Rattlesnake fern is a perennial fern with a single, fine-textured (not leathery) leaf. Like other grape ferns, all of what appears aboveground is actually a single leaf that has two parts: a vegetative (leafy) portion and a fertile (spore-bearing) portion. The vegetative portion is pale green, sessile (stalkless), broadly triangular in outline, and 3–4 times pinnately (feather) compound; it is so deeply compound that it can seem like a whorl of 3 leaves. The upright stalk of the fertile portion rises from the joint where the leafy portion is attached, and it can be up to 1.5 times as long as the shared stalk below. At the top of the fertile portion, tiny, ball-shaped spore cases are borne in a twice-pinnately compound cluster. Spores mature May–July.
Unlike other grape ferns, the leaves of this species are not evergreen. New leaves develop during the spring and do not overwinter; they wither in late summer.
Similar species: Missouri’s two other grape ferns (both Botrychium species) have the leafy portion and fertile portion apparently arising from the ground on separate stalks (however, they do share a single stalk low to the ground). Rattlesnake fern is the only one where the shared stalk rises well above the ground, with the leafy portion clasping the stem at a point usually some inches above the ground, and the stalk of the fertile portion rising from that juncture.
Height: 6–24 inches.
Habitat and Conservation
This is the most common grape fern in Missouri. It occurs in various kinds of forests and woodlands and in shrubby thickets in prairies. In the loess hill prairies of northwestern Missouri, rattlesnake fern plants are often dwarfed, existing as small vegetative leaves for several years, then finally developing spores when only 6–8 inches tall.
This fern was long considered a member of genus Botrychium, but recent molecular (DNA) studies have shown this plant to be different enough from other grape ferns that it deserves to be in a separate genus, Botrypus. In addition to having a different number of chromosomes compared to other grape ferns, it also seems to have acquired some DNA from a mistletoe species at some time in the very distant past — possibly via connections with a root-parasite mistletoe. This “horizontal gene transfer” must have happened very, very long ago, since rattlesnake ferns all over the world possess these remarkable mistletoe DNA sections.
Like other fern groups, members of the adder’s tongue family have a two-part life cycle. The part we usually see is the sporophyte, a plant that produces spores. When the spores germinate, they develop into gametophytes. In the adder’s tongue family, the gametophytes are cylindrical to nearly spherical. They usually live underground, and they lack chlorophyll — so they’re not green, and they do not nourish themselves through photosynthesis. Instead, they take nourishment from a mycorrhizal fungus that lives in the soil. This fungus attaches to them soon after germination. The underground gametophytes produce eggs and sperm, which unite and develop into a new sporophyte. Young sporophyte plants often still have the gametophyte attached to the stem base, which allows you to see this seldom-seen portion of the life cycle.
There are different explanations for the name “rattlesnake fern.” Some people say it was named because it usually grows in places where rattlesnakes live. Others say that the clusters of spore-bearing capsules resemble a rattlesnake’s rattle.
In the Himalayas, this species grows large and succulent, and it is boiled as a food plant there. It has also been used medicinally in various places across the world.
This fern is rarely cultivated because of its dependence on certain types of soil-borne fungi. Without the fungi, the germinated spores fail to thrive and adult ferns suffer. Please enjoy this fern in its natural habitat.
The science of phylogenetics uses DNA sequences to shed light on the evolutionary history of various species, which provides information on the true relationships among species. It is very much like the paternity and other genetic testing that people do. Before DNA testing was available, people could only judge relatedness — of any organisms, including people — by using relatively large characteristics, such as leaf shape, number of stamens or petals, blood type, nose shape, hair color, and so on.
Like its relatives in the adder’s tongue family, rattlesnake fern depends on mycorrhizal fungi to provide it with nutrients through its life cycle. These fungi live in the soil and attach to the fern’s roots. Thus anything that disrupts the soil, or changes its chemistry or moisture cycle, can harm the fungi, which ultimately harms the ferns.
Wildlife probably browse on the leaves of this plant, and insects probably feed on its leaves, stems, or sap, but little information is available about the plant-animal interactions with rattlesnake fern.