A Morphogenetic Basis for Plant Morphology

Abstract: The Principles Considered

The paper is meant to explain and, where possible, briefly to substantiate the following central principles:

• 1. The study of morphology has been based on the concept of homology or the assignment of different structures to one category. Categories based on intuitive groupings have been successful to a degree that merits explanation.

• 2. A possible definition of homology that would include both ontogenetic and evolutionary considerations relates not to mature structure but to shared developmental processes.

• 3. In the development of plant organs, processes occurring early in primordia generally show a wider homology than any others. (Therefore, categories such as “leaf” may be defined as groups whose members share some very early primordial states.)

• 4. Since different processes cannot be expected to evolve at the same average rate, the conservative ones should be used as a basis for morphology. Developmental programs operating early in ontogeny are usually conservative from an evolutionary point of view because when they change there are many important consequences often leading to a disruption of the functional integrity of the mature structures. In plants this conservatism is apparent in meristematic stages of organ development and not in embryos or seedlings.

• 5. The evolution of controls for the location, duration and timing of developmental processes could be expected to restrict plants to a limited number of morphological organ categories and to various intermediate organs: the observed facts can therefore be accounted for.