Of the 4 different populations of stem cells found in the Arabidopsis thaliana root, the Columella Stem Cells produce the cells found at the root tip. Part of the process of those cells maturing is to produce starch granules, which in the root tip sense the gravity vector. The starch granules, among other physiological processes, directs the root to grow in a downward direction. Image by Adam Fisher. Research lab: Dr. Ross Sozzani, Plant and Microbial Biology.