Day 3 was Friday, and our seeds had been growing since Monday.
Experiment 1. What growing medium is best for chia seeds?
Click on the thumbnail to see the partial results:
The "flower picture" was painted with chia seeds on a paper towel. Those seeds grew almost 1/2 inch in 4 days. But the seeds in the dish of soil, pictured here, also grew very well. The sponges must have contained an antiseptic substance which prevented growth of bacteria (and seeds) because the seeds failed to grow.
Experiment 2. What concentration of salt is lethal for chia seeds?
We were hoping to find the salt solution in which 50% of the chia seeds would die. Why? Because teachers of AP Biology need to offer their students a demonstration of the concept of LD50 but they would rather not cause the death of animal subjects.
Results: we narrowed it down somewhat. All seeds failed to germinate at concentrations of 2% and higher. At 1% they all germinated, but growth was stunted.
We tried concentrations between 0%, 1% and 2%, but our results were not conclusive.
Experiment 3. How much water can chia seeds absorb, and over what time period?
This photo compares chia seeds with alfalfa seeds, placed in a flask of water and stirred frequently. Chia seeds are #2.
This result came in less than 30 minutes!
We had hoped to quantify this by draining the water and weighing the drained seeds. However, we ran into technical difficulties because the chia seeds simply couldn't be separated from the water. Next time we'll try using a filter paper and funnel.
Experiment 4. Microscopic studies
We had access to a dissecting microscope fitted with digital imaging software, so we grabbed a couple of photos.
This is how 2 day old seedlings look.
And these images show what happens to dry seeds as they begin to absorb 
water over a 25 minute period: