KSN - Vol 23, No 2 - An Ecological Look at Weeds COVER PICTURE: A barbed-wire fence
separates a grassland from a cultivated field
edged with Japanese brome and annual
sunflowers.

Volume 23, Number 2 - December 1976

An Ecological Look at Weeds

by Paul G. Jantzen

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ABOUT THIS ISSUE

Published by Emporia Kansas State College

Prepared and issued by The Division of Biology

Editor: Robert J. Boles

Editorial Committee: James S. Wilson, Gilbert A. Leisman, Thomas Eddy, Robert F. Clarke

Online format by: Terri Weast

The Kansas School Naturalist is sent upon request, free of charge, to Kansas teachers, school board members and administrators, librarians, conservationists, youth leaders, and other adults interested in nature education. Back numbers are sent free as long as supply lasts, except Vol.5, No.3, Poisonous Snakes of Kansas. Copies of this issue may be obtained for 25 cents each postpaid. Send orders to The Kansas School Naturalist, Division of Biology, Emporia Kansas State College, Emporia, Kansas, 66801.

The Kansas School Naturalist is published in October, December, February, and April of each year by the Kansas State Teachers College, 1200 Commercial Street, Emporia, Kansas, 66801. Second-class postage paid at Emporia, Kansas.

"Statement required by the Act of October, 1962: Section 4369, Title 39, United States Code, showing Ownership, Management and Circulation." The Kansas School Naturalist is published in October, December, February, and April. Editorial Office and Publication Office at 1200 Commercial Street, Emporia, Kansas, 66801. The Naturalist is edited and published by the Kansas State Teachers College, Emporia, Kansas. Editor, Robert J. Boles, Department of Biology.


THE AUTHOR

Paul G. Jantzen teaches biology and chemistry at Hillsboro High School, Hillsboro, Kansas 67063. He holds an M.S. degree in biology from Emporia Kansas State College. This is but one of several articles he has written in the field of science education. The photographs are also by the author. 


An Ecological Look at Weeds

by Paul G. Jantzen

In How to Know tbe Weeds, H. E. Jaques (1959) wrote that "the word 'weed' may have many definitions such as an unwanted plant, an unsightly, useless, or harmful plant; any plant growing in cultivated ground to the injury of the crop or desired vegetation; plant growth that gives an unfavorable appearance to an area, etc."

The term weed is most often defined simply as a plant out of place. A.C. Martin (1972) complained that this definition reflects human bias. Actually, he continued, "pest plants are out of place only in terms of man's purposes. In nature's scheme, they often serve useful functions, and judging by their success in wide-open competition with other plants, they are anything but out of place." H. G. Baker (1974) used his own, less biased, more objective definition which says essentially that a weed is a plant whose populations grow spontaneously, in a specified geographical area, predominantly in situations markedly disturbed by man. This definition recognizes that certain plants have evolved to occupy the weed niche. And it correctly infers that numerous plants of prairies and woods are not weeds.

In this issue we will look at weeds from a broad, ecological perspective and encourage the view that man is only one component in a complex system that involves interactions among a multitude of organisms, including weeds. As a child grows to maturity, its exclusive concern for self develops to include an ever-wider circle of people. Likewise, society's basis of concern must expand from humankind to the whole natural community and the total ecosystem, and from biospheres to the entire Planet Earth.

The Origin of Weeds

Weedy characteristics were developed in certain plants by the same forces that developed the unique traits of various other plants. Presently existing plant varieties originated by differential reprodUction. That is, certain plants that were adapted to a specific environment were more likely to live long enough to produce offspring than those that were poorly adapted. And those offspring that inherited the successful combinations of traits were more likely to be parents of the next generation. Since the beginning of agriculture, this natural selection of parents has been supplemented by artificial selection to favor the development of varieties most useful to man for food or fiber. Man's use of hybridization, increased mutation rates, and other techniques now emerging are further increasing the rate at which new varieties of organisms develop.

Agriculture is characterized by monocrop cultures. A farmer decides that in a certain area he wants only wheat plants. He directs all available energy, soil minerals, water, and carbon dioxide into producing wheat kernels. The organic  wholeness, diversity, and balance of the natural community are replaced with an unlikely, unstable condition whose maintenance requires effort. Part of this effort involves the removal of renegade plants -- any plants that would compete with desired species for light energy, water, minerals, and carbon dioxide. These plants are discouraged by tillage, mowing, burning, and other means. This frequent disturbance is one of the features of an agricultural ecosystem. Certain plants are ready to take advantage of these disturbance conditions, perhaps having been preadapted by natural disturbances such as flooding, wind storms, landslides, fire, or animal activity. These opportunists, through natural selection, further developed this role into a specialty. Many of our most troublesome weeds developed through natural selection in older countries where agriculture has existed for many centuries. The constant disturbance of soils associated with agriculture was the guiding factor in the selection process that produced weeds.

Problems Caused by Weeds

Weeds are of particular concern to farmers. Financial losses due to weeds have been estimated to exceed the combined losses from diseases and insect pests of both livestock and crops. These losses are caused in various ways. Weeds reduce crop yields by competing aggressively for space, water, minerals, and light. Some weeds such as dodder grow parasitically on cultivated plants, absorbing from them water and organic food. The presence of weed plants reduces the value of hay and cotton. Weed seeds may lower the value of agricultural seeds with which they are mixed. Some weeds impart off-flavors to milk, butter, cheese, and eggs. Cockleburs lower the value of wool. Poisonous weeds may kill livestock or slow their growth rate. Other weeds injure livestock and cause human discomfort by means of barbed awns, hooks, and burs. Some weeds harbor insect pests and plant disease organisms. Cultivation for control of weeds may damage the crop itself or may reduce soil quality. Weeds make farms unsightly and perennials may reduce property value. Piled-up Russian thistles may destroy fences; pincture-vine burs can puncture bicycle tires and shoes. All of these effects prompt farmers to make huge investments in labor and weed control machinery and chemicals which increases the cost of crop production and reduces net income. Non-agricultural outdoor industries, managers of recreational areas, and lawn and garden owners also spend staggering amounts of time and money to control weeds. Certain weeds affect human health. Many persons become severely allergic to the "sap" of poison ivy and the pollen of ragweeds. Each year some persons experience severe illness or death from eating poisonous portions of certain weeds.

Characteristics of Weeds

How are weeds able to compete so successfully with domestic plants? Why are they so difficult to control? What characteristics do weeds possess that fit them so specifically for disturbed habitats? Baker (1974) compiled a list of characteristics for the "ideal weed."

1. The ideal weed 's germination requirements are fulfilled in many environments. In curly dock and several members of the goosefoot family there are differences in germination requirements in various seeds from the same plant. In cockleburs, two seeds with different germination requirements are produced in each fruit.

2. Seeds of the ideal weed exhibit great longevity and germination mav be interrupted. Many weed seeds remain dormant in the soil for many years and then germinate when conditions are favorable. In an experiment investigating the longevity of seeds buried 20-108 cm deep in the soil, 71 species germinated after one year. After 38 years, 91 percent of the Jimson weed seeds still germinated. Mullein seeds have had a 72 percent germination after 70 years. Lotus seeds found in the deep mud of a cold Manchurain lake were found to be viable after approximately 1000 years of dormancy. (Reports of seeds germinating after having been buried with Egyptian mummies for several thousand years have been largely discredited.)

3. The ideal weed grows rapidly tnrough vegetative phase to flowering. This enables weeds to flourish in conditions that are favorable for only a short time and produce several generations when favorable conditions are prolonged.

4. The ideal weed produces seeds continuously while growing conditions permit. Dandelions flower and bear seed in Kansas throughout most of the year (March through June and from September into December). Domestic wheat produces seeds during only a 2-3 week period in specific areas.

5. The ideal weed is self-compatible but not completely self-pollinating or asexual.

6. When cross-pollinated, the ideal weed uses wind or unspecialized visitors.

7. The ideal weed has very high seed output in favorable circumstances. Klingman (1961) tabulated the numbers of seeds produced per plant. Common ragweed produced 3,380; curly dock, 29,500; shepherd's purse, 38,500; lambsquarters, 72,450; redroot pigweed, 117,400; and common mullein, 223,200. Other authors have reported much larger numbers.

8. The ideal weed produces some seed in a wide range of environmental conditions. In Kansas, horseweed blossoms have been observed on 0.3 meter plants along a dry, packed field road as well as on 2.5 meter plants in moist, mulched soil.

9. The ideal weed has adaptations for both short-and long-range dispersal. Russian thistles and other "tumbleweeds" drop their seeds under the parent plant but may also break off near the base and be blown for miles scattering seed all the way. Other weeds, such as bindweed, entwine themselves around such tumbleweeds and their seeds are likewise scattered over great distances. Rice et al (1960) attributed the earlier invasion of abandoned fields by the weedy triple awn grass than by little bluestem partially to the tendency of the fruits of triple awn to be tumbled for long distances by winds while those of little bluestem were not carried much over two meters from the parent plants even with the high wind velocities common in Oklahoma.

10. If perennial, the ideal weed has vigorous vegetative reproduction or regeneration from fragments. Field bindweed
not only produces seed but also reproduces by rhizomes (underground stems). The root of a dandelion or dock plant cut in half may produce two plants.

11. If perennial, the ideal weed is brittle and not easily pulled from the ground. Have you tried to pull out a dandelion by the roots?

12. The ideal weed has the ability to compete with other species by special means such as choking growth, rosette growth-form, or the production of chemical inhibitors. The flat rosette of the common dandelion allows it to compete effectively with grasses. It avoids the blades of the mower and tile teeth of grazing animals. It withstands the treading action of livestock. It may have access to higher concentrations of carbon dioxide which is heavier than air. A number of weeds have demonstrated the ability to exude growth-inhibiting substances from their roots or leaves.

Minor weeds possess a few of the above traits while major weeds have developed many of them. It is probably fortunate that none have developed all of them. One can understand the difficulty in controlling a major weed especially when it is carried to a new area where its natural enemies are not present. Of the 10 plants officially designated as noxious weeds in Kansas, only two, bur ragweed and pignut, are native, most of the others having originated in Europe.

Migrations of Weeds

The modes of seed dissemination are easily available in the literature and will not be given thorough treatment here. Weeds have developed on their seeds certain accessories such as hooks, floats, parachutes, wings,  adhesives, and tempting fruits which aid in their dissemination by wind, water, or the fur, skin, and digestive tract of animals. Farm machines used in harvesting, tilling, and mowing carry seeds, rhizomes, and stolons from
one field to another where they become established. Weeds have probably been spread most widely through the transporting of crop seeds, grain feeds, hay, and straw. Many weedy species from abroad were introduced in America by early explorers through the seeds scattered in the fodder they brought for their horses. The explorers and early settlers no doubt carried seeds in the dried mud clinging to their boots and agricultural machinery. There were weed seeds present as impurities in the seeds they brought for their fields and gardens. Many seeds probably came in packing material and in the ballast of ships.

In addition to these unwitting modes of dissemination by man's activities were the deliberate transplanting of species from one country to another by botanists, explorers, and travelers for botanical and ornamental purposes.

Benefits of Weeds

In the section entitled "Problems Caused by Weeds," most of the items listed involved ways in which various weedy plants affect man's agricultural activities, his sense of beauty, his comfort and well-being. There are many ways in which individual weeds are beneficial to man. But the value of organisms cannot be viewed only from the bias of man's desires. Each individual in a natural community contributes to the wholeness of the ecosystem. And
perhaps the very act of being establishes a basic worth.

weeds shade the ground preventing over-heating of soil

Weeds shade the ground preventing the over-heating of soil organisms. Weed-covered soil at left reads 37 degrees C while the uncovered surface soil at the right reads 43 degrees C. This photograph was taken on a September forenoon. Afternoon readings in bare soil easily exceeded the range of the thermometer.

see caption below

Velvet-leaf seed pods being transpor ted by a disc.

Weedy plants carry on the same complex activities and functions as do their more respected relatives. Weeds convert solar radiation, water, carbon dioxide, and soil minerals into compounds which serve as food for herbivorous animals. In natural communities, the leaves, stems, roots, flowers , fruits, and seeds of various weeds are eaten by a multitude of organisms. Thus they contribute to the first link of the food chain. Flowers of weeds may provide nectar and pollen for bees and other insects. Recent years have seen a surge of interest in using weeds and other wild plants for human consumption and they are found to be nutritious and tasty. Weeds provide shelter for numerous organisms. Weeds bound the soil preventing wind and water erosion in bison trails and wallows of the past and continue to do so on new earthern dams, livestock paths, and new hlghway rights-of-way of the present. Weeds prevent the sun's rays from over-heating the soil and endangering the lives of soil
organisms. In arid to semi-arid climates, an open canopy of broadleaved weeds boosts grass production because their roots extract moisture from below the grass roots and the taller weeds moderate wind velocities, sunlight intensity, and evaporation. When decomposed, weeds are converted into humus which enriches the soil
and enhances its ability to absorb water. Weeds bring soil minerals from the subsoil to the surface and, when the weed dies, those minerals are made available to seedlings. Weeds play a significant role in conditioning the soil as a disturbed area is abandoned and allowed to progress toward a climax stage of succession. Pasture weeds are often a sign of overgrazing and the kinds of weeds present indicate the degree of degeneration of rangeland. Forage grasses weaken under heavy or ill-timed grazi ng and unpalatable range plants increase in number. If grazing pressure is further increased, invading weeds become established and change the character of the range. In the past, as weeds hybridized with closely related crop plants, they contributed certain desirable, inheritable traits to presently cultivated plants. The chemical inhibition of competing plants observed in wild barleys is present as well in domestic barley with which they have probably had genetic exchange. Many weeds are attractive. The knotweed that carpets trampled areas on farmsteads, along roadways, paths, and alleys is as attractive as many grasses and requires little or no care. Poison ivy was cultivated as an ornamental in Europe during the seventeenth century. Weeds furnish a wide range of hues with which to dye wool for the hobbiest. Various weeds yield compounds that have been used in the healing arts, as stimulants, astringents, cathartics, and emetics. One cannot predict what new uses man will find for weeds in the future.

Biological Control of Weeds

There are six principle methods of weed control. Mechanical control usually involves the use of machines which disturb, bury, or cut off weeds. In crop competition, the best crop production methods are employed to so favor the desired crop that weeds are not given a chance to become established. These methods include proper planting time, seedbed preparation, soil fertility, and optimum spacing. Crop rotation reduces weed growth because drilled grain crops with their characteristic weeds are alternated with row crops with a different weed community which is controlled by cultivation. Fire is sometimes used to remove undesired plants, especially in
grasslands. Chemical herbicides have been widely used in recent decades in the form of soil sterilants, contact
chemicals, and growth regulators. In biological control, man employs an organism that is a natural enemy of the target weed but harmless to desired plants. The natural enemy might be an insect, a disease organism, a parasitic plant, a rodent, a grazing animal, or indeed any organism.

A classical example of biological control of a weed is the case of the prickly pear cactus in Australia. In 1787, Governor Arthur Phillip brought cochineal insects, and prickly pear plants for them to feed on, from Brazil to New South Wales. He imported the insects to produce cochineal with which to dye his soldiers' coats red. Cochineal insects are found in Kansas within the protective, white webby masses adhering to the joints of prickly pears. Squeezing the webby mass results in the discharge of the red dye. With Australia's suitable soil and climate, birds to distribute the seeds, and no apparent enemies, the cactus spread quickly and took over forests and farms. By 1925, 60 million acres of fertile land were heavily infested. In 1920, the Australian government finally accepted an earlier suggestion that entomologists search for insects that depend on the prickly pear for food. About 145 eligible insect species were found and subjected to starvation tests in which various crop plants were substituted for the natural food of the insects. Of the 18 safe species (those found not to use crop plants as food) several were introduced to the cacti. A moth from Argentina emerged as the most important enemy of the prickly pear. Its larvae burrow into the cactus stem and feed on the internal tissue, weakening the plant for quick attack by fungi and bacteria. The first eggs were attached to cactus plants in 1926 and, by 1929, hundreds of acres of land had been cleared. The moth population has now decreased to a small fraction of its former numbers due to the scarcity of the cactus. But the moth remains present and retains its capacity for rapid increase if the prickly pear population suddenly multiplies.

St. John's wort, or Klamath weed, is present in controlled numbers in Kansas. But in 1940 the weed heavily infested five million acres of grazing land in California. Introduction of a leaf-eating beetle from France brought its population under control. Within the last 15 years additional weeds have been investigated in the United States for possible control by insects. Among them are bindweed, Canada thistle, and puncture vine.

Although biological control has usually been employed only after other methods have failed, there are now numerous cases in which a natural enemy has been found to control weed popula tions. Biological control of weeds is more permanent than other methods and is often much less expensive. But the risks are high. Generally, exhaustive inquiry is followed by conservative action. So far, no insect introduced for weed control has become a serious pest.

Weeds and Diversity

In keeping with current ecological thought, the goal of biological control is not eradication, but the reduction of weed populations to economically harmless levels. The eradication of any species would violate a cardinal principle of ecology, the importance of diversity. In addition to contributing stability to the natural community, diversity is the raw material of natural selection. Natural selection, and therefore the continued development of new and better adapted varieties, depends on variability in the population undergoing the ehange. Whether for natural selection, or humanly directed selection, the preservation of genetic variability must have high priority. litis (1972) stated that "in a cultivated plant... variability is usually greatest in its evolutionary 'cradle region,' where wild, weedy, and primitive cultivated forms tend to mingle in... heterozygous hybridizing populations..." In such often remote areas, variability has persisted even if highly specialized forms developed elsewhere. Iltis continued: "Today, 'progressive" agriculture, the 'Green Revolution,' and massive technology, often blindly conspiring with greed, hunger, population pressure, and ignorance, deliberately replace this low-yielding primitive diversity with high-yielding inbred uniformity... The only way we can save the dynamic evolutionary potential of a crop is to protect the diverse 'ancestral' genotypes in their cradle region." Providing this genetic bank is one of the significant roles that the native weedy species of today can contribute to the future. And it is a strong argument for the preservation of numerous, sizeable natural areas -- weeds and all.

Investigating the Characteristics of Weeds

see caption below

A biology student uses a hoola-hoop to
define the study area for determining the
population density of a patch of weeds.

see caption below

Measuring the growth rate of a dandelion
stalk. A s teel rod fitted into the end of a meter stick is thrust into the ground to support the stick.

see caption below

Giant ragweeds form a solid population by
producing a substance which inhibits the
growth of its competitors.

see caption below

A student collects leaves of giant ragweed
to investigate possible presence of chemical
inhibitors.

see caption below

Tomato seeds three days after being placed
on damp filter paper illustrate chemical
warfare among plants. The filter paper on the
left was saturated with water, that on the right with an extract of giant ragweed leaves.

1. Build a circular frame of heavy wire or a piece of old garden hose with a circumference of 2.51 meters to enclose a convenient 0.5 m2. Use the frame to outline spaces in which to determine the population density of common weeds in areas which are frequently disturbed such as the edges of cultivated fields, alleys, gardens, roadsides, footpaths, salt-lick or water supply areas for livestock, overgrazed pastures, or areas subject to occasional flooding. For comparison, include a few relatively undisturbed areas. For each location, record a brief
description, general soil conditions, use, and the population density of specific weeds in terms of plants per
m2. Ask students to suggest common conditions among the areas with the highest weed densities. This might be a good way to introduce them to a study of weeds and lead them to understand Baker's definition of the term "weed."

2. Collect all the seeds from an individual weed plant and set up a germination test. Plant the seeds on three layers of paper toweling cut to fit and placed in petri dishes. Fill each dish with water. After the toweling is saturated, pour out excess water and deposit seeds on the top layer of toweling. Put the lid in place and store in darkness. Each day, record the number of new germinations. Gather cocklebur fruits and determine and compare the germination times of the two seeds in each. What might cause differences in germination time in the different seeds from the same plant or from the same fruit? Of what advantage to a given species might be the different germination times or different germination requirements in various seeds? (See No. 4 for dormancy
conditions.)

3. Obtain seeds of various weedy species and allow them to germinate in a variety of temperatures, light intensities, moisture amounts, and kinds of substrate. Do the same with several farm or garden seeds. Which
seem to be most adaptable? You may use an old refrigerator with adjustable temperature control and a
laboratory incubator or improvised "light bulb incubator."

4. Seed germination does not always immediately follow maturity. The time of delayed germination due to unfavorable temperature or moisture conditions is termed "rest period." Dormancy is a non-germination state resulting from "blocks," or physiological mechanisms that delay germination even when conditions of temperature and moisture seem favorable. The seeds of common ragweed mature in late summer. If its seeds germinated immediately at maturity, the seedling would be killed by winter frosts. This catastrophe is prevented by a dormancy period which is broken only by exposure to low temperatures. Following exposure to low temperature, a warm period encourage s germination. The dormancy of the upper seed of a cocklebur fruit can be broken by increasing the oxygen supply or by breaking the seed coat. Dormancy of pigweed, wild mustard, and shepherd's purse can be broken by drying at 43 degrees C (110 degrees F.), mechanical injury, or removal of the seed coat. Cheat germinates at a temperature of 20-25 degrees C (68-77 degrees F) after either a primary dormancy of four to five weeks, or exposure to a temperature of 15 degrees C (59 degrees F) or below. Leafy spurge, hoary cress, horsenettle, wild oats, and field bindweed germinate best with plenty of oxygen. Light, and even the specific color of light, effects the dormancy of some seeds. The mechanisms of dormancy are just beginning to be understood.

Treat various seeds of a weed species to periods of darkness, light, coldness, warmth, dryness, and cracking of the seed coat. Then do germination tests to see if any of these conditions improved either the speed or the percentage of germination.

5. When the bud of a dandelion first appears, record the time and distance from the base of the bud to the bottom of the stalk. Repeat these observations at regular time intervals. Calculate the growth rate in mm/hr or mm/day. Do the same with shepherd's purse and other available weeds that send up a flowering stalk. A motion picture record or a series of still photographs might also be used to illustrate rapid growing rates.

6. Record the total seed production time observed or noted in the literature for some common weedy species like dandelion, chickweed, and others, and compare these time intervals with those of plants grown as agricultural crops or wild non-weedy plants of prairies and woods. What do these results have to do with weeds' ability to compete with other plants?

7. With a hand lens, study the flowers of common weedy species and try to determine whether pistils and stamens are mature at the same time. That is, are pollen grains issuing from the anthers at the same time that pollen grains are adhering to the stigma of the same flower? How does this relate to self- and cross-pollination? What are the advantages of each kind of pollination?

8. Study the flowers of weeds and try to determine whether the stigma of the pistil is openly available to pollen brought in by wind or any sized insect, or if specialized mechanisms or procedures may be required for pollination. What might this have to do with the role weeds play in an agricultural community?

9. Find a weedy plant which is about ready to disseminate its seeds and count all the seeds on that plant. Or, if your plant is a thistle or an other composite, count all the seeds in one average-sized head and multiply the number of seeds per head times the number of heads on the plant. Observe a dandelion plant from early spring through late fall and count the total number of seeds it produces during one year.

10. Record as many different conditions or environments as you can in which you see dandelions (or some other weed) flower and produce seeds.

11. Devise ways to measure the distance that certain weed seeds are dispersed. Study the seeds of various weeds with a hand lens or a stereoscopic microscope and determine at least one way that each kind is disseminated. In September, transplant a snow-on-the-mountain plant to a can or flowe r pot and keep it in the classroom until it disseminates its seeds. Measure the distance the seeds are thrown.

12. Chop up one plant of Burmuda grass, Johnson grass, quackgrass, field bindweed, Canada thistle, pokeweed, dandelion, dock , or purslane. Allow the parts to air dry for from one to several days and place them in potted soil to see if any of the parts develop roots or show other signs of life and growth.

13. Devise a way to test whether the carbon dioxide concentration of air is greater at ground level than at heights of 10 cm, 20 cm, and 100 cm. Try it on a calm day.

14. Using a blender or a mortar and pestle, grind up pieces of the rhizomes or the roots of quackgrass, adding 100 ml distilled water per 10 g fresh plant material. Use this material (either filtered through several layers of cheese cloth or left as prepared) to water alfalfa seedlings planted in soil. Have a control culture of alfalfa seedlings to which just water is added. Do the rhizomes or roots of quackgrass produce a substance that inhibits the growth of alfalfa?

Try extracts of the roots of Jimsonweed or the stems, leaves, or roots of annual, perennial, or giant ragweed, annual sunflower, horseweed, mugwort sage, milk spurge, flowering spurge, or Johnson grass on tomato, alfalfa, milo, and other field or garden seeds planted on three layers of filter paper or paper toweling in petri dishes. Saturate the paper with extracts in the experimental dishes and with water in the control dishes. Observe the percentage of germination and the rate of seedling growth in both.

Try the effect of possible inhibition by volatile substances by placing horseweed (or other) leaves in a petri dish with field or garden seeds planted on paper toweling saturated with plain water. Keep the leaves from making contact with the seeds, toweling, or water. To use a large number of leaves or whole plants, place the open dish with its seeds on a piece of hail screen bent to form an inverted U and flattened. Place the assembly in a can which has a lid to contain possible fumes emitted by the plant.

Relate the results to the ability of certain weeds to compete with other plants.

15. A weed specialist recently suggested the possibility that the musk thistle may release a substance which renders nearby forage grasses less palatable to cattle. Devise and carry out ways to test this hypothesis.

16. Make a survey to determine the most common and most troublesome weeds in fields and gardens of your area. Using references like Fernald (1950), Britton and Brown (1970), Gates (1941) , Barkley (1968)  Stevens
(1961), or other manuals, determine the national origin of most weeds in your area. Relate your findings to the development of weedy characteristics and to the time that large-scale agriculture has existed in various counties.

17. Conduct a thorough study of one kind of weed. Either start from seed or find individuals already growing. Use direct observation and library resources to learn all you can about that species. Determine its growth rate; usual habitat; maximum, and optimum temperature and water requirements; methods of reproduction (sexual and vegetative); flowering time; modes of pollination and seed dispersal; natural enemies; methods used to compete with ot her plants; characteristics fitting it for disturbed soils; national origin; ecological role; uses by man for food, fiber, medicine, or cosmetics; methods of control.

RESOURCE PERSONS

Knowing the name of a plant is helpful in learning more about it from the literature. But some plants are difficult to identify with certainty even with good illustrations, descriptions, and dichotomous keys. If you need help in identifying the weeds in your area, see the weed supervisor or agricultural agent of your county. Teachers of agriculture and biology of local high schools and colleges may be of aid. Or, send pressed specimens (including flowers or fruits if possible) to the herbarium of any state college or university in Kansas.

REFERENCES

Baker, H. G. 1974. The evolution of weeds. In Annual Review of Ecology and Systematics, ed. by R. F. Johnson. Annual Reviews, Inc ., Palo Alto, Calif. This paper includes an extensive bibliography.

Bare, Jamet E. In press, Kansas Wildflowers and Weeds, Regents Press of Kansas, Lawrence.

Barkley, T. M. 1968. A Manual of the Flowering Plants of Kansas. Kansas State University Endowment Association,
Manhattan, Kansas.

Bonner, James. 1949. Chemical warfare among plants. Scientific American 180(3) :48.

Britton, N., and A. Brown. 1970. An Illustrated Flora of the Northern United States and Canada. 2nd ed. Dover Publications, New York.

Brooks, R. E . 1975. Poison ivy and poison oak in Kansas. Bulletin of the State Biological Survey of Kansas, No. 4.

Controlling Weeds and Brush on Range Land. 1975. Cooperative Extension Service, Kansas State University, Manhattan.

DeBach, Paul ed. 1964. Biological Control of Insect Pests and Weeds. Reinhold Publishing Corp., New York.

Fernald, M. L. 1950. Gray's Manual of Botany. 8th ed. American Book Co., New York.

Furrer, John D. 1975. Lawn weeds and their control. North Central Regional Extension Publication No. 26. Color photos of 45 species of lawn weeds and each is described briefly. Available from county extension offices.

Gates, Frank C. 1937. Grasses in Kansas. Kansas State Board of Agriculture. Topeka.

____. 1941. Weeds in Kansas. Kansas State Board of Agriculture, Topeka. Out of print but available in libraries. A replacement for this volume is now being prepared by T. M. Barkley.

Iltis, Hugh H. 1972. The extinction of species and the destruction of ecosystems. American Biology Teacher 34(4): 201.

Jantzen. P G. 1973. Canopy-coverage method compares pasture and prairie. American Biology Teacher 35(6): 322.

____. 1976. Investigating the ecological role of weeds. American Biology Teacher 38(3): 157.

Jaques. H. E. 1959. How to Know the Weeds. Wm. C. Brown Co., Dubuque, Iowa.

Kansas Noxious Weed Law. 1975. Kansas Department of Agriculture, Weed and Pesticide Division, Topeka.

Klingman, Glenn C. 1961. Weed Control: as a Science. John Wiley and Sons, New York.

Martin, Alexander C. 1972. Weeds: a Golden Nature Guide. Western Publishing Co., Inc ., Racine, Wisconsin.

Nebraska Weeds. 1968. Bulletin No. 101·R. Nebraska Department of Agriculture, Weed and Seed Division, Lincoln. Color photos. $5.00 for out-of-state sales.

Newman, L. H. 1966. Man and Insect: Insect Allies and Enemies. Natural History Press, Garden City, N.Y.

Nilson, E. B., O. G. Russ , W. M. Phillips, and J. L. Condray. 1975. Chemical Weed Control in Field Crops, 1976. Kansas State University , Manhattan.

Pasture and Range Plants. 1963. Phillips Petroleum Co. Public Information Service Division, 404 Phillips Bldg., Bartelesville, Oklahoma 74004. Price, $6.50.

Pollock, B. M., and V. K Toole. 1961. Afterripening, rest period , and dormancy. In Seeds: The Yearbook of  Agriculture. U.S.D.A.

Quick, C R. 1961. How long can a seed remain alive? In Seeds: The Yearbook of Agriculture. U.S .D.A.

Rice, E. L. 1967. Chemical warfare between plants. Bios 38:67.

_____. W. T. Penfound, and L. M. Rohrbaugh, 1960. Seed dispersal and mineral nutrition in succession in abandoned fields in central Oklahoma. Ecology 41 :224.

Sondheimer, Ernest and John B. Simeone, ed. J970. Chemical Ecology. Academic Press, New York.

Stevens, W. C. 1961. Kansas Wild Flowers. 2nd ed. University of Kansas Press, Lawrence.

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