Disclaimer: What? Pokemon belongs to Nintendo? Oh darn.
A/N: Edited to remove sarcophytes.
Topics in pokemon biology: Plant-Pokemon Symbiosis
It seems evident that plants and animals are different. Separate. Plants don't move and make their own food from sunlight, and animals move around and take their food from other living things. But in truth, the division isn't so simple. Some animals look like plants, and even photosynthesize. The reason for this confusion is plant-pokemon symbiosis.
Plant-pokemon symbiosis is a surprisingly common and variable phenomena. It ranges from complex plants growing in or on a pokemon, to pokemon that house simple algae in their tissues. Many different groups of pokemon have evolved this symbiosis, from mammals to reptiles to invertebrates.
The implications of these kinds of creature are more than just academic. How do the animal and plant tissues not attack each other with immune responses? How do they recognize the alien tissues as their own? Teasing out the secrets of plant-pokemon symbiosis may be key to understanding immune responses in general.
Please note that not all pokemon classified as "Grass" types by the Pokemon League are symbionts. This is not a scientific classification, and many of the pokemon so classified only resemble plants as camouflage.