Horsetails are the last survivors of plants that once grew 30 metres tall
The humble horsetail in a damp ditch is the only living genus of a plant group that dominated late Paleozoic forest floors for over 100 million years. Some of its extinct relatives were trees 30 metres tall whose remains fill Carboniferous coal seams. Its spores even crawl and hop on tiny springs.
Equisetum is the sole surviving genus in its family, a group of spore-bearing vascular plants found almost worldwide. Around 18 species are accepted, though some researchers count 25, and hybrids form readily within each subgenus. Both main subgenus names, one Latin and one Greek, mean horse hair. Molecular classification now places horsetails among the ferns, a surprising home for such odd-looking plants.
Their shape is distinctive. Green, hollow, jointed stems carry the photosynthesis, ridged with anywhere from 3 to 40 ribs, while the leaves are shrunk to tiny scales fused into sheaths around each joint. Botanists realised in 1999 that these little leaves are not primitive but were reduced from larger ones. Branches, when present, oddly sprout below the leaf whorls. Most species stand 20 to 150 centimetres, but tropical giants go further: the Mexican giant horsetail has been recorded at 8 metres.
Reproduction happens in cone-like strobili at the stem tips. In the field horsetail, pale, unbranched fertile shoots push up early in spring before the green ones. Each spore carries four ribbon-like elaters that coil and uncoil with humidity, making the spores crawl and hop away once the capsule splits. The spores are green, short-lived, usually lasting two weeks at most, and sprout immediately in humid conditions.
Chemically, horsetails share a cell-wall sugar, mixed-linkage glucan, with grasses, apparently evolved independently, and possess an enzyme for modifying it not known in any other plant. Fossils of modern-type Equisetum date back to the Early Jurassic of Patagonia and Australia, and silicified Late Jurassic specimens from Argentina already look fully modern. Today some species are treated as invasive weeds. The plants contain thiaminase and are toxic, although people have prepared them as food and folk remedies.
Source: Equisetum
