Male sterility in plants can be controlledby nuclear genes or cytoplasm or by both.Three different mechanisms for control ofmale sterility in plants are: (i) Genetic malesterility; (ii) Cytoplasmic male sterility; (iii)Cytoplasmic genetic male sterility. In Maizethe genes for cytoplasmic control of malesterility are located in cytoplasm. Nicotiana sylvestris flowers onlyduring long days and N. tabacum flowersonly during short days. If raised in thelaboratory under different photoperiods - they can be induced to flower at thesame time and can be cross fertilized toproduce self -“ fertile offspring. What isthe best reason for considering N. sylvestris and N. tabacum to be separatespecies? [2003]