Leaves that Close at Night, by Amy

It all started when I went out to look at a special wild tree, called a Palo Verde tree, near our home.  It was about two weeks ago, and I went in the late afternoon.  Usually the leaves on this little tree look like this: wide open, tiny leaves that reach to the sunlight.

Palo Verde leaves in sunlight

But on that afternoon, they looked like this: folded up against each other.

Palo Verde leaves in shade

 They were in the shade, as the sun was beginning to set; and I remembered that I had seen them half-closed on another day when I visited in the afternoon.  You can see it again in this picture.

Palo Verde leaves in shade

It was a fascinating question: do leaves close at night?  Do they close in the shade?

I asked some friends on my garden blog, and the answer was "Yes!"  Some leaves close at night, just as some flowers close at night.  Botanists call this "nyctinasty".  (Botanists are the scientists that study plants.)  They don't know yet what advantage it gives for the plant.

Although no one seems to know why it happens, botanists do know how it happens.  At the base of the leaves there are miniscule cell structures that contain extra water.  As this water is pumped in and out of the cells, the leaves open or fold.  The cells act as little hinges.

It is most common in plants from the pea and bean family, called the Legume family.  This is actually a very large group which includes many plants we don't think of as peas or beans!  Here in the desert there are several very fine plants that are Legumes.  The wild Palo Verde tree is a Legume.  So is my beautiful young Fairy Duster shrub.  Its leaves also close in the shade and at night, like this...

Fairy Duster leaves in afternoon shade

Fairy Duster leaves in afternoon shade

I wondered whether plants from northern climates close their leaves as well.  I learned that they do.  Even some garden beans close their leaves at night.

I went out to look at my young sweet pea plants.  Although I am growing them in my desert garden, they are originally from cooler climates, so I thought I would like to see what they were doing in the evening.

Sure enough, some of their leaves were folded!  But I am not sure yet whether these are folded for night, or just because they are young!

Sweet pea leaves in afternoon shade