I had germinated seeds in a paper towel and had transplanted them into soil. After a week I checked the plant and it had not sprouted, and, after digging one up, the tap root has mostly receded back into the seed. Is there any way I can save the plants?
Over the years, I have found that germinating seed on a paper towel is a great idea for some seed and not others. I prefer to sow seed directly into either germinating mix or fine vermiculite.
I direct sow large seed such a the curcubits, melon, nasturtium and dolichos into a container or a cell, where they can develop a good root system. These seedlings do not transplant easily unless the soil ball can remain intact.
I do not transplant plants with taproots; beets, carrots, asclepias, and radishes. Those seed are put directly into soil where they grow because plants with taproots transplant extremely poorly, at best.
I don't transplant the seedlings grown in germination mix until they have their first set of real leaves. When we transplant too soon, the result isn't pretty.
Sometimes seedlings will "damp-off" when they are too wet or in too little light. When this happens, the stem simply collapses or dries up. When this happens, there is no survival. We try to grow seedlings in as much light (not direct sun) as possible while keeping the soil barely moist. This gives the plant short, sturdy stems.
Temperature is also a key element in seed germination and seedling growth. Seed packets should give an optimum germination soil temperature. Differing day/night temperatures depend on plant variety. While there is "wiggle-room", most seed and seedlings will perform best when kept within their daytime maximum and nighttime minimum temperature range.