This is one of 30 lessons from the NSTA Press book Scientific Argumentation in Biology. The lesson engages students in an argumentation cycle in which they evaluate three alternative claims regarding where most of the matter that makes up the stem and leaves of a plant originates. The lesson engages students by presenting the phenomenon of small sunflower seeds growing into tall flowering plants. Students are guided to design and carry out an investigation that will allow them to collect evidence needed to construct an argument defending one of the three claims. As they construct this argument, students build and apply knowledge of photosynthesis and biosynthesis in plants. This lesson is intended for middle or high school students. Teachers are encouraged to refer to the preface, introduction, student assessment samples, and appendix provided in the full book for important background on the practice of argumentation and resources for classroom implementation. This resource has also been reviewed for use in high school, see appropriate Performance Expectation for more information.