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What features make one plant able to withstand dry spells better than another with relatively similar structure? For instance, one of my Rudbeckias is wilting from drought at the moment, and an Oenothera next to it is not yet showing signs of dryness.

Or like jewelweed (Impatiens capensis), which wilts even while the soil is damp, in full sun, and ragweed (Ambrosia artemisiifolia), which will grow in very dry locations without being phased.

Is it caused by a faster transpiration rate in some plants than in others?

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The difference may be related to how the plants fix carbon. While all plants convert CO₂ and H₂O to glucose and oxygen, there are at least three pathways that are used to do it. The C3 pathway is older and less efficient than the CAM and C4 pathways. Many drought tolerant plants use the CAM or C4 pathways because less water is needed. The C4 pathway is better than C3 in drier climates, hotter climates, and when CO₂ or nitrogen are limited. The hard part about this answer is that I'm not a botanist. Wikipedia lists some plants that use C4, CAM, and C3, but there is no comprehensive list, so I can't match up all the plants you mentioned in your question. In general, a higher percentage of monocots, especially grasses, use C4, but a wider diversity of dicots use C4. But you still can't draw conclusions about families of plants, because while corn and sorghum use C4, rice and barley use C3. Ragweed, a plant you say grows well in dry conditions, is a member of Asteraceae, which includes many plants that use C4. However, rudbeckias are also members of Asteraceae, but don't do as well in dry conditions. The C4 pathway has appeared and disappeared in several different plants, making any general claim hard to make. Here are some links that might help:

If you know more about plants than me, that last link might be helpful, it's about the evolution of C4 pathway in plants and includes lists, but uses the formal species names I just don't recognize.

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Plants in drier conditions usually have reduced surface area, thick waxy cuticle covering the epidermis, reduced number of stomata, and water storage tissues that presides in its roots and leaves.

This means that even if the plants are similar, one of them might express more of one of the features shown above.

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  • $\begingroup$ But ragweed (one of my examples) has a large, thin leaf, barely any storage area, and no waxy cuticle, and withstands quite a bit of drought. Jewelweed, although it has a much reduced leaf area, and thick, watery stems, needs constant moisture. $\endgroup$ – J. Musser Jul 28 '14 at 1:57
  • $\begingroup$ i think this is a good answer inside, which is that there are lots of adaptations that cause drought tolerance. i would avoid citing any particular adaptations as universal, though many are common. $\endgroup$ – shigeta Jul 31 '14 at 1:54

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