Complex sentence: Fern ally is a term for a diverse group of vascular plants that are neither flowering plants nor ferns and which reproduce by shedding spore to initiate an alternation of generations.
Split into simpler sentences: Fern ally is a general term covering a somewhat diverse group of vascular plants that are not flowering plants and not true ferns. Like ferns, these plants reproduce by shedding spores to initiate an alternation of generations.