In most dicot plants the xylem is concentrated in the innermost region of the central cylinder. When a cross-sectional view of the root is observed, the protoxylem occupies an area in the shape of a cross or star, the ends of which touch the pericycle. The protofloema is found in the vertices formed by the "arms" of the cross.
The basic concepts of genetics described in the preceding chapters can be applied to almost any eukaryotic organism. However, some techniques, such as test crosses, can only be performed with model organisms or other species that can be experimentally manipulated. To study the inheritance patterns of genes in humans and other species for which controlled matings are not possible, geneticists use the analysis of pedigrees and populations.