Atmospheric CO2 has been increasing rapidly since the late 1800s, largely due to human activities. Recall that CO2 enters leaves through stomata and can then be used for photosynthesis. However, transpiration occurs as a result of water evaporating through stomata. How have plants responded to elevated CO2 levels? Which of these structural features can help to limit water loss in plants that occupy dry habitats? a. abundant companion cells and sieve-tube elements b. stomata that are located in pits on the undersides of their leaves, or narrow, needlelike leaves c. extensive networks of xylem and phloem d. stomata that are located on the top surface of leaves, or broad leaves

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1. To address how plants have responded to elevated CO2 levels, it's important to understand that with more CO2 available, plants can afford to keep their stomata closed for longer periods. This reduces water loss through transpiration without sacrificing the CO2 needed for photosynthesis.
2. Considering the structural features that help limit water loss in dry habitats, option 'b' is correct. Stomata located in pits on the undersides of leaves or having narrow, needlelike leaves can reduce water loss by minimizing the surface area exposed to dry air and protecting stomata from direct wind.
3. Option 'a', abundant companion cells and sieve-tube elements, relates more to the transport of nutrients and sugars in plants, not directly to water conservation.
4. Option 'c', extensive networks of xylem and phloem, are crucial for the transport of water, nutrients, and sugars throughout the plant but do not directly limit water loss.
5. Option 'd', stomata on the top surface of leaves or broad leaves, would actually increase water loss, especially in dry environments, making it an ineffective adaptation for water conservation.