What Are Those Funny Looking Bumps on the Tree Leaves?
—Michael Penziner, FRNC Docent
We bet you've seen them hundreds of times and never once paid any attention to them. The illustration shown here is basically conical "hats", but they do come in different sizes and shapes, depending on which plant they are found on. These swellings are called galls. So the question remains, what are they? If you've been following these columns, you've noticed that we often talk about the relationship of insects to plants, how they sometimes have a symbiotic relationship. Certain insects are associated with certain plants: the insects eat the leaves, or they lay their eggs on them, or their larva eats them, and in return they may act as pollinators. In any event, these galls are as a result of a certain species of insects laying their eggs on a certain species of leaves, which in the case of this illustration happens to be the leaves of the witch hazel tree. Basically what happens is that in early spring the insect lays its eggs on the leaf and the leaf reacts through hormonal spurts by growing extra tissue around those eggs. The tissue provides protection for the larva as well as food for the larva.
The variety of shapes that these galls take ranges from the little bumps on the leaves that you see here (right insert), to the much larger oak apple (yes, they are shaped like little apples on the twig). In the case of the galls which can be found specifically on the oak tree, they are caused by different insects and differ one from the other slightly but are distinguished by their common names such as oak apple, oak marble gall, oak artichoke gall, and the acorn cup gall. In the world of plants these galls can appear on leaves, on leaf stems, on midribs, on flowers, and on twigs, depending on which host plant we are looking at. If you remember to do it, this autumn find some goldenrod and look at the lovely gall to be found on the top of some of the stems (see left insert). They do no harm to the plant because the amount of nutrient they take is so small as not to affect the plant growth.
The next time you take a walk in the woods, see how many different kinds of galls you can find. Maybe a lot or maybe not. But each walk will be interesting now that you know what they are.
