Hierarchical Aspects of Plant Development

A universally accepted concept in biology is that organisms have a hierarchical organization such as the cells-tissues-organs-organism series with other levels both above and below this sequence. While this idea is simple to illustrate there is considerable difficulty in providing a definition of each level and distinctions between adjacent levels. When is a group of cells just a group of cells and when is it a tissue, and what makes a group of tissues an organ and how are organs related to constitute an organism? Generally it is thought that there must be some intergrating feature(s) by which a collection of cells, tissues and organs, respectively, become tissues, organs and organism but the identification of the intergration features and their specific roles has not been easy.