Plant morphology
Phytomorphology is the study of the physical form and external structure of plants.[1] This is usually considered distinct from plant anatomy,[1] which is the study of the internal structure of plants, especially at the microscopic level.[2] Plant morphology is useful in the visual identification of plants. Recent studies in molecular biology started to investigate the molecular processes involved in determining the conservation and diversification of plant morphologies. In these studies transcriptome conservation patterns were found to mark crucial ontogenetic transitions during the plant life cycle which may result in evolutionary constraints limiting diversification.[3]
Modern Innovations[edit]
Rolf Sattler has revised fundamental concepts of comparative morphology such as the concept of homology. He emphasised that homology should also include partial homology and quantitative homology.[19][20] This leads to a continuum morphology that demonstrates a continuum between the morphological categories of root, shoot, stem (caulome), leaf (phyllome), and hair (trichome). How intermediates between the categories are best described has been discussed by Bruce K. Kirchoff et al.[21] A recent study conducted by Stalk Institute extracted coordinates corresponding to each plant's base and leaves in 3D space. When plants on the graph were placed according to their actual nutrient travel distances and total branch lengths, the plants fell almost perfectly on the Pareto curve. "This means the way plants grow their architectures also optimises a very common network design tradeoff. Based on the environment and the species, the plant is selecting different ways to make tradeoffs for those particular environmental conditions."[22]
Honoring Agnes Arber, author of the partial-shoot theory of the leaf, Rutishauser and Isler called the continuum approach Fuzzy Arberian Morphology (FAM). “Fuzzy” refers to fuzzy logic, “Arberian” to Agnes Arber. Rutishauser and Isler emphasised that this approach is not only supported by many morphological data but also by evidence from molecular genetics.[23] More recent evidence from molecular genetics provides further support for continuum morphology. James (2009) concluded that "it is now widely accepted that... radiality [characteristic of most stems] and dorsiventrality [characteristic of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!."[24] Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties.”[25]
Process morphology describes and analyses the dynamic continuum of plant form. According to this approach, structures do not have process(es), they are process(es).[26][27][28] Thus, the structure/process dichotomy is overcome by "an enlargement of our concept of 'structure' so as to include and recognise that in the living organism it is not merely a question of spatial structure with an 'activity' as something over or against it, but that the concrete organism is a spatio-temporal structure and that this spatio-temporal structure is the activity itself".[29]
For Jeune, Barabé and Lacroix, classical morphology (that is, mainstream morphology, based on a qualitative homology concept implying mutually exclusive categories) and continuum morphology are sub-classes of the more encompassing process morphology (dynamic morphology).[30]
Classical morphology, continuum morphology, and process morphology are highly relevant to plant evolution, especially the field of plant evolutionary biology (plant evo-devo) that tries to integrate plant morphology and plant molecular genetics.[31] In a detailed case study on unusual morphologies, Rutishauser (2016) illustrated and discussed various topics of plant evo-devo such as the fuzziness (continuity) of morphological concepts, the lack of a one-to-one correspondence between structural categories and gene expression, the notion of morphospace, the adaptive value of bauplan features versus patio ludens, physiological adaptations, hopeful monsters and saltational evolution, the significance and limits of developmental robustness, etc.[32] Rutishauser (2020) discussed the past and future of plant evo-devo.[33] Our conception of the gynoecium and the search for a fossil ancestor of Angiosperms changes fundamentally from the perspective of evo-devo.[34]
Whether we like it or not, morphological research is influenced by philosophical assumptions such as either/or logic, fuzzy logic, structure/process dualism or its transcendence. And empirical findings may influence the philosophical assumptions. Thus there are interactions between philosophy and empirical findings. These interactions are the subject of what has been referred to as philosophy of plant morphology.[35]
One important and unique event in plant morphology of the 21st century was the publication of Kaplan's Principles of Plant Morphology by Donald R. Kaplan, edited by Chelsea D. Specht (2020).[36] It is a well illustrated volume of 1305 pages in a very large format that presents a wealth of morphological data. Unfortunately, all of these data are only interpreted in terms of classical morphology and the qualitative homology concept, disregarding modern conceptional innovations.[37]
Including continuum and process morphology as well as molecular genetics would provide an enlarged scope.[38]