Chapter 31.5, Problem 1CS

Core Skill: Modeling The goal of this modeling challenge is to propose a visual model that compares the density of leaf veins between ferns and another plant group.

Modeling Challenge: Figure 31.21 provides a model of the process by which fern leaves having branched vascular systems (euphylls) are hypothesized to have evolved from ancestors with branching stems. Imagine that the leaves of some other plant group evolved similarly, but from stem systems that were twice as highly branched. In other words, when flattened, the stem system ancestral to this other plant group had twice as many branches per unit area. Assuming that the branch density of ancestors is directly related to vein density of leaves in descendant plants, draw a pair of models that compare the leaves of ferns and this other plant group, emphasizing the vein density in each type of leaf. How does the venation differ in the two leaf models?

Figure 31.21 Lycophylls and euphylls. (a) Most lycophylls possess only a single unbranched leaf vein with limited conduction capacity, explaining why lycophylls are generally quite small. (b) Euphylls possess branched vascular systems with greater conduction capacity, explaining why many euphylls are relatively large. (c) Fossil evidence suggests how pteridophyte euphylls might have evolved from ancestors with branched-stem systems.