1. inner layer of cells surrounding the vascular tissue in plants guard cells 2. a root system of many similar-sized branches of roots solar tracking 3. cells that form the opening of stomata fibrous root system 4. a biogeochemical cycle of Earth that recycles nitrogen in the air and soil and to and from living organisms taproot 5. the intake of oxygen and release of carbon dioxide by living organisms respiration 6. a behavior of some plants in which they readjust their positioning to follow the Sun nitrogen cycle 7. a root system of one, long, main root that goes straight down into the ground; may have small lateral roots extending from it endodermis