I’m sure most readers of our blog are familiar with the orchid family (Orchidaceae), the plant family that provides us with the highest number of new species discovered in our reserves. The plant family which has given us the next-highest number of new species is much less familiar to the public, and so I am going to write a series of posts on this family, the melastomes (Melastomataceae). Most people, especially in the temperate zone, have never even heard of this family, but it is one of the largest families of Neotropical woody plants, both in number of species and number of individuals. If you stand anywhere in any of our reserves, you will almost always be able to see at least two or three species of melastomes without even moving. They are mostly shrubs or small trees, though a few are herbs. They are one of the first plant families that tropical biology students learn, because they are so easy to recognize and so common.
Melastomes have very unusual flowers that are easy to recognize, with four to eight petals and usually 5-10 large and complex anthers. The anther tips are often bright yellow. Most plants’ anthers are yellow from the pollen they bear, but the yellow of the anthers of the melastomes is just pigment; the actual pollen is hidden inside the hollow anthers. In most melastomes, pores or holes develop at the tips of the anthers, and a visiting bee vibrates them like a salt shaker to get the pollen out. The vibrations are generated by the bee buzzing its wings in a special way while holding onto the anther, leading to the name “buzz pollination” for this pollination syndrome.
Buzz pollination of the melastome Blakea subvaginata by a carpenter bee. Lou Jost.
This family is also easy to recognize even without flowers. With the exception of one genus, the leaves in neotropical melastomes have very distinctive ladder-shaped veins. There are several large almost-parallel veins running from the base of the leaf to its tip, with many thinner cross-veins running between the big veins, and perpendicular to them. Leaves from some of the melastome species in my yard are shown below; these show the typical vein pattern on a range of leaf sizes and shapes, though as we will see in future installments, there are more unusual ones.
Another easy-to-see trait of this family is that the leaves are arranged on the stem in pairs, one opposite the other. This contrasts with most other plants, whose leaves are arranged alternately along the stem. Again, as we’ll see later, there are some unusual exceptions.
The leaves of this family, with their ladder-shaped veins, can only be confused with those of the stinging nettle family (Urticaceae). The two families can usually be distinguished from each other vegetatively because most members of the stinging nettle family have alternate leaves or, if they are opposite, one leaf of the pair is much smaller than the other. Melastomes usually (but not always) have two equally-large leaves forming each pair.
Though the leaves share these macroscopic features, they have a wide variety of microscopic ornamentations. The leaves often have hairs or complex branching spines and/or glands and scales and other growths. We’ll see some of these unusual leaf ornaments in future posts. For now, here are 5mm sectors of the tops and bottoms of several different melastome species, to give you a taste of the variety of their leaf architectures at the microscopic level.
Above: Tops and bottoms of some melastome leaves. Each section is about 5mm across. I strongly encourage you to click any of them to enlarge them, so you can really see the complexity of their structures. Photos: Lou Jost/EcoMinga
In future posts I will look at some of the important melastome genera and species in our reserves.
All articles in the series:
Lou Jost, Fundacion EcoMinga.