The scientific classification of Oatgrass and Cereus includes the entire classification chart that includes kingdom, family, clade, tribe etc. A kingdom is the second highest taxonomic rank. A taxonomic rank is the level that a living organisms are placed in hierarchical level of arrangement of life forms. All the living organisms are divided into kingdoms. Plant kingdom is a main classification of living organisms that include all plants. There are six kingdoms which are: Plantae, Animalia, Fungi, Protista, Aschaea/Archaeabacteria and Bacteria Eubacteria. Oatgrass and Cereus kingdom and all other plants kingdom is Plantaea.
Plant family is the group of plants which have something in common. Oatgrass and Cereus family is a major factor of Oatgrass and Cereus Scientific Classification. Plant family helps you narrow the search of your plants. It also gives you idea about, how the plant looks, how the seed will be like etc. These two plant families can be different or same. Oatgrass belongs to the family Poaceae and Cereus belongs to the family Cactaceae.
While comparing scientific classification, Oatgrass and Cereus genus and other classification are also important. Also, when you compare Oatgrass and Cereus, other factors should also be taken into considerations like order, subfamilies, tribe, clade etc. First plant's genus is Danthonia and other plant's genus is Selenicereus. Oatgrass tribe is Aveneae and Cereus tribe is Hylocereeae. Oatgrass clade is Angiosperms, Commelinids and Monocots and order is Poales whereas Cereus clade is Angiosperms, Core eudicots and Eudicots and order is Caryophyllales. Every plant have subfamilies. Oatgrass subfamilies are, Danthonioideae and Cereus subfamilies are Cactoideae. In this way you can compare scientific classification of Oatgrass and Cereus and other products also.