TOPICS:  Plant reproduction and ecology, seed structure and dispersal

LEVEL:  High school

TIME:  1 class period for field work and 1 class period for lab work.  This activity must be done in late fall or early winter so that seeds are still abundant. 

ADVANCE PREPARATION:  Order Tanglefoot^TM from a biological supply company.  Construct 6 or more seed collectors.  Use a large, institutional-size vegetable can with one end removed, and nail it on a two foot stake.  Mark the directional points on the top of the can (NSEW) and cover the sides of the can with plastic sheeting or several layers of plastic food wrap. Locate an area within walking distance of school where remnants of winter weeds are abundant .

MATERIALS: Petroleum jelly may be substituted for Tanglefoot^TM, and may be easier to handle.  Its disadvantage is that it loses its stickiness in sub-zero temperatures .

SAFETY NOTE: Advise students to dress adequately for winter weather conditions.  Warm boots, hat and mittens/gloves are essential .

STEP C: Stakes should stand alone in deep snow; you may need a hammer to pound stakes into frozen ground .

STEP D: Collectors should be left outside until an adequate sample of airborne seeds is collected.  This may take from one day to two weeks .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: Species will vary with the area sampled.  Weeds having very small seeds, or seeds with wings or parachutes are likely to be dispersed by wind to the collector .

STEP F: Debris on the plates could hamper seed counts, and unidentifiable seeds may be collected.  If abundant, unidentifiable seeds may be counted in groups as species A, B, an so on.  You may find insects on the collectors as well.

QUESTIONS 9-13 require higher order thinking skills.  We recommend that you discuss these questions in class before students write out their answers.  Question 12 may require library work .

2. Answers will vary with the area sampled.  The most common winter weed species in an open field will probably include goldenrod, wild carrot, aster, milkweed, etc .

3. Ragweed and curly dock may be eaten by birds and small mammals.  Their seeds are relatively large and numerous .

4. The parachutes from such seeds as milkweed, aster, thistle, etc. may provide nesting material for animals .

5. Burdock and beggar-ticks attach easily to clothing.  Animals also serve as agents of dispersal by attachment of seeds to their fur .

6. Prevailing winds will vary locally.  That side of the collector with the most seeds will indicate wind direction .

7. Answers will vary according to the area sampled, the wind speed, and the time of year.  Student predictions may or may not agree with the class data .

8. Seeds that are dispersed by winter winds may be very small and lightweight (mullein), have parachutes (milkweed), have flattened surfaces that serve as wings (curly dock), or a combination (aster, goldenrod) .

9. Successional species typically produce small seeds in large quantities.  Since successional species are not permanent residents of an area, they must continually colonize new areas to survive.  To do this they disperse their seeds over a wide area, much of which may be unsuitable for germination or growth.  Natural selection favors successional species that produce large numbers of seeds, which increases the chances that at least some seeds will reach suitable areas for colonization.  Small seed size allows greater numbers of seeds from the same investment of available energy .

10. Climax species typically produce larger seeds in fewer numbers.  Climax species establish themselves by outcompeting successional species for available resources such as light, nutrients, and space.  Climax species have the ability to reproduce themselves on the same undisturbed site indefinitely.  The emphasis of natural selection has been on increasing the probability of individual seed survival at the expense of seed quantity.  Investment of available energy in large seed size results in fewer seeds being produced.  The large seeds store more nutrients which increases the chances of survival of the individual seedlings .

11. The winter weeds in an open field are successional species that typically produce large numbers of small seeds .

12. (500 mi)/[(11000 yr)/(30 yr/gen)] is about 1.3 mi/generation.  Acorns are large and heavy, and are dispersed by animals or water.  A mile or more is considerably greater than the distance chipmunks or squirrels carry acorns for storage.  One study suggested that jays may transport acorns for up to 4 km (2.4 mi.) Another dispersal possibility is northward flowing streams.  Early human inhabitants may have contributed to the spread of oaks, but by the time the first people arrived, the oaks may have nearly attained their present distribution .

13. Ash, maple, and pine produce winged seeds for dispersal by wind.  Such seeds travel different distances depending on wind speed, seed weight and shape, wing area, etc.