Nicotiana sylvestris flowers only during long days and N.tabacum flowers only during short days. If raised in the laboratory under different photoperiods, they can be induced to flower at the same time and can be cross-fertilized to produce self-fertile offspring. What is the best reason for considering \(N\).sylvestris and N.tabacum to be separate species?

The process by which $\mathrm{N}_2$ is reduced to $\mathrm{NH}_4^{+}$ is called nitrogen fixation. Nitrogenase enzyme catalyzes this reduction. It is only carried out by prokaryotic microorganisms. Principal $\mathrm{N}_2$-fixers include certain free living cyanobacteria in symbiotic associations with fungi in lichens or with ferns, mosses, and liverworts, and by bacteria or other microbes associated symbiotically with roots, especially those of legumes. About 15 percent of the nearly 20,000 species in the fabaceae (Leguminosae) family have been examined for $\mathrm{N}_2$ fixation, and approximately 90 percent of these have root nodules in which fixation occurs. So without active nitrogenase enzyme there will be no $\mathrm{N}_2$ fixation in legumes.