Plant parts: processes, structures, or functions?

Abstract

"...it has become increasingly accepted that in plants it is better to consider an organism and what are perceived by many as its component parts at any one time as a snapshot in a dynamic transformation of growth, maturity and senescence…"-Mabberley & Hay, 1994 : 122. ABSTRACT. It is usually taken for granted that plants are composed of a series of discrete parts that can readily be compared both within a single plant and between closely or even distantly related organisms. The biological meaning of 'part' needs to be made more rigorous such that the naming of a part or the homologising of parts among organisms constitutes a scientific hypothesis that is testable, at least in principle. I explore three alternative approaches to defining parts. The parts-as-structures approach holds that parts (or "phenes") are aspects of an organism that would not have been formed during development in the absence of causal genetic factors. A structural phene hypothesis is refuted by the lack of a hypothetical genetic deletion that would ablate just this part of the organism. The parts-as-functions approach focuses on pieces of the organism that could have been functionally different if one or more gene in the genome were mutated. A functional phene hypothesis is refuted by showing that all mutations affecting the phene have pleiotropic consequences. The part-as-process approach equates parts with developmental modules, making it especially helpful for addressing the concept of serial homology. I conclude that, depending on the context, parts are best understood sometimes as structures, sometimes as functions, and sometimes as processes, but in each case we need to develop rigorous concepts rather than falling back on human perception as the ultimate arbiter of part-ness.

EvoDevo: Past and Future of Continuum and Process Plant Morphology

“…It was not the very versatile Goethe (being poet as well as scientist), but mainly the German plant morphologist Wilhelm Troll (1897-1978 [65], who accepted [66] (p. 246): "that leaves, shoots, and the like are fundamental building blocks of the plant archetype: the starting point (in the ideal sense rather than the modern temporal/evolutionary sense) from which all plants are derived." Traditional botanists such as Wilhelm Troll were used to neatly distinguish foliage leaves (phyllomes), stems (caulomes), and roots as the three most obvious structural categories of vascular plants, as distinct and invariant modules that build up higher plants.…”

“…Classical plant morphology as understood by Wilhelm Troll was based on essentialism, which is also known as conceptual realism [46]. According to David Baum [66]: "Essentialism implies that parts, a few more or less universal types, are the universal building blocks of vascular plants".…”

“…Agnes Arber 1930 [86] (pp. 308-309) Baum [66] published a stimulating essay on the key question: What are plant parts? He focuses on the "dynamic nature of development" and claims-at least for multicellular plants such as vascular plants-that "parts are not sharply defined things like the bolts and nuts and gears of a car, but repeating eddies or vortices in developmental flows."…”

“…Baum, as an enthusiastic proponent of EvoDevo, presented three complementary perspectives to understand the nature of plant parts such as leaves (or what botanists usually accept as "leaves" in vascular plants): (i) parts-as-structures (linking structures to their genetic causes), (ii) parts-as-functions (linking traits to responses to selection), and (iii) parts-as-processes (allowing to better recognize, e.g., "leaves" along a stem as being serially homologous). Baum [66] (p. 254) concluded: "Plant parts can properly be considered structures, functions, and processes".…”

“…Due to these possibly hard-wired psychological biases, it is very hard to resist the tendency to perceive plants as objects, drawing us to a language of nouns not verbs." David Baum (2019) [66].…”