One possible explanation....there are probably others!

An increase in the size of a plant structure is usually an indication of an increase in ploidy. Almost all ferns are polyploids, having more than 2 sets of chromosomes per nucleus, such as triploid (N=3), tetraploid (N=4), pentaploid (N=5), etc. The number of chromosome sets can run into the hundreds in some plants.
Those plants with an odd number of chromosome sets (N=3, N=5, N=7, etc.) are sterile and cannot reproduce by sexual processes. Sterile ferns do not produce sporangium and spores.

ladywindsurfer,thanks for the input,maybee this particular fern will turn out to be sterile,will check next year,sometimes the leaf or frond remains evergreen and turns a bronzish color in the fall-winter then following year a new leaf comes up right along side of it and the original leaf starts to yellow and die back sending up a new frond and a short time after that, the plant sends up a fertile stalk or sporphore with the greenish -yellow grape
like cluster of sporangia as in photo below
{{gwi:615756}}