USA Biology Olympiad (USABO) Practice Exam

Which combination of features exemplifies xerophytic adaptations?

1. Stomata open during the day

2. Thick waxy cuticles and hairy leaves

3. Reduction in leaf area and rolled leaves

4. All of the above

The correct answer is: Reduction in leaf area and rolled leaves

The correct choice highlights xerophytic adaptations, which are specialized features that enable plants to survive in arid environments where water is scarce. Xerophytes evolve several strategies to reduce water loss and maximize moisture retention. The option focusing on a reduction in leaf area and rolled leaves is particularly relevant. A reduction in leaf area decreases the surface available for transpiration, which is the process where water vapor is lost from the plant to the atmosphere. Additionally, rolled leaves can help to create a microenvironment with higher humidity compared to the surrounding air, which reduces water loss further. This adaptation effectively minimizes exposure to the sun and wind, both of which can increase evaporation rates. In contrast, if stomata were to open during the day (the first option), this would generally not be conducive for xerophytes since it would allow moisture to escape into the dry air. While the second choice does mention some relevant adaptations—such as thick waxy cuticles that reduce water loss and hairy leaves which can trap moisture—these do not encompass the entire spectrum of xerophytic features in the same direct way as the reduction in leaf area and rolling of leaves, which are very specific adaptations to conserve water. Thus, the focus on reduced leaf area and leaf rolling