We are fortunate that we can refer to their work and save ourselves alot of time when we try to germinate seed. Manual of Woody Plants Hartmann, H., Kester, D., and F. Davies. Plant Propagation Principle and Practices. Numerous seed catalogues and information sheets. Seeds of Woody Plants of North America.
All humans are members of the same species.
Members of different species within a genus look less alike but are still similar. They still share a majority of their genes but have more genetic differences. At each step up the hierarchy this relatedness becomes more obscure and individuals from different branches have fewer and fewer genes in common. Therefore when we compare two species from different genera within the same family, they do not look so much alike. Sexual reproduction that leads to genetic diversity.