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Xerophytes

plants, xerophytic, dry, structures, reduction, conditions and water

XEROPHYTES, (from Gk. >71:iv, xeros, dry ciarr6y, phyton, plant). Plants whose structure specially fits them to withstand excessive transpiration, especially plants of deserts and sandy or roeky habitats, to which forms the term was Previously restricted. Many recent authors, however, have extended the term to all plants, e.g. those of marshe,.. peat bogs, mi.., which show structures typical of desert plants. Sehimper defines xerophytes as plants which in habit physiologieally dry areas, by which phial Se the plants' inability to get water from the soil is emphasized rather than the actual qunntits of water present. For example, a frozen soil is physiologically dry, as may also be a peat bog and a salt the first self-evidently, the last two because osmotically acting substatices may partially inhibit the natural processes of absorption and conduction of water by plants.

The inure prominent xerophytic structures may be defined as follows: First, reduction in sur face as compared with plants of other areas. This reduction may be permanent, as in the ease of cacti and desert shrubs, or where leaves are entirely absent, reduced to very small dimen sions; or temporary, as illustrated by plants which shed their leaves at the beginning of a season, dry because of heat, as in deserts, or cold, as in high latitudes and altitudes. Second, vertical placement of the foliage organs, illus trated not only by the cacti, in which the chloro phyll work is done in the stems, hut also by the so-called compass plants, where the leaves assume a profile or vertical position instead of the hori zontal. Third, the annual habit is typified by a short life cycle restricted to the rainy season. Fourth, bulb and tuber plants, in which the vegetative development is conspicuous during the rainy season, but ceases during the dry pe riod. Fifth, ligneous tissue. In many xero phytic areas a very high proportion of the plants are trees or shrubs. Sixth. storage organs, as in succulent plants such as cacti, various sedums, and aloes. Seventh, modifications of tissue; for example, hairs as in some alpine plants, such as edelweiss; thick-walled epidermis due to eutinization; waxy deposits, varnish or mineral crusts on the epidermis; cork in woody plants; abundant palisade cells; reduction of air spaces. occurrence of stomata mainly or wholly on the lower surface, away from the light; below the surface, as in the carnation; and protection of stomata by hairs, as in the oleander.

It will be seen that most structures can be re garded as advantageous, since they protect plants against excessive transpiration. For example, if the cork and cutin layers, which are relatively impermeable to water, be removed. the loss of water is tremendously accelerated. Some of these structures can be shown by experiment to be the direct result of xerophytic conditions, although in most cases the exact stimulating cause is not clear. Doubtless the most potent factor is a dry atmosphere. Experiments show that dry air as compared with moist air accelerates the develop ment of cutin, cork, collenehyma, bast, waxy tis sue, incrustations, and lignin. These structures may be produced experimentally in mesophytes grown under xerophytic conditions. It seems, therefore, that many plants have a high degree of plasticity, enabling them to adapt themselves in ease of need to xerophytic conditions. It. must be remembered, however, that such adaptation must be in harmony with physical and chemical laws, and that the plants can in no way overstep the bounds prescribed for them. II is quite likely that the structures whose development has been found to be accelerated in xerophytic conditions are in reality caused mechanically by excessive transpiration, and a consequent deposition of sub stances such as cutin, suberin, lignin, etc., which might otherwise have been held in solution. This deposition, which is thus clue to a mechanical cause, is, however, of direct advantage to the plant. 3lesophvtes which are transferred to xerophytic conditions a•pproach normal xero phytes in the reduction of leaf and stem surfaces. This reduction, while of advantage to the plant, may also doubtless be referred to mechanical or climatic causes. For example, the available food materials may be less abundant in the xero phytic soil, the absorption may be less, and transpiration may be great. All of these con ditions would probably tend to cause reduction. See LEAF.

The xerophytic formations of the world may be subdivided into two great classes: climatic, oc curring over wide areas, and edaphie, occurring over limited areas. For examples of the former, See ALPINE PLANT; ARCTIC REGION; DESERT