Today we will do some mindless-chores to set up future lab exercises. We will sow some spores on Knopp's agar, and some seeds on Hoagland's medium. These liquid media contain the minerals necessary for plants to obtain from the soil around them. Growing plants without soil is called hydroponics. In this case we are solidifying the liquid with a very small amount of Phytagel (a substance similar to agar). The mineral macroelements include P, K, N, S, Ca, Fe, and Mg and microelements include B, Mn, Zn, Cu, Mo. The phytagel is used at 0.8 grams per 250 mL. We will learn to use a dissection microscope properly to be sure the spores we have sown have indeed entered the container of gel.
Into 10 cm Petri dishes, approximately 25 mL of autoclaved Knopp's phytagel medium were allowed to solidify on cooling. We distributed, as sparingly as possible while still being sure some spores were sown, the spores of Polytrichum moss in one dish, and the spores of Ceratopteris fern in another dish. The dishes were placed in light to stimulate germination...with the cover of the Petri dish facing upward and toward the fluorescent lamp. (opposite the orientation used in Microbiology!).
Into one 10 cm Petri dish, lined with a disc of 9cm diameter filter paper, about eight seeds of Roman bean (Phaseolus vulgaris), and eight seeds of popcorn (Zea mays) were sown and moistened liberally with tap water (but not submerging the seeds!).
Four Petri dishes were labeled (White, Red, Far-red, or Dark) and lined with a 9 cm diameter disc of filter paper moistened with 5 mL of distilled water. Fifty (not 49 or 51!) seeds of Lactuca sativa 'Salad Bowl' were sown in each dish. The dish labeled dark was wrapped in heavy duty aluminum foil within 10 seconds of the seeds landing on the moistened paper in the dish. The other dishes were unwrapped. The dish marked white was exposed to light from a compact fluorescent lamp 40 cm above the seeds in the dish. The dish marked red was placed inside a wooden box fitted with a red plexiglas filter under the same fluoresent source. The dish marked far-red was placed inside a similar box fitted with both a red and a blue filter, but under an incandescent lamp suspended 40 cm above the seeds. The foil-wrapped dish was placed underneath this dish under the far-red light. The purpose of this was to control for temperature differences.
Hoagland's medium was prepared with Phytagel, autoclaved, cooled, and, while still liquid, 200 μL of PPM (Plant Preservative Mixture™) was added to 200 mL of medium. The medium was distributed to 3-oz plastic bathroom cups to solidify. The medium was covered with an inverted 7-oz plastic bathroom cup to form a "large volume" culture dish. Upon solidifying, three seeds of Brassica rapa 'Petite' (a Wisconsin Fast Plant variety) were sown on the surface and pressed about 1 mm below the surface of the gel with forceps. In theory PPM prevents fungi and bacteria in the cups and on the seeds and forceps from growing. We shall see how effective this mixture is.
In the greenhouse, after a brief tour, friable (moist enough to form a ball when squeezed together, dry enough for the ball to crumble at a touch) soil was prepared and packed moderately in two 11-cm pots up to the ridge. Eight seeds of popcorn (Zea mays) were planted in one pot and eight seeds of Roman bean (Phaseolus vulgaris) in the other pot. The seeds were covered with more friable soil to within a centimeter of the rim. The pots were labeled and placed on-line with the watering system in a sunny greenhouse. While in the greenhouse, we also harvested a few fertile fronds of various ferns in the collection in the greenhouse. Small pieces of frond that were shedding spores (neither too young nor too old) were placed into envelopes for drying, shedding, and collecting spores for sowing later into Petri dishes of Knopp's agar as before.
Hopefully we have observed the major size difference between spores and even very small seeds (Lactuca and Brassica). Larger seeds (Phaseolus and Zea) are considerably even larger! Clearly the methods for dispersal of these spores and seeds will use different mechanisms and likey very different vectors! Some species are better adapted (have protoadaptations) for long-distance dispersal than others.
This page © Ross E. Koning 1994.
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