Xerophytes: Categories and Physiological Adaptation of Xerophytes | Plant Adaptation

Let us make an in-depth study of the categories of xerophytes and its physiological adaptation.

The xerophytes are classified into three categories. The three categories of xerophytes are: (1) Ephemeral Annuals (2) Succulent Perennials and (3) Non-Succulent Perennials.

Inherited adaptations to abundance or scarcity of water show the pronounced effects of moisture as an environmental factor. Many plants have so modified during the course of evolution that they are able to thrive under conditions where the available soil water is comparatively small in amount and where plants without special adaptive modifications would speedily perish. Such drought-loving plants are known as xerophytes and possess several types of structural and functional modifications which result in an ability both to draw water from the soil and to retain it in the plant tissues.

Structural Peculiarities of Xerophytes:

Chief structural peculiarities of xerophytic plants are as under:

1. The root system is very well developed in proportion to the shoot. In Cacti, however, the root system if feebly developed. In certain cases, as in Asparagus, the roots become fleshy and store water and food.

2. There occurs a great degree of variation in the form and structure of leaves. In plants, such as Agave and Dianthus caryophyllus, which are capable of growing under more or less dry conditions, the cells of the mesophyll are very closely packed. Cuticle is very thick to check excessive transpiration. Sometimes a waxy coating (e.g. Salix glaucophylla) or numerous vescicular hair (e.g., Atriplex canescens) may be present on the epidermis.

In Calatropis, both waxy coating as well as vesicular hair occur on the leaf surface. Epidermis is usually multilayered (e.g., Nerium and Ficus elastica) and possesses sunken stomata covered with numerous hair (e.g., Nerium). In the mesophyll of the leaf, the palisade tissue is usually very well developed. Sclerenchymatous hypodermis may also be present (e.g., Pinus).

In majority of the xerophytes, the leaves are thick fleshy with water storage tissue. Leave may be very much reduced, sometimes so greatly reduced that they take the shape of scales or needles (e.g., Casuarina, Pinus, Equisetum). Leaves may even disappear entirely (e.g., Opuntia) and the function of photosynthesis is taken up by the stem itself. Sometimes, as in resurrection moss, the leaf surface is only temporarily reduced to overcome the drought.

3. In woody xerophytes the cork is very well developed in the stem. Such an adaptation is of great importance in the conservation of water supply. Stem may, sometimes, be covered with spinous outgrowths (e.g., Carthamus, Argemone and Solanum xanthocarpum).

In some plants the stem is greatly reduced, or it may be modified into phylloclade, or cladodes, e.g., Cactii, Ruscus, Muehlenbeckia, Asparagus etc. (see Figs. 2.13 to 2.16).

4. Oil and resin glands are often present.

5. Watery sap or latex may also be present (e.g., Cactii and Euphorbias respectively).

6. The vascular system is well developed and differentiated. The xylem possesses broad and large vessels with very much thickened walls.

7. Mechanical tissue, like bast fibres, is extensively developed.

Physiological Adaptation of Xerophytes:

1. The osmotic concentration of the cell sap is usually higher than among plants growing under less arid environment.

2. According to Maximov (1929), although the net rate of transpiration per plant is much reduced in xerophytes but the rate of transpiration per unit area is much greater. These plants control the excessive loss of water during transpiration by reducing total transpiring surface,

3. Although the rate of photosynthesis per unit area is much rapid but starch-sugar ratio is usually lower in these plants (Levitt, 1956; Iljin, 1957).

4. The amount of bound-water (i.e. water adsorbed on the surface of colloidal particles) per unit dry weight of the plant tissue is comparatively greater (Whitman, 1941).

5. According to Iljin (1957), the protoplasm in these plants is less viscous and more permeable.

6. Xerophytes have greater potentiality to resist wilting.

In a more scientific sense it will be more correct to say that the so-called drought- loving plants (i.e., xerophytes) are, as a matter of fact, drought evading and drought enduring plants.

Categories of Xerophytes:

The xerophytes may be classified into three broad categories as under, and the plants included in each category do not essentially have similar morphological and physiological characters:

I. Ephemeral annuals.

II. Succulent perennials.

III. Non-succulent perennials.

I. Ephemeral Annuals:

These are small drought-evading plants that grow in the arid zones, These plants are so adapted as to complete their life-cycle within the shortest possible time during rainy season. They have no anomalous morphological or physiological characteristics. Examples of ephemeral plants are— Carthamus oxycantha, Solanum xanthocarpum, Suaeda fruticosa, Tribulus terrestris, Trianthema monogyna.

II. Succulent Perennials:

The succulent plants are highly specialized xerophytes. Among the plant families containing succulents are the cactus, spurge, milkweed, lily, and amaryllis. The chief characteristic feature of such plants is that the bulk of the plant body is composed of water storage cells, which supply the plant during periods of drought, when water can not be obtained from the soil. The succulent organs are generally the stem or leaves, rarely the roots.

If the stem is succulent then leaves are reduced or absent, if the leaves are succulent, the stem is generally much reduced. The volume of the shoot is great in proportion to the surface exposed, and this combined with a well-developed cuticle and other features, retards the rate of water loss so that the transpiration rate is low. The succulent are said to resist, rather than endure, drought.

Succulent may originate as a direct result of aridity, but all succulent plants are not necessarily xeric. The succulent plants illustrate clearly the principle that unrelated plants, under similar environmental conditions, may develop striking similarities in external form.

Succulents, depending upon the succulent organ, may be divided into two categories:

(A) Succulents with fleshy stem.

(B) Succulents with fleshy leaves or Malacophyllous xerophytes.

(A) Succulents with fleshy stem:

In xerophytes with succulent stem, the leaves are reduce to spines or are altogether absent. The function of leaves is also taken up by the stem thus greatly reducing total transpiring area. Common examples of such succulents are Opuntia, Echinocactus, Cercus, Euphorbia royaleana etc. These plants have several characteristic features to resist drought. For instance in Opuntia phylloclade (i.e., succulent stem, the epidermis is thick-walled and covered with a very thick cuticle; the epidermis is followed by multilayered, collenchymatous hypodermis. The cortex is chlorenchymatous. Outer 3 to 4 cortical layers consist palisade cells with chloroplast.

The inner cortical cells are succulent and mucilaginous; these cells are very thin walled and with or without intercellular space. This tissue stores large amount of water that can be used by the plant during drought period. Almost similar arrangement of tissues is found in Cereus and Euphorbia royaleana. Fluted stems, such as those of giant cactus (Carnegiea gigantea) store huge amount of water and undergo characteristic expansion and contraction during moist and dry periods, respectively.

(B) Succulents with fleshy leaves or Malacopyllous xerophytes:

Malacophyllous xerophytes are characterized by reduced stem and succulent leaves. Succulent leaves are usually small in size and are often more or less cylindrical in shape. Such leaves posses prominent water storage tissue consisting of thin walled parenchymatous cells. The peripheral cells of the leaf possess chloroplasts.

The amount of chlorophyll gradually decreases from periphery to the centre. There is not much of difference in the photosynthesizing peripheral cells and the cells of the water storage tissue except that the latter do not possess chloroplasts and their cell walls are made up of cellulose. In succulent leaves, the epidermis is quite often covered with a waxy coat, and in addition some possess a thick cuticle (e.g., Agave).

Salsola kali-tenuifolia may be quoted as an important example of malocophyllous xerophytes. In this plant leaves are succulent and somewhat cylindrical (see Fig. 2.17).

The leaf of Salsola show marked distinction between peripheral chlorenchymatous cells— whose function is photosynthesis, and central water storage tissue. Cells of the water storage tissue are large, thin-walled and mucilagenous. The epidermis is covered with a thin cuticle.

In certain plants, such as Peperomia, these are the peripheral cells which store water. In the leaf of Peperomia, two or three layers of cells just below the upper epidermis constitute water storage tissue. In Mesembryanthemum crystallinum, some of the epidermal cells become inflated and swell out beyond the epidermal line. These epidermal cells function for water storage.

Some other common examples of malacophyllous plants are Senecio. Aloe, Yucca, Bryophyllum, Kalanchoe, Tradescantia, Begonia, Sempervivum assimile, Haworthia etc.

III. Non-Succulent Perennials:

Non-succulent perennials include herbaceous forms, mostly grasses, as well as woody species. Many of these trees are more or less evergreen and their leaves show xeromorphic modifications, which enable them to survive during the period of drought. Not all non-succulent xerophytes have xeromorphic modifications; for instance, the leaves of many desert shrubs are thin, and the rate of transpiration is high when water is available.

Such plants have extensive root systems which penetrate deeply into the soil, and they frequently shed their leaves quickly when there is any scarcity of water and they begin to wilt, then produce new leaves when the period of drought is over. Like xerophytes with xeromorphic modifications, they endure drought during rainless period.

Some of the important xeromorphic modifications of non-succulents are summarized below:

1. The root system is extensive: Roots penetrate deep into the soil and establish contact with sub-soil water. For instance, in Prosopis and Alfalfa the roots penetrate as deep as 60 and 130 feet respectively into the soil.

2. The osmotic concentration of the cell sap is usually very high which helps in preventing irreversible changes in protoplasmic colloids which might take place under extreme desiccation.

3. They considerably reduce the rate of transpiration.

This is achieved through various kinds of adaptations, such as:

(i) Rolling of leaves: in many grasses, such as Agropyron (Fig. 2.18), the leaves roll upward and inward, due to decreased turgidity of bulliform cells (or motor cells). In Ammophila arenaria also there occur longitudinal furrows on the upper surface of the leaves, which enable them to fold upward and inward and thus the stomata that are present in the furrows are covered (see Fig. 2.19). The lower surface of the leaf is thickly cuticularized. This adaptation brings down the rate of transpiration to almost nil,

(ii) Certain plants produce special set of leaves which are particularly suited to drought conditions,

(iii) Some plants, such as Euphorbia splendens, shed their leaves soon after the rainly season is over.

4. The leaves are usually heavily cuticularised. In addition a waxy coating on the surface of the leaf may also occur (e.g., Salix glaucophylla).

Stomata may be present in pits (i.e., sunken stomata) covered with hairy out growth (e.g., Nerium, Fig., 2.20).

Presence of multilayered epidermis, such as in Nerium, gives rigidity to the leaf. In some plants the leaves are trichophyllous i.e., covered with hairy outgrowth (e.g., Calatropics, Eleagnus etc).

5. In some non-succulent xerophytes, such as Casuarina, Capparis aphylla, Ephedra, Equisetum, the leaves take the shape of needles or scales. In these microphyllous plants the role of leaves is usually performed by the stem. For instance, in Casuarina the stem has prominent ridges and furrows (see Fig. 2.21). The stomata are situated at the bases of the furrows and are also covered by multicellular hair.

The epidermis is covered with a thick cuticle and is followed by a multilayered patch of sclerenchyma in each ridge. The most characteristic feature is the presence of chlorenchymatous palisade cells in the cortex, function of these cells is decidedly to carry on photosynthesis in place of leaves. Thus, the stem of Casuarina not only shows the features of a xerophytic stem but also that of a xerophytic leaf.

Almost similar features, as exhibited by Casuarina stem, are also shown by the stem of Capparis aphylla. In this plant the leaves are scaly and shed soon after rainy season. Mere also the functions of the leaves are performed by the stem which shows combined characters of a xerophytic stem and a leaf.

In Capparis aphylla stem, the epidermis is covered with a thick cuticle. Sunken stomata are present in the epidermis. Presence of myrosin cells, which are laticiferous cells, in the hypodermal region is another characteristic feature (see Fig. 2.23).

The cortex, like that of Casuarina, is made up of chlorenchymatous palisade ceils-whose function is to carry on photosynthesis in place of leaves.

Furthermore, the endodermis consists of stone cells which is an important xerophytic character.

There is maximum possible lignification of tissues.

6. In most of the non-succulent plants the size of the cells is small and the vacuoles are also minute. This adaptation probably helps to prevent pulling away of the protoplasm from the cell walls under conditions of excessive drought and desiccation.