Xerophytes are a group of plants that, in the process of evolutionary changes, have adapted to the deficit of moisture in the environment. It is not homogeneous in terms of physiological characteristics. In some, the processes of transpiration are reduced, while in others it is enhanced. Xerophytes have different ways of overcoming drought. P. L. Genkel developed a classification of flora that can tolerate a prolonged absence of moisture.
Succulents
This group includes plants that have adapted to store water in tissues and organs. Bright representatives of xerophytes are cacti and Crassulaceae. A sufficient amount of moisture accumulates in fleshy stems (euphorbia, cactus) and leaves (aloe, juvenile, stonecrop, agave).
Characteristic signs of succulents:
- Reduced surface from which evaporation of moisture occurs.
- Leaves are reduced.
- Thick cuticle restricting transpiration.
- The root system is shallow, but abundantly overgrown.
- There is little cellular juice in the roots.
Succulents occur in areas where the period of heavy rains gives way to prolonged drought. To reduce water loss, stomata open only at night. Lack of water plants of this type do not tolerate well. They are more adapted to heat than to drought, during which they sparingly spend the fluid stored in the tissues.
Euxerophytes
Real xerophytes are plants that can significantly reduce moisture loss when it is deficient. In the process of evolution, euxerophytes received the following adaptations at the cellular level:
- Increased cytoplasm elasticity.
- Reduced water content.
- Increased moisture retention.
- Increased viscosity.
All this helps to absorb moisture from almost dry soil. Sometimes the underground parts and stems of euxerophytes are overgrown with a cork layer. A thick layer of the cuticular membrane covers the leaves of xerophytes. Plants of this group have stomata protection in the form of:
- The recesses in which they are located.
- Resin and wax caps.
- Folding leaves into a straw.
Representatives of euxerophytes: saxaul, sand acacia, aristide, some species of wormwood, etc.
Hemixerophytes
If we analyze the meaning of the word “xerophytes”, we can see that it is derived from the Latin words “dry” and “plant”. Therefore, it is part of the flora adapted to moisture deficient habitats.
What are xerophytes of this group and why are they unique? Hemixerophytes are distinguished by developed devices for extracting water from great depths. Their roots go far underground and branch heavily. In the cells of underground organs there is a negative water potential and highly concentrated cellular juice.
The listed features help to extract moisture from huge volumes of soil. If the aquifer does not lie very deep, then the root system can reach it. The abundance of branching veins on the leaves minimizes the time for the delivery of moisture from the roots to the cells.
In this type of xerophytes, transpiration occurs more intensively than in others. Due to this, the leaves are cooled, and even in heat, photosynthetic reactions occur in them. This is well manifested in steppe alfalfa, wild watermelon, cutter and sage.
Pseudoxerophytes
False xerophytes are plants whose life is so short that they do not find the dry period of the year. The vegetation stage in them coincides with the rainy season. Adverse conditions they experience in the stage of bulbs, seeds, tubers or rhizomes.
Poikyloxerophytes
Poikiloxerophytes include plants that are not able to regulate water metabolism. They are waiting for the dry period in a state of suspended animation. At this time, metabolism does not occur or is very slow.
Ferns, part of algae, most of lichens and some angiosperms belong to poikiloxerophytes. This group is distinguished by the ability of the protoplast to condense to a gel state. After that, being dry to the touch, they continue to live. With the onset of the rainy season, these plants return to their normal state. Losing water for them is not a pathology.
Xerophytes: signs and features
The anatomy of the leaf largely depends on the tier in which it is located. Dependence was called the law of Zalensky, by the name of the physiologist who discovered it. With increasing height above the ground:
- The size of the cell is reduced.
- The stomata clearance decreases.
- The density of veins and stomata increases.
- It becomes more palisade parenchyma.
- Increased transpiration and photosynthesis rate.
The reason for the discovered pattern lies in the deterioration in the supply of moisture to the leaves located on the top. A similar pattern is observed for plants growing in a dry climate. Xeromorphic structure is characteristic for leaves of species adapted for drought.
The transpiration coefficient characterizes how rationally the moisture is consumed. The degree of openness of stomata equally affects both evaporation and the accumulation of dry matter.
Attempts to make plants more resistant to drought are being carried out by genetic engineers and breeders. Other methods are suggested:
- Presowing hardening of seeds: drying them after soaking.
- The application of fertilizers that help retain moisture in the cells.
- Treatment with hormonal drugs.
- Agrotechnical techniques (rolling, spring harrowing, etc.).
Scientists in the development of methods rely on the experience of xerophytes. Studying their structure and metabolic processes, they offer ways to adapt cultivated plants to adverse conditions. As a result, drought-resistant varieties appear in agriculture.