Plants are interpreted as structural hierarchies which are real systems organized through descending constraints. Types of hierarchical groups in plants are (a) cluster by integration, (b) support through attachment, (c) enclosure by encasement (d) dissipative by input of energy and (e) control through variable state switching. Most plant hierarchies are mixtures of these types which explains a number of paradoxes in plant morphology. The traditional means of identifying levels, i.e., cell, tissues, organs, uses a compositional group which is not a (...) hierarchical group but a similarity feature and so is inadequate for describing hierarchies. Hierarchies can be defined by set theory which is more a description of cognitive than real hierarchies and therefore is of little value in describing plant organization. The hierarchical description of a plant emphasizes the immediate physical status of organization which provide, in turn, a physical explanation of development. (shrink)