James T. Costa

Darwin’s “seed salting” experiments were aimed at determining, first, how long seeds might retain their vitality after exposure to saltwater, and second, how long they can float versus sink in saltwater. He performed several versions of this experiment using seeds from many species, including garden vegetables, common weeds, and tropical plants.

A. Materials

Saltwater: prepare anywhere from 1 to 10 gallons (3.8 to 38 liters) of saltwater, depending on the scale of your experiment. We’re interested in approximating seawater. Like Darwin, you can readily make artificial seawater with commercially available salt and mineral preparations. You can purchase your salt mixture from any of the numerous online aquarium supply retailers (Instant Ocean® and Natural Sea Aquarium Salt Mix from Oceanic Systems, Inc. are less expensive brands), or from a local pet shop or aquarium supply store. Or, make your own with this recipe:

Into every 3 gallons (11.4 liters) of fresh water, dissolve:

• 10^1⁄2 ounces (298 grams) pure (not iodized) table salt
• 1^1⁄2 ounces (43 grams) magnesium chloride
• 1 ounce (28 grams) Epsom salts
• ^1⁄2 ounce (14 grams) plaster of Paris

Chemical and mineral mixtures such as these give freshwater all the qualities of seawater, making it suitable for marine life. If you were setting up a saltwater aquarium to support fish, we would have to pay attention to salinity and quality of the freshwater used. Since we’re simply floating seeds in our saltwater, there is no need to worry about that. By the way, even if you live close to the ocean it is still preferable to use artificial saltwater, as ocean water is full of algae and other microorganisms that will die and decay, fouling the water.

B. Other materials

• Notebook and pencil
• Seeds of at least six species (e.g., assorted vegetable and wildflower seed packets)
• 500 ml (17 fl. oz.) beakers or flasks (or even mason jars), one for each plant species. The bottom of a 1- or 2-liter (1- to 2-quart) beverage bottle will work too.
• Pipette or turkey baster
• Spoon
• Forceps or tweezers
• Plastic wrap
• Labeling tape and marker
• Potting soil
• For seed planting: Petri dishes or similar chambers, or one or more germination flats (ideally, partitioned into individual planting units) or paper cups, with potting soil

1. Prepare the saltwater in an aquarium tank, carboy, or large ask, depending on volume, and keep at room temperature. Measure 300 ml (10 fl. oz.) saltwater for each beaker or flask. (If the vessel does not have a 300-ml (10 fl. oz.) measurement mark, make one with a marker or tape as a reference for maintaining the water level.) Choose a location out of direct sunlight where the flasks won’t be disturbed. Place the beakers or flasks there and, once positioned, using fingers or forceps carefully place (not drop) 10 seeds of a single species onto the water surface in one vessel, labeling it with the date, time, and plant species. Repeat for each of the remaining seed species, one species per beaker. (Don’t worry if your seeds sink: this situation is addressed below.) Loosely cover the mouth of each vessel with plastic wrap; some water will evaporate, and a pipette or baster can be used to replenish evaporated water. To do this, carefully dispense water along the inner wall of the vessel so as to minimize agitating the water surface. (If you have a large number of participants doing this experiment it may be desirable to set up duplicates of the experiment at several different stations, comparing the results of each group later.)

2. Record the status of the seeds daily, logging for each vessel the number of seeds still floating and the number that have sunk. This can be continued indefinitely, as Darwin did by removing seeds at intervals to see how long they would remain viable after prolonged exposure to saltwater, but it will be more practical for most people to run the experiment for a set period of time—1, 2, or 3 weeks, or whatever you prefer. Larger groups with replicates of each seed species might harvest one replicate per species after, say, 1 week of exposure, a second replicate after 2 weeks, etc. Alternatively, all replicates can be floated for the same period of time in order to obtain descriptive statistics on seed performance among replicates.

3. At the conclusion of the time-span determined for the experiment, record the number of floating versus sunken seeds for each species. First retrieve the floating seeds from each vessel using the spoon, taking care not to sink any seeds. Then retrieve the sunken seeds, making sure to keep each species separate and the still-floating and sunken seeds of each species separate. Rinse seeds in fresh water and plant the seeds using one of the two following methods:

Paper toweling method. Use separate dishes for floaters and sinkers of each species. Place one piece of paper towel on the bottom of the dish, saturate with water, and place seeds. Saturate the second piece of paper toweling and place over the bottom piece, covering the seeds. Place the lid on the dish to seal in moisture. Ensure that all dishes are labeled (species, floater versus sinker, date).

Soil method. Carefully plant each set in adjacent units of the germination flat. (If the flat does not have individual planting units, use string to grid off the flat.) Plant floating and sunken seeds of each species separately. Repeat as necessary, water, and cover the flats or cups with plastic wrap.

4. When all the seeds are planted, expose dishes, germination flats, or cups to indirect sunlight (not direct sunlight) or a growth lamp. Monitor daily and record numbers of seeds of each category (species, floating versus sunken, etc.) germinating.

Record, for each species and each saltwater exposure time (1 week, 2 weeks, etc.):

(a) Number and percentage of floating seeds germinated versus non-germinated
(b) Number and percentage of sunken seeds germinated versus non-germinated

5. At the conclusion of this experiment tabulate the data, noting percentage of seeds sinking and remaining afloat for the experimental period, and the percentage of each category germinating. The results can be related back to Darwin’s original puzzle: How do species colonize remote islands? Two sets of conclusions can be drawn from our Darwin-inspired experiment, relating to flotation and viability in saltwater. First, the seeds of some species do remain afloat for an extended period. What was the duration of your experiment, and how far might they be carried by currents in that time? In a letter to Hooker, Darwin pointed out that “many sea-current go a mile an hour: even in a week they might be transported 168 miles: the Gulf-stream is said to go 50 & 60 miles a day.” Indeed, this is likely an underestimate for some currents: with a maximum estimated surface velocity of 5.6 mph, the Gulf Stream could carry floating seeds over 130 miles per day! Try your hand at calculating this.

6. The sunken seeds might seem less informative for Darwin’s purposes. However, they too provide insights, in showing to what extent seeds might remain viable after saltwater exposure whether floating or sinking. What proportion of your sunken seeds germinated as compared to the floating ones?

From the book Darwin’s Backyard by James T. Costa. © 2017 by James T. Costa. Used with permission of the publisher, W.W. Norton & Company, Inc. All rights reserved.