Xerophyte

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As xerophytes (from ancient Greek ξηρός Xeros "dry" and φυτόν Phyton "plant") an organizational type will plant called, which are adapted to extremely dry locations. More precisely, they are plants that are adapted to a complete scarcity of liquid water. This scarcity can be due to climatic factors, such as in arid to semi-arid climatic zones , where water is generally lacking, or in arctic-alpine zones, where water is not available in liquid form that the plants need. The water shortagecan also be attributed to the nature of the soil, such as sandy soils (low water retention capacity), clay soils (compacted material, water flows off the surface) or limestone (often many furrows and cracks, water seeps away quickly). The forms of adaptation to the scarcity of liquid water are called xeromorphies . Xerophytes thus separate them from mesophytes and hygrophytes , although the boundaries cannot be clearly identified. The halophytes have morphological adaptations that resemble those of the xerophytes, since the presence of NaCl (table salt) causes a physiological dryness ( water potential ).

Many xeromorphies are based on the reduction of water loss through perspiration. Most of the perspiration occurs when the stomata is opened . However, this transpiration drives the long-distance transport of the water in the plant ( perspiration suction ), so plants are dependent on the opening of the stomata. Xerophytes are adapted to this problem.

Xerophytes can be found in many plant families, such as sour grasses ( Cyperaceae ), thick-leaf plants ( Crassulaceae ) or daisy plants ( Asteraceae ).

Classification

Abraham Fahn and David F. Cutler classify the xerophytes as follows:

Drought escaping (evade dryness)

They are adapted to the conditions in arid regions or at high altitudes in that they go through all development stages (sprouting, flowering, fruiting) within a very short vegetation period, within a few days or weeks.

Drought resisting

"Drought resisting" can still be divided into drought evading and drought enduring .
These herbaceous plants survive unfavorable environmental conditions due to subterranean plant organs , while the aboveground plant parts die off in the dry season.
By means of deep root systems, plants can tap into the groundwater in the region and prevent water stress from occurring in the first place.
  • Drought enduring (withstand drought)
The water storage cells of the succulents can store a lot of water in their vacuoles in a short time and continuously release it to the water-consuming cells in the assimilation parenchyma (photosynthetically active tissue). They must be able to fold their cell walls in an orderly manner so that they are not destroyed in the event of severe water loss.
Succulents store water in the rainy season, their small suction roots usually die off in the dry season, so that no more water is absorbed during the dry season. Exceptions are the hydathodes of the Crassulaceae, which can absorb water distributed on leaf blades. During this phase, succulents use their water reservoirs very sparingly.
  • External water storage
The external water reservoirs include the cisterns of the Bromeliaceae (rain form), those of the antler ferns ( Platycerium ) and the root network of the nest fern ( Asplenium nidus ). Since they are epiphytes , their roots have no contact with the ground and are therefore dependent on rainwater or humidity as sources of water absorption. They absorb the water through suction scales on the leaf blades.

Further adjustments

Further important xeromorphic adaptations are strongly thickened epidermal cells up to multilayered epidermis . Often these plants have a powerful cuticle (e.g. Clivia nobilis ) or, in addition, outgrowths of the cuticle. These outgrowths or hairs do not represent an enlargement of the transpiring surface because they are dead and filled with air. As a result, they often appear as a gray to white coating (e.g. Aechmea fulgens , Peperomia incana ) and reflect some of the light and thus reduce the temperature in the leaf. Since most of the transpiration occurs at the stomata, the creation of windless spaces around the stomata by sinking (e.g. oleander leaves) or the formation of wax tubes around this area reduces perspiration . The steam hoods over the stomata can no longer be removed so easily. In general, a reduction in the transpiring surfaces is advantageous through e.g. B. leaf shedding, reduced leaves to thorns or the formation of compact forms as in Echinocactus grusonii . Special metabolic pathways are also considered adaptations to dry locations, such as B. CAM (crassulacean acid metabolism) and C4 syndrome .

See also

literature