Thursday, August 1, 2013

Of Snake Grasses with Siliceous Stems

Horsetails have a long evolutionary pedigree stretching back over 300 million years to the Carboniferous Period. Their extinct relatives grew to the size of trees, and along with the giant ferns and clubmosses of the day they formed our coal beds. All extant Horsetails are members of the genus Equisetum and some species such as the Mexican Giant Horsetail – Equisetum myriochaetum – still attain a respectable height.

Horsetails reproduce in a manner similar to their allies, the ferns, by alternating generations of sporophytes and gametophtes. The familiar plant we encounter on a walk through the woods and fields is a sporophyte, producing spores in cones composed of hexagon-shaped plates and borne on the ends of fertile stems. Should the spores land in a place favourable to growth they will grow into gametophytes, fertilization occurs and a sporophyte is produced, completing the cycle. A short, oversimplied summary of a complex and fascinating life history, here's the story in its entirety at the International Equisetological Association.

There are two sub-genera within Equisetum, the Horsetails and the Scouring Rushes. By far the most common of the first group is the Field Horsetail (Equisetum arvense), which in our area can be found growing in wet, sandy soil along the trans-Canada Trail. It's form can be quite variable, but as a rule the branches of the sterile stems are upswept. The pale brown fertile stems have no branches and appear in the spring before the sterile stems.

Fertile stems in early spring.

A closer look at the delicate, graceful cone.

Sterile stems sprouting later in spring.

A patch of sterile stems.

Another group – note the ascending branches.

Studying the profiles of the stems in cross-section is useful in separating the various horsetail species, and illustrations may be found in a good field guide such as the Peterson Field Guide to Ferns: Northeastern and Central North America, 2nd Edition. There are also some excellent photos of stem cross-sections and Equisetum anatonmy in general at APHOTOFLORA.

Incidentally, Equisetum spp. have silica embedded within their stems and the plants were – and sometimes still are – used to scour pans and as an abrasive, hence their common name Scouring Rush. And since they can also take up gold when its in a solution, horsetails are often used to check the levels of the valuable metal. But getting back to studying the stems in section ...

A collage of a couple of Field Horsetail stems seen in cross-section. As per the illustration in the field guide, the central cavity is a quarter to a third of the stem diameter, but the surrounding vallecular canals should be only a third to a quarter the width of the central cavity. The canals of these two specimens are larger than this, in the bottom image nearly the diameter of the central cavity. Were it not for the proportion of the stem sheath to the first branch internodes, one might suspect this stem belongs to a Marsh Horsetail (Equisetum palustre).

It looks like this is indeed Field Horsetail ... the first branch sections or internodes are definitely longer than the dark-tipped, lance-shaped stem sheaths, which tend to occur in pairs. (The lowest branch internode of the Marsh Horsetail would be shorter than the adjacent stem sheath.)

A closeup shot of the green branch teeth with the next section of the branch removed.

Except that the fertile stems bear cones, the fertile and sterile stems of the Water Horsetail (Equisetum fluviatile) are similar to one another. The plants in the following three photos were encountered at the Stoco Fen; no surprise, as this species prefers wet, flooded areas.

The branches can vary considerably in length and number – some stems may bear only a few branches, such as this specimen from Stoco Lake, or even none. The stem walls of Water Horsetail are very thin, hollow and easily compressible, with the central cavity comprising 80% or more of the stem diameter.

The Wood or Woodland Horsetail (Equisetum sylvaticum) is easily distinguished by whorls of branches that in turn rebranch. These specimens were growing, as one might infer from their name, in moist forest soil west of town. A cone borne on a fertile stem in mid-May ...

... a sprouting sterile stem ...

... and a mature sterile stem in late July.

Horsetails can be difficult to distinguish from one another, as, for example, this patch growing in a damp wooded area along the Moira River. They meet most – but not all – of the criteria to qualify as Meadow Horsetails (Equisetum pratense), a plant often confused with the Field Horsetail.

The overall aspect of the Meadow Horsetail is delicate and spidery with long, thin branches growing in whorls. They are horizontal or drooping, unlike the upswept branches of the Field Horsetail. (Sometimes it's necessary to be creative with backgrounds, as it can be virtually impossible to find a subject framed against something other than a confusing tangle of vegetation.)

Closeups of the stem sheath and branch teeth – however, the stem teeth of the Meadow Horsetail should have white margins. But the branch teeth are triangular and clasping as described in the Peterson Field Guide, unlike the narrow teeth of the Field Horsetail. The first branch segments are shorter than or equal to the adjacent stem sheath.

The central cavity is relatively large – the guide states a sixth to a third of the stem diameter – with the vallecular canals being small compared to the central cavity. (Compare to the Field Horsetail, where the central cavity is a quarter to a third of the stem diameter, and the vallecular canals are a third to a quarter the width of the central cavity.) To be fair it's necessary to point out that the stem of the Meadow Horsetail should be more strongly ridged. However, on balance the evidence points to these plants being Meadow Horsetails.

The Common Scouring Rush (Equisetum hyemale) can be found in countless numbers growing in marshes, damp woods and clearings along the trans-Canada Trail. As a rule the strongly ridged stems have no branches, earning scouring rushes the surrogate name "Snake Grass".

Occasionally the main stem will bear a few branches.

A combination of symmetry, art and beauty emerging as a result of the force of natural selection.

Found growing along a marsh near Sulphide, the strongly contrasting black and white sheaths of the Variegated Scouring Rush (Equisetum hyemale) are eye-catching and distinctive. Variegated Scouring Rushes are much smaller than their cousin the Common Scouring Rush, and tips of the cones are very sharply pointed.

Large patches of Variegated Scouring Rush, often mixed with Common Scouring Rush, can also be found in drier, wooded habitats along the trans-Canada Trail east of Tweed.

A cross-section of the tiny stem – the diameter is perhaps two millimeters and this is really pushing the limits of the camera's macro.

There are a couple of other local Equisetum species, images will be uploaded as they become available in the future.