Question

in intrastelar ground tissue system which is not included?​

Answer 1

Answer:

The below mentioned article provides an overview on the ground tissue system of plants.

This system includes all the tissues excepting the epidermis and the vascular bundles. Thus it is the largest or the most exhaustive system, which begins from the layer next to epidermis and continues right up to the centre of the organs in cylindrical bodies.

Ground tissue system is heterogeneous in nature, consisting of diverse types of cell elements adapted to carry on different functions. In the axis of higher plants the vascular bundles occupy a restricted position inside the stele or central cylinder.

ADVERTISEMENTS:

The ground tissues occurring out­side the stele, and, in fact, surrounding it, form the cortex, what may be called external or extrastelar ground tissue. Similarly there are internal or intrastelar ground tissues inside the stele, e.g., pith.

Both external and internal ground tissues are further differentiated to specialised zones. These tissues are derived from the ground meristem of the embryonic region.

Cortex:

Cortex constitutes that part of the axis which envelopes the stele. It begins from the layer next to epidermis and solely consists of primary tissues. During growth in thickness secondary tissues are added to the cortex, forming what is known as secondary cortex.

The cortex may be rather thin, consisting of only a few layers of cells, or it may be quite massive. It is essentially composed of parenchyma which is the most predominant cell type.

ADVERTISEMENTS:

Collenchyma very often occurs in the superficial regions of herbaceous stems and other rapidly elongating bodies like leafstalks and floral stalks. It may be present as discrete patches at the ridges in the bodies with wavy outline, at the corners in the angular stems, or as continuous bands consisting of a few layers of cells beneath the epidermis.

Sclerenchyma fibres are also common as continuous sheets or in patches. In some organs sclerenchyma forms a band of supporting tissue just beneath the epidermis.

The term hypodermis is applied to that zone of protecting or supporting tissues com­posed of collenchyma or sclerenchyma. The paren­chyma cells of the cortex are loosely-arranged isodiametric ones usually con­taining chloroplasts.

Scle­reids, resin ducts, oil-cavi­ties and laticifers may also occur. Starch, tannins and crystals are the common inclusions. The cortex of root is more homogeneous, as it consists of only parenchyma cells.

Though cortex is often composed of various types of cells, naturally with different functions, but it is primarily a protective layer. Other functions like support, photosyn­thesis, storage, etc., are secondary.

Endodermis:

It is a uniseriate zone composed of modified parenchyma cells which are compactly set, so that intercellular spaces are absent. Endodermis is considered to be the innermost layer of cortex; but some anatomists are of opinion that it is the limiting layer of stele.

Though ontogenetic studies are not conclusive on this point, its intimate association with the vascular system is undeniable.

The endodermal cells are elongate ones with long axes parallel to the vascular tissues. In cross-section the cells are essentially tabular in shape (Fig, 568).

A very distinctive feature of the endodermal cells in the presence of a waxy sub­stance like suberin in form of bands or strips on the radial and transverse walls. These are known as Casparian strips or bands (Fig. 568 A, B & C), so named after Caspary who first noted them in 1865. The bands also show lignin reactions.

ADVERTISEMENTS:

In some cases the radial and inner, and often, all walls (Fig. 568 E & F) become very much thickened, so much so that the lumen of the cells may be obliterated. These walls are also strongly suberised.

In this type of endodermis, what is common in monocotyledonous roots, thin-walled cells occasionally occur here and there. These cells, called passage cells or transfusion cells (Fig. 568E), are supposed to facilitate flow of water and solutes.

In roots passage cells are located just opposite to the protoxylem vessels. The terms primary and secondary have also been attributed to the thin-walled and thick-walled endodermis respectively.

The first type is common in pteridophytes and most dicotyledons and the second type in the monocotyledons. In an endodermal cell the cytoplasm usually remains firmly attached to the Casparian strips (Fig. 568C), so much so that they do not easily separate even on plasmolysis.