The tracheid-vessel element transition in angiosperms involves multiple independent features: cladistic consequences

Area of Science:

• Plant anatomy
• Evolutionary biology
• Wood formation

Background:

• Current definitions of tracheids and vessel elements rely on outdated light microscopy, neglecting advanced SEM and TEM data.
• Existing classifications are primarily based on eudicots and predate thorough studies of basal angiosperms.

Purpose of the Study:

• To re-evaluate the definitions of tracheids and vessel elements using comprehensive microscopy data.
• To identify and characterize evolutionary changes in these water-conducting cells across vascular plants.
• To propose refined criteria for classifying intermediate forms and evolutionary transitions.

Main Methods:

• Analysis of data from light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
• Comparative study across diverse angiosperm groups, including basal angiosperms.
• Examination of six key characters involved in the evolution of tracheids and vessel elements.

Main Results:

• Identified limitations in current, overly simplistic definitions of tracheids and vessel elements.
• Recognized evolutionary changes in six characters during the transition between tracheids and vessel elements in various vascular plants.
• Observed numerous taxa with incipient vessels, not meeting all criteria for fully developed vessels.
• Highlighted increased conductive area and pit area on end walls as key indicators of intermediacy.

Conclusions:

• Vessel presence/absence should not be a simple binary character in cladistic analyses.
• Refined criteria are needed to accurately classify intermediate forms between tracheids and vessel elements.
• A more nuanced understanding of water-conducting cell evolution is necessary, incorporating advanced microscopy and broader taxonomic sampling.