On the Structural Relationships of Branch Roots and Their Parental Root Axes in Secondary Growth

Four aspects of the structural effects of branch roots on their parental root axes are described: (1) lesser accumulation of secondary xylem on the side that is toward the branch root, (2) formation of depressions where branch roots emerge, (3) accumulation of parenchyma along the loci where branch roots are formed, and (4) lobing of the parental axis in coordination with branch root attachments. Thus, cell proliferation and accumulation and cell differentiation around the parental root axis vary with the locations of branch root attachments, with the latter relating spatially to the differentiation of protoxylem in the parental root axis. The examples reviewed show that these structural features vary independently of each other, in combinations that result in alternative morphological outcomes.