Shoot Apex Found in the Leaf of a Plant | Botany

In this article we will discuss about the shoot apex found in the leaf of a plant.

The terminal portion of shoot that occurs just above the youngest leaf primordium is designated as shoot apex. The shoot apex is the region of the primary organization of shoots and exhibits indeterminate or unlimited growth. It continuously produces leaf primordia at definite plastochron in a genetically predetermined manner.

Sometimes shoot apex loses the capacity of unlimited growth and forms tendrils or thorns (ex. Ulex europaeus and Bougainvillea etc.). The meristem of apical shoot with indeterminate growth becomes determinate when it forms an inflorescence or flower.

Shoot apex exhibits great variation in size and diameter as seen in longitudinal section and in scan electron micrograph. Commonly the shoot apex is in the form of mound. It may be in the form of a slender with blunt-tipped cone (ex. grasses, Elodea and Hippuris etc.). The shoot apex of Hibiscus syriacus and Drimys is slightly concave.

The diameter of shoot apex exhibits rhythmic fluctuation during plastochron. The diameter may be about 40 microns (Syringa), 500 microns (Phoenix canariensis) and 700-900 microns (Trichocereus spachianus). Cycas revoluta exhibits shoot apex with 3.5 millimeters in diameter. Generally large shoot apices are observed in ferns, cycads and cacti.

Shoot Apex of Pteridophyta:

In Psilotum and Equisetum etc. the apical meristem exhibits a single apical cell (Fig. 7.11). The cell is three sided (ex. Salvinia and Selaginella) or four sided and occurs on the surface layer of meristem. One side of apical cell is directed to the surface of apex and forms the outer surface of the shoot apex. This side of apical cell is triangular or square in outline and never divides.

The other sides are cutting faces, pointed downward and divide in an orderly fashion. The three sided apical cell has two whereas the four sided apical cell has three cutting faces. All cutting faces divide and form a large packet of cells. These cells on differentiation form the different segments of shoot.

So this apical cell is regarded as ‘structural and functional unit of apical meristem’ in pteridophyte. The apical cell is genetically sound as its derivatives form the primary organization of shoot apex. The apical meristem of Selaginella exhibits 2-5 apical initials (Fig. 7.13).

In spermatophyte, according to the classical concept, the shoot apex is composed of promeristem/primordial meristem. Promeristem consists of undifferentiated mass of cells and the cells have equal morphology. Later investigations reveal that there exist complex zonations in the promeristem.

The zones are distinct and can be distinguished on the basis of:

(1) Cell size,

(2) Thickness of cell wall,

(3) Nuclear size,

(4) Staining properties,

(5) Relative frequency of mitoses, and

(6) Planes of cell division.

Before illustrating the shoot apex of spermatophyte it may be useful to have a precise description of the following terms that are used to designate the zones of meristem (Fig. 7.12):

i. Surface meristem:

It consists of a group of cells called apical initials that occur on the outermost region of shoot apex. Apical initials divide by both anticlinal and periclinal division.

In gymnosperm the derivatives of apical initials are regarded as the initiating zone of shoot apex. In angiosperm surface meristem commonly divides by anticlinal divisions only. The derivatives form epidermis of shoot apex. In this sense surface meristem represents tunica of angiosperm.

ii. Central mother cells:

The cells occurring in median position just below the surface layer compose central mother cells zone. The cells are distinguished by their large-size, thick primary wall with conspicuous vacuolated cytoplasm. The cells divide by oblique and transverse planes of cell division. The derivatives produce the lateral peripheral zone consisting peripheral meristem/flank meristem and basal zone consisting of rib meristem.

iii. Rib meristem:

The cells composing rib meristem zone may occur at the terminal region of apical meristem (ex. Cycas) or at the lower region below the central mother cells of apical meristem (Ginkgo, Opuntia etc.). Rib meristem is composed of vertical series of transversely dividing meristematic cells. The derivatives of this meristem differentiate into pith cells of a stem.

iv. Flank meristem:

It is also termed as peripheral meristem and appears as a cylinder enclosing the zone of rib meristem. The cells of flank meristem contain dense cytoplasm. Surface meristem occurs on the peripheral side of flank meristem.

Flank meristem produces cortex, procambium and leaf primordium. Flank meristem increases by its own derivatives. In gymnosperm surface meristem donates cells to flank meristem. In Ginkgo and Opuntia flank meristem is supplemented by cambium-like transitional zone.

v. Cambium-like transitional zone:

This zone is cup-shaped and occurs in between central mother cell-zone and rib —and flank meristem. The cells of this zone divide frequently usually by periclinal division and donate cells to rib —and flank meristem.

Shoot Apex of Gymnosperm:

The followings are the three types of shoot apex as illustrated by Popham. Fig. 7.13 & 7.14.

i. Cycas type:

Example:

Cycas revoluta:

This type includes the following zones:

(a) Surface meristem,

(b) Rib meristem and

(c) Flank meristem.

ii. Ginkgo type:

Example:

Ginkgo biloba, Sequoia sempervirens etc.

This type includes the following zones:

(a) Surface meristem,

(b) Central mother cells,

(c) Cambium-like transitional zone,

(d) Rib meristem, and

(e) Flank meristem.

iii. Cryptomeria-Abies type:

Example:

Cryptomeria japonica, Abies concolor and Taxus baccata etc.

This type includes the following zones:

(a) Surface meristem,

(b) Central mother cells,

(c) Rib meristem and

(d) Flank meristem.

Shoot Apex of Angiosperm:

The shoot apex of angiosperm was illustrated previously in histogen theory and tunica-corpus theory.

Popham (1952) regards the following two types of shoot apex in angiosperm (Fig. 7.14):

i. Usual angiosperm type:

It includes the following zones:

(a) Surface meristem,

(b) Central mother cells,

(c) Rib meristem and

(d) Flank meristem.

Rib —and flank meristem appears to be confluent with central mother cells. The usual angiosperm type can be correlated to tunica-corpus theory. Surface meristem represents tunica and the corpus is represented by central mother cells, rib-and flank meristem.

ii. Opuntia type:

Example:

Opuntia cylindrica, Xanthium pennsylvanicum and Liriodendron tidipifera etc.

It includes the following zones:

(a) Surface meristem,

(b) Central mother cells,

(c) Cambium-like transitional zone,

(d) Rib meristem and

(e) Flank meristem.

The Opuntia type can be correlated to tunica-corpus theory. Surface meristem represents tunica and the corpus is represented by central mother cell-zone, cambium-like transitional zone, rib-and flank meristem.