The Compound Leaves of Serjania atrolineata

A couple of years ago I started using a plant press to save specimens from plants I could not identify. Recently I had the opportunity to visit the Smithsonian Tropical Research Institute in Panama City, and I took a couple of samples along for possible identification. I did not know whether anyone would have the time or desire to look at my plants, so I took along only a couple of them. I was in luck. Carmen, one of the workers in the Herbarium, not only gladly looked at my plants but also keyed them out and pulled out some of the Herbarium’s own specimens so I could see them for comparison. It was a neat experience. She identified this plant as Serjania atrolineata, a member of the Sapindaceae family. More about Serjania in a bit. Back when I was collecting the plant, I took a quick look at the leaf arrangement. I thought I saw leaflets coming off a stem in groups of three. So I made a note to myself that these were “trifoliate” leaves. I interpreted the grouping in this image as follows: these were opposite compound leaves with a trifoliate leaf at the end of the stem (a terminal leaf, I thought).

It’s important to know whether a leaf is simple or compound if you want to identify the plant. If only one leaf arises from the stem, it is a simple leaf. If you see a leaf stalk arising from the stem and you see that it supports several leaves (called leaflets in this case), then you have a compound leaf. Knowing the difference between compound and simple leaves now, can you see where I was wrong in my initial notes? A look at the pressed sample gives you a big clue. The leaf shown within the red rectangle is the true leaf, consisting of 9 leaflets.

When you zoom in on the leaf (image below), you’ll see I’ve labeled the stem and the rachis . A rachis is the leaf stalk or petiole of a compound leaf. When I first looked at this plant, I had assumed the rachis was the stem and that each group of 3 leaflets was arising from the stem. This is where I was wrong.  

In the pressed sample, the difference between the stem and the rachis is perfectly clear. It’s also clear that the compound leaf of the Serjiana is rather special. Imagine that there were a single leaflet within each circle above. Then we would have a compound leaf consisting of 3 leaflets and it would be called a “trifoliate” or a “ternate” leaf. Instead we have 3 leaflets within each circle. This kind of leaf arrangement is called biternate, which is defined as “doubly ternate with the ternate divisions again ternately divided” (Harris & Harris). I have to admit that this kind of terminology really intimidated me when I first started trying to identify plants. Fortunately, in the Harris and Harris book each term is illustrated. Once you look up the word “ternate” and learn that it is a 3-part leaf and then come back to “biternate” and look at the illustration, the definition becomes clear. Now that it’s clear, I hope, that the leaves of Serjania are compound, with the special “biternate” structure, the next step is to determine the arrangement of the leaves along the stem. The importance of leaf arrangement Knowing the difference between a stem and a rachis is crucial if you want to learn the leaf arrangement of a plant. Leaves arise from a stem. Leaflets arise from the rachis. The location from which leaves arise on the stem is called a node. If two leaf stalks arise from one node, the leaves will be on opposite sides of the stem, and the arrangement is called opposite. If only one leaf stalk arises from the node, the next leaf along the stem will be on the other side, and the arrangement is called alternate.

In the above image you can see that only one leaf stalk – in the case of Serjania, the leaf stalk is a rachis – arises from each node. Opposite each rachis, though, is a tendril. Serjania is a liana (woody vine). In the initial observations I made two years ago, I thought the trifoliate leaflets were the leaves. I had never heard of biternate leaves and so I thought the leaves were opposite whereas they are actually alternate. No wonder I had never been able to identify this plant! I hope I’ve learned something about observation and about recognizing compound leaves since then, especially since I spent a little time describing compound leaves in a post on a plant from the bean family the following January. Meantime, now that the plant has been identified by a professional, I’ll just summarize where Serjania atrolineata fits into the scheme of things. Family: Sapindaceae (the soapberry family) Serjania atrolineata is in the Sapindaceae family. Most genera in this family are trees, but most species are lianas. The tendrils of the climbers do not coil except at the tips (see image below). There is a small amount of latex in the stem. Leaves are alternate. Flowers, at least in our area, are small (Gentry). That latex, or milky sap, “…may contain mildly toxic saponins with soap-like qualities….” (wikipedia).

Serjania is a large genus in the family There are about 375 species in the genus Serjania but just about the only information I can glean from the internet on the genus is that extracts from certain species are used as fish poisons (NCBI, TICO, Economic Botany). The poison is apparently enough to stun the fish but not enough to cause toxicity to the humans who eat them. The name Serjania comes from the Rev. Father Phillipp Sergeant, an 18th century monk skilled in botany (Botanary). Serjania atrolineata – the (dark line?) The prefix atro means “dark” and -lineata means “line” or “lined.” Maybe the purplish stem in the image below is the dark line? Or is it dark from the treachery of its poison? I don’t know whether S. atrolineata is one of the “fish poison” species, but if it is, the toxicity comes from this milky sap.

At any rate, for our corner of the savanna, the neatest thing about Serjania atrolineata is its sophisticated compound leaves. It’s an attractive liana among many at the edge of the woods, and I don’t think I’ll ever mistake it for anything else, now that I know its leaves.