (1947) theory on phyllotaxy. The mitotically inactive central apical zone is termed meristeme d’attente (waiting meristem).
The results of the study of natural and colchicine-induced polyploid chimeras favoured the concept that divisions do occur in this zone.
But the histological investigations indicate that cell divisions in the central zone of mother cells take place. Ball (1960) observed that the cells in the surface layer including those at the summit of the apex frequently divide. It was also supported by the DNA synthesis data with radio- labelled precursors. West and Gunckel (1968) with cytohistochemical studies found no major difference in RNA synthesis between the central apical cells and that of the flanking zones.
Lyndon (1976) concluded that in most of the plants there exists a gradient in the rate of cell division and growth from a minimum at the summit of the apex to a maximum in the region of leaf initiation.
Essay # 3. Gymnosperm shoot apex:
Recent cytohistological studies indicate that the gymnosperm shoot apex contains a complicated arrangement of groups of cells which are characterised by their size and nucleus, differential staining pattern, relative wall thickness, and the frequency and orientation of the plane of cell division.
The cell division in the surface of the gymnosperm shoot apex is anticlinal and periclinal. This top layer represents the initiation zone and has been termed surface meristem. At the summit of the apex the cells divide periclinally and these cells are the apical initials. A distinct zone of central mother cells is the striking feature of the gymnosperm shoot apex. The cells of this zone are relatively larger, polyhedral, irregularly arranged and thick walled in the angles of the cells.
Electron microscopic studies have revealed that the apical initials and the central mother cells of Pinus banksiana contain numerous lipid bodies and their nuclei contain very little amount of heterochromatin. Both types of cells contain numerous vacuoles and large light-stained nuclei. These features of the apical mother cells and the central mother cells help to the appearance of the cytohistological zonation of the shoot apex.
Other apical regions also develop as a result of the diagonal and horizontal divisions of the central mother cells along the sides and the base of the central mother cell zone. Thus peripheral or flank meristem is formed laterally and the rib meristem zone at the base.
Based on the structure and development, Popham (1952) distinguished three main types of gymnosperm shoot apex:
1. Cycas Type:
In Cycas type (Fig. 5.45C) there are three meristematic zones:
(a) The Surface Meristem:
The cells of this meristematic zone divide anticlinally, periclinally and diagonally. They are not uniform in size and the apical initials are distinguished in the centre of this zone in the seedling stage. Epidermis and other meristematic regions arise from this zone.
(b) The Rib Meristem:
This meristem is situated centrally in the apex below the surface layer. Cells of this meristem are arranged in vertical rows. They divide periclinally, anticlinally and diagonally near the base. Pith develops from this zone.
(c) The Peripheral Meristem:
The cells of this meristem are elongated. Cell division takes place within the zone itself. Further addition of cells to this zone takes place from the surface layer and from the periphery of the rib meristem. The derived cells of this zone differentiate into the cortex, the procambium and the leaf primordia.
2. Ginkgo Type:
In Ginkgo type (Fig. 5.45D) there are five meristematic zones as:
(a) The Surface Meristem:
This zone remains at the summit of the apex. Cells of this zone are large, polyhedral and irregularly arranged; they divide in various planes.
(b) The Zone of Central Mother Cells:
Beneath the surface meristem this zone is located centrally.
(c) The Rib Meristem:
Just beneath the cambium like transitional zone the rib meristem is distinguished. The cells of these zones are usually arranged in rows. From this meristem pith of the stem develops.
(d) The Peripheral Meristem:
The peripheral meristem forms a cylinder surrounding the rib meristem and is a continuation of the cambium-like transitional zone. In this zone, the number of cells increases by division in the meristem itself as well as by the addition of cells from the surface meristem and the cambium-like transitional zone.
(e) The Cambium-Like Transitional Zone:
The cambium-like transitional zone is cup-shaped, relatively narrow and characterized by frequent cell divisions. This zone forms a transitional zone between the central mother cells and the rib and flank meristems.
Most of the divisions are periclinal in relation to the central mother cells. So the cells are added to the zones below it. The cortex, leaf primordia, procambium and in some cases the outer region of the pith develop from this zone.
3. The Cryptomeria-Abies Type:
In this type (Fig. 5.45E) four meristematic zones are distinguished. Excepting the cambium-like transitional zone all other zones are remaining same like that of Ginkgo type. In Pinus montana, Sequoia, Metasequoia glyptostroboides, Abies concolor, Taxus baccata, Ephedra altissima and Cryptomeria japonica. This type shoot apex is found.
Essay # 4. Shoot Apex of Pteridophytes:
In pteridophyte shoot apex there are one or more readily distinguishable initials which — by division and re-division —give rise to all the remaining cells of the apex. Where there is a solitary initial at the apex it is called the apical cell and, if the number is more, they are called apical initials. In Selaginella (Fig. 5.45B) apical initials occupy the shoot apex, whereas solitary apical cell is found in Equisetum, in some ferns and in the members of Psilotales.
The single apical cell is generally tetrahedral in shape and its base is directed towards the apex surface. The apical cell divides in such a way that the new cells are formed on all sides except the surface.
The apical initials of pteridophytes are usually four-sided, but in Salvinia and Azolla and sometimes in Selaginella, they are only three- sided. In Salvinia and Azolla new cells are added on three sides while in Selaginella they are formed on two sides only. There is a general belief that in ferns the species with solitary apical cell are evolutionarily more advanced than those which possess apical initials in their apex.
Branching:
Branches on the shoot originates at the apex. In many pteridophytes and in a few angiosperms branching is initiated by the equal division into two of the solitary apical cell or group of apical cells in such a way so that two apices are formed leading to a mode of dichotomous branching.
In remaining pteridophytes and in most seed plants lateral axillary buds are initiated in the axils of the leaf primordia. From these lateral axillary buds branches develop later Accessory buds may also originate from part of the peripheral meristem of the axillary buds. Some branches, of course, may initiate extra- axillary buds from which tendrils may develop.
Reproductive Apex:
After a certain period of vegetative growth, under either the influence of the environmental factors or by the influence of internal factors or by both, the vegetative apex is replaced by the reproductive apex which produces a flower or an inflorescence. The classical concept is that the flower is a modified shoot.
It is generally thought that that at the time of flowering the vegetative shoot apex, terminal or lateral, undergoes various physiological and histological changes and becomes directly transformed into a reproductive apex. This may develop either a flower or an inflorescence. Like the vegetative shoots there are plastochronic size fluctuations in the floral apex also during successive stages of development of bracts, perianth, stamens, and carpels.
Some floral apices may retain the cytohistological zonation of the vegetative apex. There is a progressive metamorphosis of the vegetative apex leading to the flower bud formation. According to Plantefol (1947) the anneau initial functions during vegetative growth in production of foliage leaves, the sepals and even the petals. Stamens and carpels derive from the meristeme d’attente.
However, most authors do not support the opinion of the existence of a “waiting meristem” which gets active only when the vegetative apex is transformed into the reproductive apex. They consider the apex as a whole to enter a new phase of development.
The main function of the shoot apex is to promote longitudinal growth of the axis, while that of the floral apex is to produce a meristematic mantle with a large surface area from which the parts of a flower(s) develop. Inside this meristematic envelope there is a rib meristem consisting of relatively large, vacuolated cells. It has been shown that the transition from vegetative to reproductive apex is gradual.
The first observable change is the increase of mitotic index on the boundary between the central mother cell zone and the rib meristem zone. Gradually, this mitotic activity spreads into the central mother cell zone where the cells decrease in size and become rich in protoplasts.
In this way all the cells above the rib meristem are added to the tunica, the cells of which are more or less isodiametric and are relatively small. Following these changes, mitotic activity and growth almost ceases in the cells of the rib meristem and the pith below it. Thus in the apex a parenchymatous pith surrounded by meristematic cells develops.
Eventually only the flower parts or the bracts, the axillary branches of the inflorescence, and the flowers themselves develop from these meristematic cells. Many cytohistological studies on the transition of the vegetative apex to reproductive state have been made. Cytoplasmic basic histone proteins, RNA, and total protein level increases in all zones of the apex during vegetative to reproductive transitional stages.
During the transitional stages following overall events occur in the apical meristem of Sinapis alba when they are treated with proper photoperiods:
(a) Rise in the mitotic index and reaching a maximum in about 30h at the onset of the long day
(b) Initiation of DNA synthesis and its culmination at the 38th hour
(c) Increase in the size of nucleolus reaching to a maximum at about the 54th hour
(d) Cell volume starts to increase and culminates at the 62nd hour
(e) Further rise in the mitotic index and culminates at 62nd hour.
Floral induction is thought to occur during the first burst in mitotic activity in coordination with the second peak of the same. The cytohistological changes during floral induction are obviously preceded and accompanied by physiological and biochemical changes.
The apical dominance of the main apex is lost with the formation of the inflorescence. The number of ribosomes and Golgi bodies is increased. The endomembrane system is also changed.
In this stage of development the apex ceases to elongate and is terminated by an inflorescence or by solitary flower. In banana, pineapple, etc., however, the elongation takes place by the activity of the rib meristem.