Sentence:  Plants' <WSD>life cycles</WSD> are an attractive subject of study for citizen science because they are relatively easy to discern. In fact, the name for the field in which we work, "phenology", literally means “the science of appearance.” The word was coined just a few years after Thoreau made his phenological observations, from the Greek "phaino" (to show or appear) and "logos" (to study). 
Meaning of the word marked with <WSD> tags:  The course of stages through which an organism passes from fertilized zygote until maturity, reproduction, and death.