<Li> Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in Proteaceae and some plants from the following families Betulaceae, Casuarinaceae, Elaeagnaceae, Moraceae, Fabaceae and Myricaceae . </Li> <Li> Stilt roots: these are adventitious support roots, common among mangroves . They grow down from lateral branches, branching in the soil . </Li> <Li> Storage roots: these roots are modified for storage of food or water, such as carrots and beets . They include some taproots and tuberous roots . </Li> <Li> Structural roots: large roots that have undergone considerable secondary thickening and provide mechanical support to woody plants and trees . </Li>

Which of these terms can be defined as a root's downward growth as a response to gravity