It’s winter and the trees, most of them, don’t have leaves on them. But it was pointed out to me by Meredith (Master Teacher here at the Museum) that several small trees on the south side of the Wetlands still had some of their leaves attached, though they were all attached to the tree in an odd way.
According to Meredith, the leaves looked as though they had been stuck onto the twigs in a manner as would a receipt on a receipt spike, as if they had been pushed onto the end of the twig. The little leaves were not actually attached to the tree but would spin freely and ride up and down on the twig with the wind.
Hmm, another mystery to solve.
First, if possible, we had to determine what kind of tree we were dealing with. There were three or four of the trees next to the boardwalk where it descends into Explore the Wild.
They were all sycamores, young sycamores (12′ – 20′) that had yet to acquire the pealing lower bark and white upper bark of mature trees. I remembered the trees from earlier in the year when they had their large, broad leaves. But there were more clues to the identity of the trees besides the remembrance of the past season’s leaves.
Leaf scars are what’s left after the leaves fall off of the tree, they are where the leaf petiole, or leaf stem, had been attached to the twig. Sycamore leaf scars are distinctive, although I had to be reminded of that by referencing a tree field guide. The scars nearly encircle the buds.
OK, so it’s a sycamore, but what are those little leaves spinning around on the twigs? I remember seeing those small “leaves” when they were green during the growing season. I was curious as to why they were so small. They looked like stunted sycamore leaves. But I simply shrugged them off as, well, stunted sycamore leaves that hadn’t grown out completely due to some insect or disease infestation. And besides, I’m sure that at the time I was too busy doing something else to dig any deeper, there’s so much going on during the summer, you have to pick your mysteries carefully.
A stipule is an outgrowth of a young leaf and is part of the base of the leaf. Although some plants do not have stipules, in those that do the stipule can take on many forms. It can be a very small tendril like structure, a spine (the thorns on acacia trees are stipules), or it may take on the shape and form of a leaf.
I have yet to discover the definitive answer as to the purpose of stipules but they may have evolved as a protective structure for young emerging leaves. The stipules that are more leaf-like in structure may actually perform some of the functions of leaves (photosynthesis).
Sycamores happen to have stipules that are very much like leaves.
Why don’t the stipules fall off the tree when the leaves do. Many of them do, but those that don’t are still on the tree during winter due to their manner of growth. As you can see in these photos the base of the stipule nearly surrounds the twig, it’s attached to the twig for most of the twig’s circumference.
The leafy part of a sycamore stipule grows both outward and around the stem. The stipule that I have in my hand at the moment had actually grown into itself, the sides had grown together and were attached, holding it onto the twig.
When the stipules dry out they retain their shape (like leaves do) and since they grow around the twig, and despite the fact that they have detached from the twig at the point of growth, they stay on the twig. Eventually they become too brittle or decay to a point where they break apart and fall to the ground. But for most of the fall and part of the winter some of the stipules remain on the tree if for no other reason than to make curious naturalists wonder what they are and why they are there.
Read enough about stipules? If so, you can stop here, I’m done. If not, you can look deeper into their structure, origin, and purpose by browsing through “The Nature and Origin of Stipules,” Ansel Augustus Tyler, at Google books where that publication has been digitized and is viewable online.