The standard I chose to use as a learning experience for my students is MS-LS1-5: Construct a scientific explanation for how environmental factors and genetic factors influence the growth of organisms.
The Science and Engineering Practice on which to focus would be Constructing Explanations. Scientists form explanations for why something happens, basing their reasoning on previous knowledge and theories from experimentation and other scientists. The students would make observations during experiments and would draw from previous knowledge of scientific theories they studied in lower elementary grades.
The Disciplinary Core Idea would be that genetic factors as well as local conditions affect the growth of the adult plant. Experiments should be set up to explore this.
The Crosscutting Concept is Cause and Effect. The idea that "for every effect there is a cause" is innate in all of us. Students will explore the idea that phenomena may have more than one cause; some cause and effect relationships in systems can only be addressed as probability. Middle school students will learn to argue from evidence when making their explanations of why some scientific phenomena occurs.