One of the common weeds in open areas of Oklahoma is the sensitive plant, genus Mimosa. Its leaves are divided into leaflets, and each leaflet into pinnules. It is called a sensitive plant because the leaves, when touched, will close up. The most famous sensitive plant (available from retailers) is Mimosa pudica, a now-widespread weed of tropical and other warm areas, although the Oklahoma plants are probably M. quadrivalvis. I have just posted a Darwin video about this plant if you would like to see it at work.
The reason I call it an evolutionary mystery is that nobody is quite sure why the leaves close up in response to touch. The leading explanation, which I dutifully included in my botany classes, is that it is a defense against herbivorous insects. An insect such as a grasshopper might land on what looks like a big juicy leaf, only to see it disappear, becoming what looks to an insect like an inedible stem.
This leaf response is a response to touch, not necessarily to an insect’s touch. The leaves also close up in response to raindrops. This opens the possibility that the leaves close up not so much to escape the attention of a herbivore but to keep from being weighed down my raindrops.
A related genus within the subfamily Mimosoideae, the genus Albizzia, is the famous mimosa tree. Its leaves do not close in response to touch. Instead, they close up each night and open each morning. The leading explanation for this, one first proposed by Charles Darwin and his son Francis (widely considered the founder of modern plant physiology), is that by closing up the leaf avoids frost damage caused by the clear night sky.
Of course, more than one explanation can be correct at the same time. The mimosa tree, by closing its leaves at night, might avoid frost damage and at the same time damage from raindrops. I have frequently seen mimosa trees which, on rainy mornings, have delayed re-opening their leaves.
The biggest problem with all evolutionary stories, this one and others to follow, is that they leave unanswered the question of my other plants do not have this adaptation. All leaves would benefit from closing up in response to an insect, but almost none of them do, except those of sensitive plants.
In this case, the herbivore and raindrop explanations may be valid within this evolutionary lineage. For whatever reason, the genus Mimosa evolved bags of pressurized water known as pulvini that quickly push the leaves closed in response to touch. Other leaves, without pulvini, cannot close up. The leaves benefit from closing up if they happen to have, for this or for some other reason, pulvini. Other plants, such as the velvetleaf Abutilon theophrasti, lower their leaves each night and raise them each morning, just like Mimosa. Abutilon has pulvini. I do not know why pulvini are not more widespread in the plant kingdom. Abutilon is in the Malvaceae family. Pulvini are also found in the Marantaceae and Oxalidaceae. Most plants do not have them. It is not that the other plant families would receive no benefit from having their leaves open and close, or raise and lower; they, perhaps by chance, do not have pulvini.
The ”adaptive stories” of evolution, such as touch-sensitive leaves, are usually very good at explaining the benefit that an organism may get from having the adaptation, but are usually not very good at explaining why other organisms do not have the adaptation.