How do we differentiate between plant species?

Earlier the scientists differentiated between plants mainly by their appearance. Today, plants are classified according to their evolution, i.e., on the basis that all members of a group must have shared a common ancestor more recently than they have with any species outside the group. Thus, the plants are broadly classified as those containing chlorophyll (the green plants) and those without chlorophyll. The green plants include the green algae and the land plants, which grow towards light against the Earth's gravity. The land plants include the mosses, the vascular plants such as ferns and horsetails, and the seed plants. These also include the conifers (gymnosperms) and the flowering plants (angiosperms).