Final answer:
Pterophytes, such as ferns, are confined to moist habitats because their reproductive cycle, particularly the fertilization process involving flagellated sperm, requires a watery medium. The sporangium structure adapted for spore release also underscores the ferns' moisture dependency for successful spore germination and ongoing life cycles.
Step-by-step explanation:
Pterophytes, such as ferns, are limited to moist habitats due to a key aspect of their reproductive cycle requiring water. Ferns, as part of the group of seedless vascular plants, do not produce seeds that could survive drier conditions. Instead, they produce spores that are dependent on a moist environment to develop into multicellular gametophytes. During the fertilization process, flagellated sperm released by the male gametophyte needs to swim through water to reach the female gamete for successful reproduction. This dependence on water for fertilization makes ferns more abundant and successful in environments that are consistently damp.
The sporangium structure of ferns also contributes to their preference for moist habitats. Fern sporangia are equipped with a specialized band of cells, an annulus, which facilitates the release of spores into the air. The sporangia's mechanism for spore dispersal is reliant on moisture; as the sporangium dries, the annulus contracts, creating tension that ultimately flings the spores out when the attached water columns break. However, these spores then need moisture to grow into a new generation of gametophytes.