The relationship of cells of organisms in plants: problem and implications of an organismal perspective

Two principal theories have been advanced to explain the origin of multicellularity in higher organisms, the cell theory and the organismal theory. A brief review of the evidence indicates that plants express the organismal concept of multicellularity, i.e., they consist of a unified protoplast that is chambered into cells rather than being the product of cell aggregation. Moreover, it has been demonstrated that the mode of "multicellular" construction in plants is altogether different from that in animals. Instead of having complete protoplast separation at telophase, protoplast delimitation in plants is by internal wall insertion. It is because of these differences in cell delimitation that cell theory concepts rely on qualitative homology criteria rather than positional criteria. However, in plants positional criteria are of greater significance in judgments of cell and organism relationships. The aim of this article is to examine the implications of the organismal concept of plant multicellularity for a variety of structural phenomena, including root contraction, cuticular differentiation, ring cell development, and embryo sac divergence. From these analyses I conclude that the procrustean and artificial nature of past interpretations have resulted from either the misapplication of homology criteria or the attempt to force plant histogenesis into animal cell models that are inappropriate. Not only do these analyses support the validity of the organismal model for plant multicellularity but they also provide a holistic framework in which to carry out future studies of plant morphogenesis.