Saving tomato seeds is a fairly simple process. Every tomato seed is covered in a gelatinous sack which contains chemicals that inhibit seed germination. This prevents the seeds from sprouting whilst inside the tomato fruit. In nature the fruit drops from the plant and slowly rots away on the ground.
The asparagus plant is dioecious meaning that it produces both male and female flowers on separate plants. Each flower has two sets of sexual organs with one set aborting as the flower matures. This leaves either all male or all female flowers on a plant. Asparagus relies on insects for pollination.