dont understand how im supposed to get sap to grow the tree. I only am gaining the currency, energy, and water. I used my free research point to purchase a new plant that gives additional water
The first (and at that time only) plant you're able to buy is the "Spore Lily" which produces sap when planted. If you head over to the Vending Machine, you should see it available for purchase there.
You can only buy the plant that gives additional water after you purchase the plant that produces sap.