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ECOLOGICAL FACTORS: 



hydrophytes, various plant plankton, only contact 

 water. Submerged anchored hydrophytes, typified by many 

 pondweeds {Potamogeton) and stonewort (Chara), are 

 attached to the substrate but covered by water. 

 Floating-leaved anchored hydrophytes, like the water lily 

 {Nymphaea), have only the upper surfaces of their 

 leaves contacting air. Emergent anchored hydrophytes, 

 such as bullrush (Scirpus) and cattail {Typha), have a 

 larger proportion of the vegetative structures above 

 water. 



Mesophytes have medium available water because 

 they are found in moist soils and climates. They 

 cannot inhabit situations that have either a great 

 deal of water or very little water. For this reason they 

 are not found in water, wet soil, or dry environments. 

 These plants have no structural specialization for 

 their way of life and have low to high water loss, de- 

 pending on environmental conditions. Mesophytes 

 are of two types, sun plants and shade plants . 



Xerophytes are plants having low available mois- 

 ture because the soil is either physically or physiologi- 

 cally dry. They usually are in dry climates but can 

 be in any climate type. As a rule they have a low rate 

 of water loss. Physical dryness of soil is related either 

 to a rock, gravel, or sand matrix, or to climate dry- 

 ness. The dry climates have desert plants that lack 

 moisture much of the year; grassland plants (prairies, 

 plains, steppes, etc.) that lack moisture some of the 

 year; and certain woody vegetation that has low or 

 discontinuous available moisture. Woody vegetation 

 includes bushes, shrubs, and small trees, grouped 

 mostly into scrub, chaparral, and woodland habitats, 

 whose plants have thick and waxy leaves — apparently 

 an adaptation for dry conditions. Physiological dry- 

 ness will be considered with the discussion of soils. 



Xerophytes are of three main types: ephemeral an- 

 nuals, succulents, and nonsucculent perennials. 

 Ephemeral annuals complete their life cycles in a very 

 short period of time. Only a few weeks may be re- 

 quired for germination, growth, and reproduction; 

 the more unfavorable periods are spent in the form of 

 seeds. These plants tend to avoid the dry season, but 

 many can withstand strong atmospheric drought if 

 their soils remain moist. Succulents have tissues and 

 organs specialized to accumulate and store water 

 during wet jDeriods and to release this water during 

 drought. Most of these plants also function at a low 

 rate of transpiration and have shallow root systems, 

 an adaptation to gather moisture from light rains. 

 Nonsucculent perennials might be called the only true 



xerophytes. Unlike the ephemeral annuals and 

 succulents, these perennials have specializations 

 that react against dryness rather than avoiding it. 

 Perhaps most surprising is that many of these plants 

 can endure permanent wilting. Although the woody 

 forms can tolerate only short perids, some herbaceous 

 plants can tolerate years of permanent wilting. Other 

 nonsucculent adaptations are extensive root systems, 

 rapid growth of roots in young plants, reduced trans- 

 piration, and smaller leaves. However, both succu- 

 lents and nonsucculents often are small and greatly 

 spaced. 



ANIMAL-WATER RELATIONS 



Most animals contain 70 to 90 per cent water by 

 body weight, and none are likely to survive the loss of 

 one third of their body water. This is probably the 

 reason why various adaptations are related directly to 

 possible body water loss. Certain of these modifica- 

 tions merely retard water loss; examples include pro- 

 tective coverings and the secretion of dry body wastes. 

 Other adaptations, actually behavior patterns, cause 

 the animal to avoid situations that would exact a high 

 water loss. For example, when desert areas are hot- 

 test and water loss would be greatest, many animals 

 aestivate or are active at night when it is cooler. Also, 

 many of the aestivating and nocturnal animals are 

 burrowing forms, the burrowing habitat providing a 

 retreat from the great heat of the sun. Another 

 specialization is extreme development of the ability 

 to produce water from foods eaten. All animals form 

 such metabolic water but not to the extent that many 

 desert forms can. In fact, some desert animals may 

 obtain all their water from foods eaten, because they 

 are not known to drink even when water is offered in 

 captivity. These adaptations to minimum water 

 environments are also correlated to temperature ex- 

 tremes, hence so-called water adaptations are just 

 as likely to be adaptations to temperature. This 

 correlation again displays the integration of ecological 

 factors and organism responses. 



Overabundance of water can be just as serious an 

 environmental stress as limited moisture. To ap- 

 preciate the problems of excess water one must un- 

 derstand the phenomena of diffusion and osmosis. 

 Diffusion is of common occurrence in mixtures of 

 different kinds of gases or different kinds of liquids. 

 It is the tendency for each component of a mixture to 

 move from any area of higher concentration to any 



