Once we have acquired the seed we process it in such a way that it will successfully germinate. This varies from species to species. For trees that have berries, like Hackberry, we gather the fruit by either picking them from the plant or sweeping them from the ground. In the fall, we can sometimes be seen crawling around in the gutters along the streets of metropolitan Milwaukee with dustpans and whisk brooms obtaining fruits. It is not an aspect of propagation I dreamed of when I was choosing my vocation. We then place the fruits in a bucket of water for a few days to let them ferment slightly.
Next, we clean them, separating the seed from the fruit. We do this because of the presence of chemical inhibitors in the fruit that will prevent the seed from germinating. The fermentation process allows the fruit to be easily separated. The procedure entails mashing the stinky fruit and seed mixture into a paste either with a food processor or a 4×4 with the corners rounded off (like a mortar and pestle). Then water is added and the pulp is floated off. The sound, solid seed will sink to the bottom of the bucket and the lighter pulp, skins, and seeds with air pockets in them (not sound seed) will float away as you slowly pour the water off. You continue the mashing and pour-offs repeatedly until all you have left is a clean seed at the bottom of your bucket.
From this point, the seed is stratified (put in a plastic bag with a 50-50 mixture of moist sand and peat) until it is time to plant. Stratification is a form of trickery to make the seeds think that they are planted in the ground but are not. Depending on the species, the stratification period can be either just a few weeks or a year and a half. American Linden and Hawthorns are two types of plants that need these long stratification periods.