Answer: The easiest plants I know of to hybridize are "FastPlants" (http://fastplants.cals.wisc.edu/). These plants have been bred to complete their lifecycle very quickly, so within a reasonably short time you can grow plants, cross pollinate their flowers, allow them to produce mature seeds, plant those seeds, and observe these offspring plants. I'm not sure about whether sweet peas would be easy to cross (remember to avoid choosing hybrids for your experiment). However, I'm concerned that they would require quite a long time to complete two life cycles. I suggest you check out the Fast Plants web site and see if they would be appropriate. Here are a few web sites you might find helpful:
"Custom-Crafted Vegetables A guide to plant breeding and seed saving in the squash and pumpkin patch"
You might enjoy the book "Breed Your Own Vegetable Varieties" by Carol Deppe.
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