Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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transport, which requires energy. Water transport, on the other hand, is mostly passive,

The leaf is the main photosynthetic organ of the plant. Its large surface area is designed

to capture sunlight. A cross section through the leaf (Figure 7.4) shows that the upper and

lower surfaces of the leaf are formed of a layer of cells called the epidermis, which is

covered by a waxy cuticle to minimize water loss. The cells between these two layers

form the mesophyll, which has two parts. The palisade parenchyma form vertical col-

umns under the upper surface. They are the main photosynthetic tissues of the leaf. The

rest of the mesophyll consists of spongy parenchyma, which has a about 15 to 40% void

volume ﬁlled with air and a large surface area for exchange of carbon dioxide, oxygen,

and water vapor.

The epidermis is punctuated by special pores called stomata (singular, stoma or

stomate). There can be as many as 12,000 stomata per square centimeter, mostly on

the lower surface of the leaf, where they occupy about 1% of the leaf surface. Each

stoma is made up of a pair of banana-shaped guard cells. The guard cells have chloro-

plasts, whereas the other epidermal cells do not. The stomata control the passage of gases

into and out of the leaf, thus affecting photosynthesis and water use. They also affect the

transport of pollutants into the leaf. When the guard cells accumulate water, they swell,

causing them to bow outward, opening the pore. Conversely, loss of water causes the pore

to close. This occurs in response to several conditions. First, and most obviously, if water

is scarce, a general wilting will cause the stomates to close, limiting water loss. This gives

the plant control over transpiration , the evaporation of water from the plant. This ‘‘loss’’

is actually necessary for the plant’s survival, giving it the means to transport minerals

Figure 7.3 Microscopic view of root with root hairs. (From Simpson and Orgazaly, 1995.)

148

THE PLANTS

Figure 7.4 Cross section of a leaf, showing layers, a vein, and a stoma. (From Simpson and Orgazaly, 1995.)

149

from the roots. A ﬁeld of corn in Kansas transpires the equivalent of 28 cm of rain per

year. A stand of red maple transpires an estimated 72 cm. These ﬁgures are far in excess

of the water actually used by the plant in its tissues or for photosynthesis.

When water is not limiting, the plant needs the stomata to open to admit CO

for

photosynthesis. When light is present, the plant actively transports salts into the guard

cells. This increases their osmotic potential, drawing in water and opening the pore.

Photosynthesis in the guard cells also contributes to their osmotic potential by producing

glucose in the cytoplasm. Temperatures above 30 to 35



C or high CO

levels can cause

stoma closing. Because of all these effects, a plant may close its stomata at night, open

them in the morning, then close them again in the heat of the midday, ﬁnally to open them

again when the afternoon cools. The arid-climate CAM plants are different: They

open their stomata at night and close them all day to conserve water. CO

is stored during

the night for use in photo synthesis d uring the day.

An interesting problem for tall plants is how they are able to move water from the soil

to great heights, as high as 111 m. Either the water is pushed by the roots, in which case

the water column would be under pressure, or it is drawn from the top, which would place

the water in the xylem under vacuum. Experiments show that when the xylem is punctu-

red, air is drawn in. This eliminates the pressure hypothesis. However, it is a simple

hydrodynamic fact that even a perfect vacuum pump cannot raise water much more

than about 10 m. The accepted explanation is called the cohesion-tension theory.

The suction is provided by evaporation of water from small pores in the leaves. The

water column is prevented from breaking because of adhesion to the walls of the

xylem. Experiments have shown that water in a ﬁne capillary can withstand a tension

of up to 26.4 megapascal (MPa). But only 2.0 MPa (about 20 atm) is needed to

lift water to the top of a tall redwood. (Actually, the roots of many plants do produce

some pressure. Th ey do so by active tran sport of ions into the xylem, producing osmotic

pressure. Root pressures as high as 0.3 to 0.5 MPa have been measured.)

Transport of sugars via the phloem is described by the pressure-ﬂow hypothesis. First,

the sugars (mostly sucrose) are moved into the phloem from leaves or storage by active

transport. This increases the osmotic potential, causing water to be absorbed into the

phloem, which generates hydrostatic pressure. The pressure causes the ﬂuid to ﬂow

toward tissues that are removing sugars. The removal of sugar at those sites causes

water to leave the phloem osmotically at that point. Most of the water then returns to

the xylem to complete the circulation. Thus, plants, like animals, can be thought of as

having a complete circulatory system, although it is not a single closed system.

7.2.2 Plant Growth and Control

Plants produce a number of chemical messengers, called hormones, which affect the way

the plant grows. There are ﬁve main hormones or groups of hormones:

1. Gibberellins promote growth, especially the lengthening of stems and formation of

fruit and leaves.

2. Auxins promote growth by stimulating cells to elongate. They control the bending

of plants toward light. They are used commercially to stimulate plant cuttings to

form roots. Auxins are produced by the growing tip of the plan t and diffuse

downward through the plant. Pruning encourages the formation of lateral buds by

150 THE PLANTS

removing the auxin-forming tip of the plant. Developing seeds produce auxin, which

stimulates the formation of the fruit. There is only one naturally occurring auxin:

indole-3-acetic acid, which is chemically similar to the amino acid tryptophan.

However, many synthetic analogs have been developed, including the herbicides

2,4-D and 2,4,5-T.

3. Cytokinins promote growth by stimulating cell division. They have an effect on

differentiation of plan t cells. Leaves that turn yellow after being picked can be kept

green longer by treatment with cytokinins. Chemically, they are derivatives of

adenine.

4. Abscisic acid has the opposite effect of some of the other hormones: It inhibits

growth and development. It is produced during water stress, causing the stomata to

close and thereby inhibiting photosynthesis. It is involved in causing the seed

embryo to become dormant so that it does not germinate prematurely. It also

stimulates. A some processes, such as protein storage in seeds.

5. Ethylene is the simple hydrocarbon H



. It stimulates maturation, promoting

the ripening of fruit and the dropping of fruit, leaves, and ﬂowers. Being a gas, it is

released to the air by plants and ripening fruit. This explains the adage ‘‘one bad

apple spoils the bunch,’’ as ethylene from o ne fruit hast ens ripening in the others.

Plants also release it when injured. Ethylene is used commercially to stimulate

ripening of fruit such as the tomato that were picked green so that they could be

transported to market while ﬁrm and less liable to damage. Ethylene is act ive at air

concentrations as low as 0.06 ppmv (parts per million by volume).

Plants also change their growth patterns in response to environmental stimuli, such as

by light and gravity. The response to gravity is called geotropism. If a potted plant is

placed on its side, the cells on the lower side of the stem will elongate, causing the

stem to bend upward. Auxin seems to be involved. The turning of plants toward the

light is called phototropism. Experiments have established that light coming from

the side, especially blue light, causes auxin to migrate to the shadow side of a stem.

This causes elongation on the shaded side, bending the plant toward the light.

Plants exhibit another behavior, familiar to those who raise houseplants, cal led photo-

periodism, in which the length of the night controls when the plant ﬂowers. Plants exhibit

one of three photoperiodism behaviors: Short-day plants set ﬂower only when the length

of the night exceeds a cri tical period, which varies from plant to plant. Examples include

some chrysanthemums, poinsettias, and strawberries. Short-day plants ﬂower in the early

spring or fall. Ragweed, for example, blooms in the fall and needs at least 9.5 hours of

continuous darkness. Interestingly, a single ﬂash of light in the middle of the night can

fool a short-day plant and prevent its ﬂowering. Long-day plants require a period of dark-

ness less than some critical value and tend to ﬂower in the summer. Sp inach, lettuce, and

some varieties of potato and wheat are long-day plants. Spina ch will bloom only if there

is less than 10 hours of darkness. In contrast to short-day plants, a ﬂash of light during a

long night can fool a long-day plant into ﬂowering. Finally, there are day-neutral plants,

such as cucumber, sunﬂower, rice, and corn, which are not controlled by photoperiod.

Photoperiodism is controlled by a membrane- bound protein complex called phytochrome

that acts as a detector for light. Plants also use phytochrome to detect if light is totally

absent, such as if the plant is shaded by other plants or by a fallen log. In response, plants

do not produce chlorophyll but instead, devote their energy to growing longer, seeking

STRUCTURE AND PHYSIOLOGY OF ANGIOSPERMS 151

TABLE 7.2 Elemental Plant Nutrients

Form in Which Typical Plant Concentration

Element Absorbed (% or ppm dry weight) Functions

Macronutrients

Carbon CO

44% Component of organic compounds

Oxygen H

OorO

44% Component of organic compounds

Hydrogen H

O 6% Component of organic compounds

Nitrogen NO



or NH

1–4% Component of amino acids, proteins, nucleotides, chlorophylls, and coenzymes

Potassium K

0.5–6% Involved in osmosis and ionic balance, stomata opening and closing, and coenzymes

Calcium Ca

2þ

0.2–3.5% Cell wall component; enzyme cofactor, involved in membrane permeability

Phosphorus H



or HPO

2

0.1–0.8% Component of energy-storing compounds (ATP), nucleic acids, several essential

coenzymes, phospholipids

Magnesium Mg

2þ

0.1–0.8% Part of the chlorophyll molecule; activator of many enzymes

Sulfur SO

2

0.05–1% Component of some amino acids and coenzyme A

Micronutrients

Iron Fe

2þ

or Fe

3þ

25–300 ppm Required for chlorophyll synthesis; component of cytochromes and nitrogenase

Chlorine Cl



100–10,000 ppm Involved in osmosis and ionic balance; probably essential in photosynthetic reactions

that produce oxygen

Copper Cu

2þ

4–30 ppm Activator or component of some enzymes

Manganese Mn

2þ

15–800 ppm Activator of some enzymes; required for integrity of chloroplast membrane and for

oxygen release in photosynthesis

Zinc Zn

2þ

15–100 ppm Activator or component of many enzymes

Molybdenum MoO

2

0.1–5.0 ppm Required for nitrogen ﬁxation and nitrate reduction

Boron B(OH)

or B(OH)



5–75 ppm Inﬂuences Ca

2þ

utilization, nucleic acid synthesis, and membrane integrity

Essential to some

plants

Sodium Na

Trace Involved in osmotic and ionic balance; probably not essential for many plants;

required by some desert and salt marsh species and may be required by all plants

that utilize the C

photosynthesis pathway

Cobalt Co

2þ

Trace Required by nitrogen-ﬁxing microorganisms

Source: Raven et al. (1992), Table 27.2.

152

the sunlight. The result can be seen in the spindly, yellow growth of grass that has been

covered by a board for some days. Some seeds require total darkn ess in order to germi-

nate; others, such as lettuce, grow only if exposed to light. The difference relates to

whether the seeds need to be buried or on the surface of the soil to germinate. This beha-

vior is also controlled by phyto chromes.

Plants also show 24-hour cycles in movements of their leaves that persist even if light

cues to day length are removed. Some plants open their ﬂowers in the morning; others

fold their leaves at night. In the absence of light cues, the timing is not precise. It ranges

from 21 to 27 hours. These cycles are called circadian rhythms and have been referred

to as biological clocks. Circadian rhythms are now known to occur in all eukaryotes,

including animals. However, they are absent in prokaryotes.

The herbicides 2,4-dichlorophenoxyaceticacid (2,4-D) and 2,4,5-trichlorophenoxyace-

tic (2,4,5- T) were active ingredients in Agent Orange, the dioxin-contaminated herbicide

used in the Vietnam war to deprive the enemy of their forest cover. The use and manu-

facture of 2,4,5-T is now banned in the United States. 2,4-D is still used in household

broadleaf weedkiller, because dicots (such as dandelion) are much more sensitive to it.

They act by causing the plant to grow out of control, plugging the phloem. Another impor-

tant herbicide is glyphosate, which is sold by Monsanto under the name Roundup.

Glyphosate effectively kills all plants, although it is nontoxic to animals. It acts by

blocking the action of a single enzyme involved in the production of aromatic amino

acids.

7.2.3 Plant Nutrition

Plants, being autotrophs, do not require organic food sources. In fact, they do not need any

organic growth factors. They can synthesize all of the amino acids and vitamins they

require. Thus, their only known nutritional requirements are the inorganic nutrients listed

in Table 7.2. Besides roles as components of organic compounds, the elements are used as

enzyme cofactors, as intermediates in electron-transfer reactions, and for regulating

osmotic pressure and membrane transport.

Nitrogen is absorbed by plants as either nitrate or ammonium. If it is in the form of

nitrate, it is reduced to ammonium by the plant. This requires energy. The ammonium

is then transferred to organic compounds such as amino acids, in a process known as ami-

nation. These processes occur mostly in the roots. Forms of nitrogen rising toward the

shoot in the xylem are almost all organic, mostly amino acids.

Plants can also take up toxic minerals such as lead or some radionuclides from the

soil. This is being exploited in the phytoremediation of contaminated land (see

Section 16.2.4).

PROBLEMS

1.1. By looking at the Table 7.2, can you ﬁnd any minerals that may be soil pollutants and

that might be removed from soil by plants? Are there any minerals not in the table

that are more toxic, yet have similar chemical behavior?

1.2. Sometimes industrial sites can be seen to have large areas of soil with few or no

plants. Other than the presence of toxic substances, what could account for this?

PROBLEMS 153

REFERENCES

Fried, G., 1990. Schaum’s Outline: Theory and Problems of Biology, McGraw-Hill, New York.

Raven, P. H., R. F. Evert, and S. E. Eichhorn, 1992. Biology of Plants, Worth Publishers, New York.

Simpson, B. B., and M. C. Ogorzaly, 1995. Economic Botany: Plants in Our World, McGraw-Hill,

New York.

Weier, T. E., C. R. Stocking, and M. G. Barbour, 1970. Botany: An Introduction to Plant Biology,

Wiley, New York.

154

THE PLANTS

THE ANIMALS

Animals are multicellular heterotrophs whose cells lack cell walls and that have a motile

stage at some part of their life cycle. They are diploid and reproduce primarily by sexual

reproduction. Some of the int eractions between animals and the environmental effects of

human activities are:

 Loss of wildlife h abitat may cause extinction.

 Overharvesting may lead to depletion or extinction of species.

 Animals may be reservoirs for human diseases.

 Control of pests may cause pollution.

The animal kingdom is divided into 33 phyla. A few of them include familiar groups or

organisms. However, quite a few are relatively unfamiliar, and most of these are small

marine organisms. About 1.5 million animal species have been named, but these are prob-

ably only a fraction of those that exist. Table 8.1 summarizes the animal phyla, grouped

according to an informal subdivision system.

8.1 REPRODUCTIVE STRATEGIES

Animals have a great variety of means of reproduction. Only a few of the simplest

animals reproduce asexually. Cnidarians, such as hydra, reproduce by budding, the

unequal division of an organism. Many invertebrates, such as sea stars, can reproduce

by fragmentation, in which the organism can simply be broken in two, with each part

then regenerating the missing portion. Asexual reproduction produces clones, individuals

Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman

155

TABLE 8.1 Summary of Animal Phyla

Phylum Description

Mesozoa Small worms 0.5 to 7 mm long, composed of only 20 to 30 cells; exclusively

marine parasites.

Porifera Sponges, 5000 species (150 freshwater) made of gelatinous matrix with

calcium carbonate or silicate skeleton. Flagellated cells pump as much as

1500 L of water per day to capture food.

Placozoa Contains a single species, Trichoplax adhaerens, a small (2 to 3 mm

diameter) plate like organism, it glides over food, secreting digestive

enzymes, absorbing the products.

Coelenterates (the Only Animals with Radial Symmetry)

Cnidaria Mostly marine, includes jellyﬁsh, corals, anemones, and freshwater hydras.

First phylum with specialized tissues (e.g., nerve cells).

Ctenophora Common name comb jellies. Fewer than 100 species known. Only biradial

phylum.

Flatworms, etc.: More Complex Organ Specialization

Platyhelminthes Flatworms and ﬂukes, turbellarians (e.g., Planaria), trematodes (e.g.,

Schistosomona), tapeworms.

Gnathostomulida Jaw worms: found in sediments, where they tolerate low oxygen levels; feed

on fungi and bacteria scraped from rocks.

Nemertea Ribbon worms: simplest organisms with a circulatory system.

Vermiform (Worm-Shaped)

Rotifera Rings of beating cilia draw food to the mouth, giving impression of rotating

wheels.

Nematoda Roundworms; includes organism that causes trichinosis.

Gastrotricha

Kinorhynca

Loricifera

These resemble rotifers, but without ‘‘wheel’’ of cilia. Together with rotifers

and nematodes, form a group called aschelminthes, and are common

aquatic organisms.

Nematomorpha Horsehair worms, include 250 species that are arthropod parasites.

Acanthocephala Spiny-headed worms; all are internal parasites that cause painful intestinal

injury.

Entoprocta Sessile organisms that superﬁcially resemble hydra, but with ciliated

tentacles.

Priapulida 15 species of cold-water marine worm.

Development of a ‘‘True’’ Body Cavity

Mollusca Bivalves (e.g., clams), octopus and squid, snails and slugs.

Annelida Segmented worms: polychaetes (marine worms), oligochaetes (e.g.,

earthworms, tubifex), leeches.

Arthropoda Insects, crustaceans, spiders.

Lesser Protostomes: Minor Ecological Signiﬁcance

Echiurida

Sipunculida

Tardigrada Water bears, common in aquatic habitats.

Pentastomida

Onychophora

Pogonophora Includes tube worms from deep-sea vent communities.

156

THE ANIMALS

that are genetically identical to the ‘‘parent.’’ It has the advantage that it can produc e new

individuals rapidly. However, it reduces genetic variablity and the ability of a population

to adjust to drastically changed environmental conditions.

Sexual reproduction is the formation of a new individual from the union of two

potentially genetically distinct gametes. The larger, nonmotile gamete is called the egg,

produced by meiotic division in female sex organs. In animals the smaller motile gam ete

is the sperm, which is produced by meiosis in male sex organs. The haploid gametes

combine in the process of fertilization to form a diploid single cell called the zygote,

which will develop into a new individual.

The most familiar form of sexual reproduction is bisexual reproduction, in which the

male and female organs that produce the gametes are on separate individuals. Species that

use this strategy are called dioecious. Almost all vertebrates, and many invertebrates, are

dioecious. Mating requires time, energy, and coordination and results in offspring that do

not share all the genes of either of its parents. Nevertheless, sexual reproduction confers

on a species a great ability to adjust to environmental change.

Some animals reproduce by parthenogenesis (virgin origin). No fertilization occurs,

although the diploid condition is restored by chromosome doubling. Some ﬂatworms, roti-

fers, insects, and even ﬁsh reproduce parthenogenetically. Among the bees, a queen may

allow only some of her eggs to be fertilized. The fer tilized eggs develop into diploid

female worker bees; the haploid males are drones. Certain strains of turkeys have been

found capable of parthenogenetic reproduction. Even in mammals, such as mice, eggs

will on rare occasions start to develop into embryos and fetuses without fertilization.

However, they have never been found to survive to full term.

Some animals are hermaph rodites, having both male and female sex organs on the

same individual. They are also referred to as monoecious. This is a reproduction strategy

that is appropriate for organisms that are sessile, such as barnacles, or live in burrows or

parasitically within other organisms, such as the tapeworm. The advantage of this is that it

makes it possible for every individual to bear young, not just half the population. Some

hermaphroditic ﬁsh actually start out as a member of one sex, then change to the other

during their life span.

Animal Weeds: Sessile Wormlike Marine Animals

Phoronida Only 10 known species.

Ectoprocta Moss animals.

Brachiopoda The ‘‘lamp shells’’, resemble bivalves.

Deuterostomes

Chaetognatha Arrowworms: wormlike planktonic predators, 2.5 to 10 cm long.

Hemichordata Wormlike organisms.

Echinodermata Sea stars, sea urchins, etc.

Higher Animals

Chordata Tunicates, lancelets, vertebrates.

TABLE 8.1 (Continued)

Phylum Description

REPRODUCTIVE STRATEGIES 157