The soil of planet Preplanis is rich in spores of a type of microscopic parasite which is able to infect vegetation, causing extreme mutations.
When Dr. Smith planted a pea seed on Preplanis soil, the parasitic infection caused it to grow to a length of five feet (~ 1.52 meters) in a single night, and develop strange tentacles in its pods. A microscopic exam soon revealed that the pod cells had been transformed into some kind of intermediate between plant life and animal life.
Details on the natural life cycle of the soil parasites are not known. It could be speculated that the transformation it triggers allows its host to develop faster, in order to spread the parasitic spores around.
Another possibility is that the host plant grows to a gigantic size to enhance its chance of being consumed (perhaps by the Giant Cyclops), helping to spread the seeds of both host and parasite alike. If true, this might mean that the soil parasite is actually more of a symbiont.
- Lost in Space, s01e04, "There Were Giants in the Earth" (1965)