Xerophytes: Types and Characteristics | Botany

In this article, we will discuss about the xerophytes. After reading this article you will learn about: 1. Types of Xerophytes and 2. Characteristics of Xerophytes.

Xerophytes:

There have been many interpretations of the term xerophyte. If we use the term in a loose qualitative way, xerophytes are plants of relatively dry habitats—dry in soil and most often also climatically. A truly ecological definition approaching as near as possible a quantitative basis is that xerophytes are plants which grow on substrata which usually become greatly depleted of gravitational ground water to a depth of at least 20-25 cm during the course of a normal season.

In dry regions all plants not con­fined to the sides of streams or lakes are considered xerophytes whereas in the regions of high rainfall, xerophytes would likely be represented only by some shallow-rooted plants in light sandy soils, by the vegetation on dry hill-tops and the cryptogamic flora of lichens, algae and mosses.

The environmental adaptations achieved by xerophytes to escape drought or to endure recurrent drought (drought is not easily defined; it refers to periods during which the soil contains little or no available water) can be either morphological (structural) or physiological or both. In order to endure dry conditions xerophytic plants must adopt all the means at their disposal, aiming constantly at two definite objectives: (i) to pro­cure as much water as they can get from the inhospitable soil and (ii) to economise on this supply of water by conservative use and keeping the loss of water from the aerial parts as also the water requirement of the plant to a minimum.

This is, however, drought resistance in the narrow sense of the term. During drought, the plant suffers, in addition to dehydration of its cells and tissues, from a considerable increase in the temperature of the body, i.e., from overheating. Thus, the lack of water caused by drought in usually accompanied by high body temperatures. Hence drought resistance of the plant includes both the abilities—first, ability to with­stand dehydration and secondly, the ability to withstand overheating. High heat-resis­tance is, however, not always linked with high drought resistance. It is necessary also to note that drought not only affects plants in different ways, but even individual species may have different types of response.

According to many authors, such as Henckel (1954, 1960) drought resistance “is a property which is formed and developed in the process of ontogenesis and is based on the whole preceding phylogeny of the plant” (Henckel, 1964). This property thus is not unchangeable and may be markedly affected by individual development of the plant.

The following definition of drought-resistance seems to satisfy reason and the results of experimental observations:

“Drought-resistant plants are those which in the process of ontogenesis are able to adapt to the effect of drought and which can normally grow, develop and reproduce under drought conditions because of a number of proper­ties acquired, in the process of evolution, under the influence of environmental condi­tions and natural selection.”

We must try to understand how drought injures and kills plants. Soil drought first of all dehydrates the cells and tissues. A slight water deficiency is generally a normal phenomenon which does not greatly impair the functioning of the plant. Only when the deficiency exceeds a certain limit, its effect on the plant becomes fatal.

This fatal level is not the same for different plants—drought-resistant plants can lose great amounts of water without any harmful effects; drought-susceptible plants, however, can only endure a slight water deficiency, which sometimes may be enough to disturb the balance between their various physiological processes.

Interesting facts with regard to the development of xeromorphism have been obtained during recent years. It was found that phosphorus nutrition, light and water deficiency, all stimulate the formation and development of xeromorphic morphological charac­teristics. All these factors affect the same aspect of plant metabolism, namely morpho­genesis, through nucleic acid metabolism—phosphorus-stimulating morphogenesis through increased nucleic acid synthesis, while water or nitrogen deficiency, inhibiting nucleic acid metabolism, may hasten tissue differentiation.

The nature of injury and death of plants caused by drought may be considered now. Ilyin (1930-1957) thought that during dehydration, cells are injured and die as a result of sudden and violent mechanical disruption of the protoplasm. This concept was, however, modified later and it was strongly suspected that actually the cause of death was the destruction of protein molecules.

The injury and the subsequent death was attributed at first to excessive protein decomposition but later, it was found that decreased protein is not a cause but a result of the injury. Stocker (1950-1960) attributes the main cause of death during drought to changes in the sub-microscopic structure of proto­plasm—loosening of modifying specific molecular connections in the sub-microscopic protoplasmic network—accompanied by changes in viscosity, permeability, electric charge as well as due to inactivation of enzymes.

Heat-resistant plants respond to high temperatures with intensive protein synthesis and intensive restorative processes. These are the results of high levels of nucleic acids, RNA in particular (complexed with protein), in these plants which stimulate protein synthesis and high respiration rates. The hardened plants as a rule maintain all their synthetic processes at a high level during drought.

Leaves of hardened plants are usually richer in starch and all tissues are higher in organic high-energy bond phosphates (ADP and ATP). The protoplasmic colloidal-chemical properties play an essential role in the resistance of plants to high temperatures—high hydrophilic viscosity, the degree of hydration of colloids and increased content of bound water are also important aspects of heat-resistance.

Hardened plants have a greater total absorbing surface in the root system. Thus the absorption of mineral elements is more intense in hardened plants—in addition to this, the plants also bear more primary roots. As a rule, roots are more resistant than leaves to high temperatures and accumulate more starch (Petinov, 1961).

It has been observed that kinetin placed on the leaf blades increases the heat resistance of tobacco plants. This effect is attributed to the ability of kinetin to promote the mobilization of amino acids (Englebrecht and Mothes, 1960).

Hardened plants have a different and specific type of xeromorphism in that the foliage area is larger with denser network of veins and ribs. Usually smaller leaves are correlated with xeromorphism but because of extensive increases in nucleoproteins, enhanced protein synthesis, cell division and differentiation of cells—the hardened plants develop this specific xeromorphic morphology concomitant with an increase in leaf area.

It is quite possible that the nature of high temperature-killing of plants is different from drought injury. A slow increase in temperature may result in ammonia poisoning; a fast increase, however, disrupts protoplasmic sub-microscopic structure, culminating in the coagulation of protoplasmic proteins.

Protective adaptive reactions against high temperatures generally consist of more rapid respiration, increased amounts of organic acids, which may bind released ammonia, and increased hydrophilic viscosity of protoplasm.

Hardening of plants against drought and high temperature falls sharply during the development of reproductive organs (crucial period). Reproductive organs are parti­cularly sensitive to drought; among them, androecium is much more sensitive to drought than the gynaecium. In mustard, cucumbers and others, drought, however, injures the gynaecium which also leads to a reduced fruit set. Under drought condi­tions, pollination and fertilisation are in general poor and in cereals, result in partially or entirely empty heads.

The most severe injuries to a plant resulting from drought occurs at the sexual-cell-formation stage—a stage at which a plant undergoes extensive proto­plasmic reorganisation because growth processes are now cru­cially directed towards the formation of reproductive organs. The change is reflected in the modification of colloidal-chemical properties of the cell—the decrease in elasticity and protoplasmic viscosity and respiration of the plant at this stage, certainly requires much more of water and is naturally very sensitive to drought.

Plants possessing structural features and adaptations similar to evergreen xero­phytes are as we know, referred to as xeromorphic. Xeromorphic characters are inherited (genetically fixed) under any environmental conditions and as such xeromorphy is found outside the desert as many evergreen trees, e.g., hydrophytic mangroves. Xeroplastic features are, however, only induced by drought conditions (characters are ac­quired) and are in most cases associated with dry habitats (characters not heritable).

Types of Xerophytes:

Three general types of xerophytes having practically nothing in common morpho­logically, physiologically and perhaps even more truly taxonomically, may be recog­nised:

1. Ephemeral Annuals:

The ephemerals are a prominent feature of vegetation of all semi-arid regions which are characterised by definite and regular, however brief, rainy seasons. These regions support a considerable flora of small annuals which complete their entire life cycle within a few weeks after the first rains have fallen. With the onset of rains seeds of such annuals germinate, quickly grow to maturity, flower and set seed, i.e., the entire life cycle is completed before the soil dries out again. The new crop of seeds set, survives through intervening dry season until the next advent of rains.

Among seeds of these desert annuals mechanisms are found which delay germina­tion of seeds until a rain of sufficient intensity has wetted the soil to such depths that seedlings can complete their life cycle with the water. This is accomplished by water soluble inhibitors which are leached out of the seeds only by a heavy rainfall. Such inhi­bitors may either be simple inorganic salts like NaCl or they may be organic substances.

The principal morphological adaptations of ephemerals are their small size (they cannot possibly grow to a big size for the duration of life cycle is so short) and large shoots in relation to root system.

Such plants have been termed ‘drought escaping’ and not true xerophytes as they do not really endure drought (they cannot endure a severe reduction of water content for extended periods without permanent injury to the cells) rather escape it. Yet it is a significant fact that by no means all mesophytic annuals can grow in the desert. It is also true that the percentage of such annuals in the flora varies directly with increasing dryness of the soil.

A great number of these desert annuals have been investigated and in most cases no xerophytic characteristics could be discovered in these plants. Most of these plants are small roundish, dense shrubs represented among others by species of Papilionaceae (Astragalus sp., etc.), some inconspicuous Compositae (e.g., Artemesia), a few Zygophyllaceae, Boraginaceae and some grasses.

2. Succulents:

Succulents also contribute a large percentage of vegetation to the flora of the most semi-arid regions. They are also frequently found in locally dry habitats such as sandy soils, sea beaches, etc., of more humid climate.

Succulence is due to the proliferation of parenchymatous cells accompanied by an enlargement of vacuoles of mature cells and a considerable reduction in the size of intercellular spaces. The succulents are a distinct group of plants, not only in structure but also in metabolism and water-economy as well (the accumulation of organic acids, pentosans and mucilage by succulents is well-known).

His structural modification enables the plant to accumulate in the proliferated tissue, depending upon the extensibility of individual cells, large reserves of water during brief rainy seasons. The cells of the water-storage tissues are rich in glue-like mucilage. This holds water very efficiently, and as cacti have got no normal leaves, there are no large Stomata-covered leaf surfaces, through which much water might have been lost in transpiration. A few stomata are seen in the epidermis of the fleshy stems and it is pro­bable that they are open only at night. The succulents shrivel during the periods of drought as they become depleted of water, again swelling up with the advent of rain.

Succulence, to be effective, must necessarily be accompanied by reduced trans­piration rates. They must also economically use the water stored in their tissue. A rela­tively thick cuticle and the feet that in many succulents, stomata are open only during night and closed during day are important factors permitting conservation of water.

In some cases, the form of the stem succulents such as cacti may approach that of a sphere; this for a given volume certainly exposes the minimum transpiring surface and is thus largely helpful in reducing transpiration rates (with the same supply of soil water, it has been estimated that a spherical cactus may lose more than 500 times less water than from an equally heavy plant of the same volume but with greater transpiring surface).

The succulents, like cacti can survive periods of many rainless months and replenish their water supply again during the following rain. To decrease their water loss they have evolved an interesting type of metabolism by which they open their stomata during night when they absorb CO₂ (dark fixation) which is stored in the cells in the form of dicarboxylic acids (e.g., malic acid). During the day, they keep their stomata closed thereby losing very little water and also transforming the dicar­boxylic acids formed at night into sugars (Crassulacean acid metabolism).

The most conspicuous succulents of all semi-desert regions belong to the cactus family (Cactaceae). The other important families of plants which include a number of succulent species are Euphorbiaceae, Crassulaceae, Liliaceae, Amaryllidaceae, Portulacaceae, etc. Succulence may occur in roots (in species of desert Pelargonium, Oxalis, etc.) but is rare.

Stem succulents are the cacti (desert plants but shallow rooted) and the leaf succulents, Agave, Aloe, Sedum, etc. Cacti are unique among all types of desert plants in having shallow, but vast spreading roots, enabling them to absorb superficial water quickly, with rootlets which are drought-deciduous. The rootlets wither away when supply of water in the surface soil is exhausted reappearing within a very short time when there is again a supply of water in the soil.

Some ecologists have excluded succulents from the group of true xerophytes as they avoid drought by means of their water reserves. It seems that the tissues of succu­lents do not have any intrinsic resistance to the harmful effects of droughts: survival of plant depends wholly on its outer line of defences—xeromorphic characters (e.g., leaves in many cases reduced to spines, ridges or protuberances, so that when the storage tissue shrinks due to depletion of water, contraction can take place without damage; thick and waxy epidermis, etc.) and its water-storage capacity.

But succulents are by far the best represented plants in the desert vegetation all over the world. It seems perfectly justi­fiable to regard succulence, found in certain true xerophytes simply as a unique mode of adaptation to cope with extreme water-scarcity conditions of the environment.

3. Non-Succulent Perennials:

Members of this category are the true xerophytes (euxerophytes)—the drought-enduring plants—which can successfully endure long and continuous dearth of water in the soil. Soil-drought conditions are usually accompanied by dry atmospheric conditions such as high temperature, low humidity and often high wind velocity, all of which favour high transpiration rates. In euxerophytes, water deficiency usually reaches 60-70% of their gross fresh weight. As a rule, water deficiency is harmful; first of all it decreases growth process as a whole especially during cell elongation. The plants which survive drought are as a rule small and weak.

Characteristics of Xerophytes:

The morphological and physiological characteristics that enable these plants to withstand drought are many but the principal ones are primarily as follows:

(1) Dehydration:

Plants respond to progressive dehydration by changing the colloidal chemical state of the protoplasm, namely by increased hydration and hydrophily of the colloids of the protoplasm (1959, 1960). It has been noted that dehydration causes the same changes in the colloidal system as does cell-aging, i.e., it lowers the water-holding capa­city and ability to swell.

(2) Rapid Elongation of Tap Root:

In arid and semi-arid regions the surface soil dries out no deeper than few cm. So all plants whose tap root system is long enough to penetrate into deeper soil and whose rate of penetration can keep ahead of the pro­gressive drying of the soil from the surface downwards are potentially capable of grow­ing in such habitats.

(3) Absorptive Capacity:

One of the most important adaptations of true xerophytes seems to be the pro­duction of extensive root system in proportion to shoot system. It not only increases the total absorptive capacity of the plant but also exposes relatively only a small proportion of the plant to the atmosphere. Many desert plants have roots, sometimes adventitious; deep enough to absorb whatever little amount of available water (during and imme­diately after rains) there may be from the moist subsoil. Well-aerated and dry soils are a boon to most xerophytes for there they allow roots to penetrate to great depths where there may be permanently available water.

(4) High Osmotic Pressure of the Cells:

It is well known that osmotic pressure of plant cells vary inversely with their water supply and consequently high osmotic pressure of the cells (which produce such high suction pressure, i.e., diffusion pressure deficit that an equilibrium between loss of water and water uptake can probably be maintained by the cells) is a characteristic feature of the true xerophytes but how effective, this factor really is in helping plants to extract more water from the arid soil, is debatable. It may only be an expression of too high solute content of the soil.

In xerophytes, the level usually develop a very large water-potential by increasing the osmotic pressure of the cells—osmotic values, as high as 100-150 atm. are some­times exhibited by such cells.

Desert perennials can withstand considerable dehydration; some of these plants being quite able to tolerate desiccation to about 50% of their dry weight (in mesophyte this value averages about 200%)—that is they can tolerate protoplasmic desiccation to a much larger extent compared to ordinary mesophytic plants. The gelatinous coating on the cell walls of the blue-green algae perhaps plays an important role here.

The cell walls of some true xerophytes exhibit little or no elasticity and which characteristic may greatly influence the water-holding capacity of the cell. In cells with inelastic walls there is essentially no change in the volume of the cell from the turgid state to incipient plasmolysis. This is believed to enable xerophytes with inelastic cell walls to adapt themselves to drought conditions as such a condition would greatly enhance the plant’s resistance to water loss through transpiration.

Thus the xerophytic habit of a plant, which may not, necessarily, involve greatly reduced transpiration, seems to be closely linked with rigid and inelastic cell-wall struc­ture, preventing cellular collapse under high water-stress. Consequently it also allows the development of highly negative water-potential for water uptake, even from very dry soils.

Thus the reduction of negative water-potential gradient during the day-light horns and the production of a positive potential gradient during the night, allow water vapour uptake as long as the relative humidity of the surrounding air is high.

(5) Ability to Reduce Transpiration Rates to Extremely Low Levels during Permanent Wilting:

The organ that is most strikingly modified in xerophytes is the leaf. Although some desert plants may have low transpiration rates, conclusive evidence points to the fact that true xerophytes transpire more freely and vigorously (actually there is a great number of stoma per unit area in xerophytic leaves) than ordinary mesophytes when the availability of water is equal.

In many desert plants, stomata very often close during the hottest hours of the day when the cooling effect of transpiration would be most advantageous. The control of water loss by transpiration is, however, strikingly different in a xerophyte compared to a mesophyte when the condition of permanent wilting sets in.

The leaf-shedding habit of many xerophytic deciduous perennials is a very efficient means of enduring transpiration for the branches, devoid of leaves, lose very little water.

Many xerophytes which retain their leaves throughout the season, decrease trans­piration rates by special structural adaptations, of course chiefly effective only during periods of permanent wilting. Plants with evergreen leaves whose epidermis (sometimes multiple or more than one layer of epidermis is observed) is heavily cutinised or with waxy cell walls show greater resistance of desiccation under conditions of extreme soil dryness.

This is most likely due to closure of stomata coupled with highly efficient cutinisation of the epidermal cells. Plants of this type are sometimes referred to as sclerophylls (hard leaved). This thick cuticle may also be advantageous in preventing the breakage and consequent damage to the leaves caused by bending of the blades by high wind generally prevalent in a desert. A shiny, heavily cutinised leaf surface reflects much of the sun’s rays, thereby reducing transpiration through reduced heat absorption.

Transpiration rates may also be reduced as a result of protection variously given to the stomata. A greater density of epidermal hairs is a prominent feature of one or both surfaces of the leaves of many xerophytes (Fig. 777). But living hairs probably increase the rate of cuticular transpira­tion by increasing the transpiring surface of the leaf but if the hairs are white in colour it might actually reduce the transpiration by reflection of large proportion of incident light.

Dead hairs, on the other hand, perhaps reduce the rate of stomatal transpiration particularly under conditions of intense sunlight or strong wind for it is supposed that such hairs impose a mechanical barrier to the effect of wind in transpiration. Dead hairs projecting outwards may keep air currents well above the stomatal surface.

Sometimes the stomatal surface is protected by a dense coating of such hairs and also by permanent revolute margins of the blade.

Peltate hairs sometimes thinly shade the upper leaf surface of some xerophytes to such an extent as to create a dead air space just above the stomata-bearing under surface of the leaf, thereby reducing transpiration rates. But it must also be pointed out that conclusive experi­mental evidence about the efficiency of a dense coating of hairs on the leaves in the reduction of transpiration rates is lacking.

Sunken stomata—Stomata, sunken in cavi­ties below the level of epidermal surface and thus also well below the level of atmospheric stress and tensions, have generally been found to be well-capable of reducing the transpiration water loss (Fig. 777). Sometimes stomata are over-arched by adjoining cells so they come to be situated in cavities protected from the wind. In some other plants stomata are sometimes plugged, however temporarily, with a depo­sition of wax, or resin.

The mesophyll of most xerophytes is characterised by the small size of the intercellular spaces especially those which are continuous with the pores of the stomata. There is often no spongy tissue but only very compactly packed palisade cells beneath both upper and lower epidermis. All these certainly contribute largely towards an actual reduction in transpiration rates.

When the drought conditions are first felt by xerophytic plants, many of them change form or position so that the amount of light received per unit area generally becomes less. Often part of aerial transpiring surface is thereby protected from direct
contact with outside air. The leaflets of many desert legumes fold upwards in such a manner that only approximately half the leaf-surface is exposed to air.

In case of plants that grow in mats, a very common form in xerophytic habitats, aggregation of stems brings about protection of the leaves. In addition, plants may often have erect or oblique leaves. Some desert grass leaves roll or fold lengthwise along the longitudinal furrows in their upper surface; these turgour movements are due to loss in turgidity of the rows of enlarged colourless cells, often termed bulliform cells (motor cells), situated at the bottom of the furrows (Figs. 778 and 779). The free surface of leaf is reduced in many desert grasses (Stipa) and in certain species of the family Ericaceae, by the inrolling of the leaf surface. The leaves of moss, Polytrichum, also fold longitudinally in a similar manner.

In certain erect and shrubby xerophytic plants, the leaf blades are permanently oriented in a vertical position so they are never subjected to full incident sunrays. The greyish or sometimes light green colour of most desert plants is thought to be of value in reflecting light rays which, otherwise, would have been absorbed and converted into heat, raising the temperature of the leaf thus promoting rapid transpiration.

It must be pointed out here clearly that anatomical features, e.g., dead hairs, sunken stomata, leaf-roll, etc., which had formerly been considered as helping to reduce trans­piration rates of xerophytes, actually benefit the plant in a, completely different way by tending to prevent stomatal closure by maintaining high humidity conditions at the leaf surface. This increased the duration of active photosynthesis by allowing gaseous exchange to continue for longer time.

Xerophytes have more intense assimilation rate than other plants for their palisade cells and chloroplasts are better developed and also can only function part-time because of the restriction of their activity during periods of water-deficit. Green tissues function efficiently only when they are turgid and their photosynthetic efficiency falls off rapidly if they are water-deficient.

As a result, the photosynthesis of plants under drought conditions is usually confined to the early morning and the late evening when the cells are comparatively turgid (Stacker, 1960). In extreme water-deficient conditions of the habitat on the other hand photosynthetic efficiency is greatly decreased.

The epidermal cells of many xerophytic leaves are in a collapsed state in extreme water scarcity conditions of soil and air but the cells straighten up whenever there is an increase in the moisture content—this bellow-like action of the epidermal cells seems to be typical of many desert plants.

(6) Reduction in Size of Leaf Blades:

It is well known that all leaf blades, pinnae and pinnules of compound leaves of desert plants are in general smaller in size and more compact, i.e., the desert flora is mostly microphyllous. Though the size of leaf blades is reduced, the blades contain a denser network of veins. The area of the leaf usually does not exceed 1 sq. cm. theoretically; presence of microphyllous leaves should reduce the amount of transpiration due to this decrease of the aggregate area of the total transpiring surface.

But very often in microphyllous plants, the small size of the individual leaf units is compensated by greater numbers with the result that nothing is really gained by the plants in the way of reducing the total transpiring surface. Thus it is evident that reduction in the size of individual leaf-lamina has no positive significance with respect to reducing transpiration.

Still microphylly is encountered everywhere in the arid desert regions and this omnipresence must be indicative of some fundamental role played by the minute leaf blades of shrubby desert plants. That is, why microphylly? When average-sized leaf blades are injured by severe atmospheric drought it is commonly seen that the marginal parts, especially those between the larger veins die first, while those in the immediate neighbourhood of the veins, degenerate last.

This pattern of injury can be explained by the facts:

(i) That because the water diffu­sion through the mesophyll is very slow (diffusion is cell by cell), only those cells which are situated near the veins can obtain sufficient water supply during the crisis brought about by drought and;

(ii) More stomata are found towards the midrib than the margins of the leaves and thus in a linear leaf blade the tip, which is the remotest part of the blade from the water supply (transpiration stream), is the first to die.

If we closely examine a leaf blade which has almost been killed, but not quite, by desiccation, we find that the pattern of tissue still remaining alive in the injured leaf blade is strikingly similar to a much dissected pinnatifid or pinnatipartite blade. It is interesting to speculate whether this evolutionary breaking up of leaf blades or reducing the size of individual leaf units by discarding much of the intervening mesophyll has been to reduce the likelihood of severe drought degeneration of leaf tissues of the desert plants.

Microphylly is sometimes a – definite advantage in the desert environment in that, as is evident, the smaller the leaf blade there is less likelihood of its being overheated when exposed to strong intensity of light commonly prevalent in deserts.

Desert plants often lack green leaf altogether. In certain plants such as Ephedra, the reduction in the size of the leaf blades has progressed so far that the leaves in Ephedra are probably vestigial (only a trace, the remaining sign that the leaf blades were there). In Australian, acacia, leaf blades have been lost entirely; the photosynthetic function is taken over by the expanded petioles (phyllodes) or stems or both.

(7) Change in the Size and Shape of the Cells:

The characteristics of the cells of xerophytic plants are that they are relatively small in size with small vacuoles.

Mosses, algae, lichens and other lower organisms adapted to dry habitats have always cells with small volume, some being only 50 or 300 or 700 cu. microns in size. The cell volume of higher plants not adapted to drought is between 2-3 million cu. microns. Consequently when desiccated small cells decrease their volumes to nearly half, compared to larger cells, where the reduction may be as high as 5-10 times. As a result, the former suffer much less disturbance compared to the latter (large cells) when a cell is desiccated and water is lost from the vacuole. There is thus a lesser readjustment of walls to each other in smaller cells than in larger ones.

Buds of higher plants are quite resistant to drought. Their cells are completely devoid of vacuoles. As these buds develop, vacuoles are gradually formed and simul­taneously the natural resistance to drought of buds decreases. Germinated seeds behave in a similar way.

Cells having a larger proportion of protoplasm and consequently smaller vacuole are least disturbed by loss of water and are also protected against injury. Cells of hiber­nating or storage organs and reproductive structures, e.g., spores, zygotes and seeds generally lack vacuoles which help them to survive through drought conditions.

The shape of the cell is also a factor in resisting drought. The spherical and cubical cells of plants not adapted to desiccation have their walls further apart compared to elongated cells of many xerophytes. Certain xerophytic mosses have cells which are much elongated with narrow cell cavity; their opposite walls are almost in contact. Thus there may be almost no change in shape of such cells when water is lost from them.

Vegetative organs of ferns and certain angiosperms can remain in a dry state and can again revive in water supply without any apparent injury to their cells. Though the cells of these organs have comparatively larger vacuoles, the protoplast of these cells is never violently disturbed as the cells are actually protected against such injury by the extraordinarily firm nature of the cell vacuoles which resist desiccation.

(8) Metabolism:

Xerophytism is expressed in many ways due to changed metabolism of the cells owing to desiccating conditions. These result, as we know, in thickened cell walls and in the formation of protective coverings and these may again be due to accelerated conver­sion of polysaccharides into their anhydrous forms, e.g., cellulose, etc.

Desiccation may also promote development of suberised cork cells. Desiccation actually promotes the deve­lopment of cork—thickness of bark is greatest in desert plants compared to plants grown in moist conditions. In xerophytes conducting vessels are, as a rule, well developed and the heavily thickened vessels are more numerous, larger in diameter and longer.

As with cutin, lignification of the vessels in xerophytes begins much earlier than in mesophytic vessels. Annual rings of growth are more pronounced in xerophytes than in mesophytes. Lignified bast fibres and sclerenchyma sometimes reach their highest achievement in the xerophytes. In xerophytic leaves often even the palisade cells are partly lignified, and accumulation of gums and tannins may be of frequent occurrence in the lignified fibres or cutinised cells, constituting the hypodermis of many xerophytes.

Desert plants usually have a dull greyish colour quite in contrast to the bright green of mesophytes and hydrophytes which may probably be due to thicker epidermal coverings and deep-seated chloroplasts or it may also be due to chloroplasts being paler and fewer in number.

Enzymatic activity is generally greatly affected by the water-deficit conditions of the tissues of xerophytic plants. The activities of enzymes such as catalase and peroxidase have been found to be relatively higher in the tissues of xerophytes (is much of the respiration of these plants peroxisomal?) compared to plants growing in normal supply of water. The same is perhaps also true of the hydrolytic activity of the starch-splitting enzyme, amylase.

In general, drought-resistant plants have higher respiratory rates than plants growing in abundant supply of water. It is probable that they maintain the high water-content of the protoplasmic colloids with water molecules obtained from intensi­fied respiration (?).

There is perhaps an inhibition of normal carbohydrate-phosphorus metabolism— corroborating the conclusion of Zholkevich (1960) that drought first of all disturbs phosphorylation process.

There seems to be a relative acceleration of hydrolytic processes (in some cases leading to accumulation of reducing sugars) with a simultaneous decrease in synthetic processes in the tissues of many xerophytes investigated. Xerophytes seem to use a lesser quantity of respiratory substrates than do mesophytes. The enzymatic equilibrium is perhaps not quite normal in xerophytic plants.

Dry air and soil conditions seem to favour production of male flowers and a ratio of 10: 1 of male and female, is sometimes exhibited.

(9) Genetically:

In the cells of many xerophytic plants there seems to be considerable reduction in the number of terminal junctions and chiasmata during the conjugating phase of reduc­tion division. The conjugation phase itself may also be inhibited to a certain extent in water-deficient xerophytic plants.

Many plants enduring water scarcity conditions of the soil have been found to be polyploids. It is probable that polyploids which show a wider range of variability in osmotic pressure values of their tissues are more suitably adapted to water-scarcity conditions than diploids.

In concluding this discussion of plants adapted to conditions of great scarcity of water, it becomes apparent that there are no definite anatomical or physiological characteristics common to all members of this ecological class. Each species seems to have solved its own water requirement (really water balance) problem by its own peculiar and even almost specific combination of adaptive characters.

Ecological literature concerning adaptation of plants for resistance to drought contains many statements which have proved to be quite untenable in the light of modern critical experimental evidence. Early views were essentially abstract, philosophical interpretations of various morphological and anatomical characters associated with xerophytes. It was sublimely reasoned that because the xerophytes had to conserve on their limited supply of water they must have developed structural adaptations to enable them to transpire very slowly.

Nothing is really further from the truth for most-xero­phytes have higher rates of transpiration (if there is any water to lose) with a greater number of stomata per unit area than mesophytes.

Another misconception, long current in ecological literature, was that xerophytes are more efficient in reducing the soil water-content—could deplete it to a lower per­centage before showing permanent wilting—than mesophytes. Critical experimentations, however, have conclusively proved that there is no valid evidence for such a belief.