A hormone that has another endocrine gland as a target. tropical rain forest The most complex of all communities, located near the equator where rainfall is abundant; harbors more species of plants and animals than all other terrestrial biomes combined. tropism A growth response that results in the curvature of whole plant organs toward or away from stimuli due to differential rates of cell elongation. tuber tuber, bump, swelling] A much-enlarged, short, fleshy underground stem, such as that of the potato.