Final answer:
Whisk ferns, or psilophytes, are the derived ferns that lost their roots and leaves through reductive evolution, performing photosynthesis via their green stems.
Step-by-step explanation:
The derived fern that lost its roots and most of its leaves is known as a whisk fern, scientifically referred to as psilophytes. These plants have undergone reductive evolution, a process where natural selection diminishes the size or complexity of structures that no longer provide an advantage in certain environments. The whisk ferns perform photosynthesis through their green stems and have small yellow knobs, which are sporangia, at the tips of their stems. Despite having lost much of the structure seen in typical ferns, recent DNA analysis suggests that whisk ferns may be closely related to true ferns, and their simplicity is the result of evolutionary reduction. Photosynthesis takes place in the green stem of a whisk fern and sporangia are found at the tip of the branch stem.