Isoetes and I

I have always felt that field biologists are primarily explorers. Even as we revel in a new discovery, our minds are already drifting off to the next expedition. We all have our own lengthy lists of locales and taxa for which we pine. For me it is usually a certain species, but with some groups I would be equally happy to find any member of the genus. The genus Isoetes (Isoetaceae) is once such group. With my feet and eyes, I have covered a lot of ground in Missouri and the Midwest. Yet, somehow I have never crossed paths with an Isoetes. I have molested many sterile Eleocharis and Juncus in the distant hope that they were Isoetes. Experience has told me, I will know one when I see one.

Friday morning, I met with the esteemed Missouri Botanist Alan Brant for some stream corridor work in the St. Francois Mountains (center of uplift for the Ozark Plateau characterized by a preponderance of rhyolite). He enticed me along on the trip with the promise of nice weather and possibility of finding some fens, seeps and springs. About a mile into our trek, at the base of a slope, Alan spotted a grove of Alnus serrulata (Smooth Alder) in a young even age cut. Within the grove we could see open grass/sedge areas. Given the geology of where we were and the dominance of Osmunda regalis, Panicum microcarpon, and Alnus serrulata we decided it was an acid seep (really more like an acid fen if such a creature exists). Once we fought our way through the dense stand of saplings and briars that tend to follow blatantly disrespectful logging practices, we explored the seep. As you may know, botanizing in the winter is mostly an exercise in forensics, as the dried and shattered remains of carices, grasses, composites, ferns and the like are the only fodder for study. So anything green, is gold. As I approached some shallow pools that were covered with a thin layer of ice and rife with salamander eggs, I saw green.

Having been prematurely excited on other occasions by what proved only to be Eleocharis stems, I was a bit hesitant to believe that what I was seeing could be an Isoetes. But as I got closer and knowing I had never seen anything so Isoetes-like as these rosettes, my heart jumped. I knew it was my first bona-fide Isoetes. I called Alan over and he verified it.

In Missouri, there are three species of Isoetes, each with a fairly wide distribution (I. butleri, I. engelmannii and I. melanopoda). Two are common and one, I. engelmannii, is listed as “probably state imperiled” (S1?).

While the best way to identify Isoetes englemannii is by the sculpturing on the megaspore, a field determination can be made by habitat, size and characters involving the leaf bases. Isoetes engelmannii is the only submerged aquatic species in the state, as this clearly was; the other two species are more emergent. Isoetes butleri is principally found on calcareous substrates (such as dolomite or limestone glades), so it wasn’t this species. Isoetes engelmannii has pale leaf bases, as did our specimen. Isoetes melanopoda, as the name suggests, has a black coloration to the leaf bases. Thus by morphology and habitat, we came up with the field determination of Isoetes engelmannii. A hard target search of the fen revealed at least 30 more stems. Given the extent of the population and Alan’s opinion that it is far more common than its S1 status indicates (a status more indicative of the rarity of habitat), I collected a small sample. Since it is a new population the sample will be sent to the Missouri Botanical Garden herbarium and an Element of Occurrence Record will be filed with Natural Heritage.

Isoetes (the Quillworts) fit into that silly lay-term “fern allies” that also includes the Horsetails (Phylum Sphenophyta), the Whiskferns (Phylum Psilotophyta) and Club Mosses (Phylum Lycophyta, as is Isoetes)(based on Raven’s “Biology of Plants”, sixth edition). Being an artificial term, “fern allies” has no real taxonomic significance other than designating the non-flowering, non-fern vascular plants. Isoetes is somewhat closely aligned with the Lycopods (Club Mosses) but is the only genus in the order Isoetales. There are roughly 150 known species of Isoetes, distributed worldwide.

Isoetes have a very unique morphology. They grow from a corm which, unlike a bulb (composed of modified leaves), is simply a very short and slightly modified stem. In the photo below the corm is the brown bulbish portion of the base.

The leaves grow from the top of the corm and the roots grow from the bottom. However, Isoetes do it in a strange fashion. The corm is more or less divided into halves. The photo below is taken 90 degrees from the previous photo.

So the corm of Isoetes is kind of like the body of a yo-yo with the leaves and roots growing out along the seam (where the string winds). In the photo below I have made a section perpendicular to the seam.

Here you can see the roots coming out of the heart-shaped bottom of the corm and the leaves out of the heart-shaped top.

The leaves of Isoetes are four-chambered in cross-section. This feature allows the leaves, once gas filled, to float. The base of each leaf flattens out like a clove of garlic. In this “clove” area the sporangia are formed. Like the genus Selaginella, Isoetes are heterosporous (meaning the “male” and “female” spores are very different). The “male” spores are called microspores and the female spores are megaspores. Unlike ferns, the sporangia of Isoetes are like little white eggs. In the photo below (be sure to click on it) you can see last years megaspores (each is about 0.5mm in diameter) erupting from the leaf base. Just to the right you can see some newly formed megaspores.

Each individual plant alternates between the production of male and female spores; the megaspores are produced in the spring and the microspores in the summer. I put several megaspores under the dissecting scope and they are indeed sculpted with ridges which is the key reproductive character for this species in our range.

My mind has been racing, since finding Isoetes engelmannii. I’ve been mentally rearranging my already booked summer schedule for a chance to hunt down the other two species. Where should I look? Perhaps I could cheat by copying label information next time I’m at MoBot. But in reality, plant finds, like many things in life, are best when they are spontaneous. I’m sure I’ll find the other two. And if I’m lucky, it will be when I least expect it.