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Classification of Plants
The newer classification system lists all of the more than 300,000 known plants in just
two phyla, the Bryophytes and the Tracheophytes. Bryophytes, the mosses and liverworts, are
usually soft and nonwoody in structure, take in water through short root-like filaments called
rhizoids, and may have stems and simple leaves but, unlike the more complex Tracheophytes, do
note have true roots or vascular tissue whose function it is to circulate water, food, and essential
minerals throughout the organism.
Tracheophytes are divided into four sub-phyla: lycopsids, which number some 900 living species;
sphenopsids, whose fossil species contributed to coal formation in the Carboniferous period,
but which have few living species; psilopsids, an extinct group of relatively simple plants,
which fossil studies show to have been more advanced than any of the mosses; and pterosids,
subdivided into three classes, the ferns, the gymnosperms, and the angiosperms.
The angiosperms, the most highly developed and complex class of plants, reproduce by means
of single and double seed leaves called cotyledons. Monocots, such as corn, wheat, lilies, and
orchids, have leaves with parallel veins, while dicots, which include oaks, maples, roses and
thistles, among others, have net-veined leaves and stems with annual growth rings.
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Adaptations of Desert Plants
Desert plant populations have evolved sophisticated physiological and behavioral traits
that aid survival in arid conditions. Some send out long, unusually deep taproots; others utilize
shallow but widespread roots, which allow them to absorb large intermittent flows of water.
Certain plants protect their access to water. The creosote bush produces a potent root toxin
which inhibits the growth of competing root systems. Daytime closure of stomata exemplifies
a further genetic adaptation; guard cells work to minimize daytime water loss, later allowing
the stomata to open when conditions are more favorable to gas exchange with the environment.
Certain adaptations reflect the principle that a large surface area facilitates water and
gas exchange. Mose plants have small leaves, modified leaves (spines), or no leaves at all.
The main food-producing oragn is not the leaf but the stem, which is often green and non-woody.
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Thick, waxy stems and cuticles, seen in succulents such as cacti and agaves, also help conserve
water. Spines and thorns (modified branches) protect against predators and also minimize water
loss.
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Leaves
Microbiological activity clearly affects the mechanical strength of leaves. Although it cannot be denied that with most species the loss of mechanical strength
is the result of both invertebrate feeding and microbiological breakdown, the example of Fagus sylvatican illustrates loss without any sign of invertebrate attack being evident. Fagus shows little sign of invertebrate attack even after being exposed for eight months in either lake or stream environment, but results
of the rolling fragmentation experiment show that loss of mechanical strength,
even in this apparently resistant species, is considerable.
Most species appear to exhibit a higher rate of degradation in the stream environment than in the lake. This is perhaps most clearly shown in the case of Alnus. Examination of the type of destruction suggests that the cause for the greater loss of material in the streamprocessed leaves is a combination of both biological and mechanical degradation. The leaves exhibit an angular fragmentation,
which is characteristic of mechanical damage, rather than the rounded holes typical of the attack by large particle feeders. As the leaves become less strong,
the fluid forces acting on the stream nylon cages caused successively greater fragmentation.
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Mechanical fragmenation, like biological breakdown, is to some extent influenced
by leaf structure and form. In some leaves with a strong midrib, the lamina break up, but the pieces remain attached by means of the midrib. One type of leaf
may break clean while another tears off and is easily destroyed once the tissues
are weakened by microbial attack.
In most species, the mechanical breakdown will take the form of gradual attrition at the margins. If the energy of the environment is sufficiently high, brittle species may be broken across the midrib, something that rarely happens with
more pliable leaves. The result of attrition is that, where the areas of the whole leaves follow a normal distribution, a bimodal distribution is produced, one
peak composed mainly of the fragmented pieces, the other of the larger remains.
To test the theory that a thin leaf has only half the chance of a thick on for
entering the fossil record, all other things being equal, Ferguson (1971) cut discs of fresh leaves from 11 species of different leaf thickness and rotated them with sand and water in a revolving drum. Each run lasted 100 hours and was repeated three times, but even after this treatment, all species showed little sign of wear. It therefore seems unlikely that leaf thickness alone, without substantial microbial preconditioning, contributes much to the probability that a
leaf will enter a depositional environment in a recognizable form. The result of experiments with whole fresh leaves show that they are more resistant to fragmentation than leaves exposed to microbiological attack. Unless the leaf is exc-
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eptionally large of small, leaf size and thickness are not likely to be as critical in determining the preservation potential of a leaf type as the rate of microbiological degradation.
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Plants and Geography
Although different plants have varying environmental requirements because of physiological differences, there are certain plant species that are found associated with relatively extensive geographical areas. The distribution of plants depends upon a number of factors among which are (1) length of daylight and darkness, (2) temperature means and extremes, (3) length of growing season, and (4)
precipitation amounts, types, and distribution.
Daylight and darkness are the keys by which a plant regulates its cycle. It is
not always obvious how the triggering factor works, but experiments have shown
day length to be a key. A case in point is that many greenhouse plants bloom only in the spring without being influenced by outside conditions other than light. Normally, the plants keyed to daylight and darkness phenomena are restricted
to particular latitudes.
In one way or another, every plant is affected by temperature. Some species are
killed by frost; others require frost and cold conditions to fruit. Orange blossoms are killed by frost, but cherry blossoms will develop only if the buds have
been adequately chilled for an appropriate time. Often the accumulation of degr-
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ees or the direction of temperatures above or below a specific figure critically
affects plants. Plant distributions are often compared with isotherms to suggest
the temperature limits and ranges for different species. The world's great vegetation zones are closely aligned with temperature belts.
Different plant species adjust to seasonal changes in different ways. Some make
the adjustment by retarding growth and arresting vital functions during winter.
This may result in the leaf fall of middle latitude deciduous trees. Other plants disappear entirely at the end of the growing season and only reappear through
their seeds. These are the annuals, and they form a striking contrast to the perennials, which live from one season to another.
Precipitation supplies the necessary soil water for plants, which take it in at
the roots. All plants have some limiting moisture stress level beyond which they
must become inactive or die. Drought resistant plants have a variety of defenses
against moisture deficiencies, but hydrophytes, which also are adapted to humid
environments, have hardly any defense against a water shortage.
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