Species are particular kinds of plants.
In practice, however, even though species are considered the fundamental units of evolution it is difficult or impossible to come up with a single basis for deciding what is a species. This is because we cannot know with certainty the genealogical relationships between individuals sampled at different times and places from a population of usually unknown size and distribution. Nevertheless, we can extrapolate from what we know about patterns of morphological, anatomical, and chemical variation, about plant genetics and breeding systems, as well as about plant distributions and plant ecology to arrive at what are in most cases useable, working definitions of species. Inevitably, in almost all of the vascular plants in this checklist the species concept employed is basically a morphological one, although in many cases it is bolstered by data from other sources. What this means is that most species will be recognizable from their morphology because they exhibit either several characteristics that are altogether unique, or else unique patterns of covariation between characteristics that individually might show overlapping variation between species.