The horsetail – a widespread group of vascular plants with an interesting history

A great part of our fossil fuels is based on distant relatives from our present horsetails. During the Carboniferous, this group of plants attained almost worldwide distribution: arborescent lifeforms up to 30m. The decay of these trees under anaerobic conditions, later additionally under high pressure and high temperatures finally formed coal, still one basis of fossil energy.

Today the horsetails are worldwide in distribution except for parts of Australasia. In Middle Europe a plant height of 1m is remarkable.

The most conspicuous morphological feature of a horsetail is the segmentation of the shoot into nodes and internodes. This is why these plants are also called the “articulate”.

                                               Equisetum telmateia                       Equisetum telmateia (note the jointed stem)

At the nodes we find quite small leaves with buds between them. The reproductive structures are aggregated in terminal cones. Depending on the species, they may grow at the top of a regular vegetative shoot or on separate shoots without chlorophyll. The cones consist of whorls of sporangiophores looking like a one-legged table.

                                                     Equisetum telmateia; terminal cone with sporangiophores                  

THE HORSETAIL UNDER THE MICROSCOPE

The horsetails give us several chances for impressive microscopical insights.

It’s the shoot which is displaying a remarkable pith cavity. Have a look on the prominent ribs in the periphery with its sclerenchyma bundles giving stability to the stem. A cross section with elder mark and razor blade will do fine.

The spores are produced in sporangia, bag-shaped structures at the bottom-side of those one-legged tables.The structure of the spores is remarkable. The spore wall is said to be laminated, with an outer layer deposited in the form of 4 bands with spatulate endings. These “elaters” are hygroscopic, attached at a common point and remain tightly coiled around the spore in case of humidity. Carefully use the fine focus of your microscope to get the complete beauty. 

At the time of ripeness, the sporangia open. Dry weather uncoils the elaters, a chance for a common dispersal in clumps as the elaters get entangled. This is quite helpful for an adjacent germination of “male” and “female” spores. The fertilization of the female archegonium by male gametes is facilitated.

This picture has to be taken without water in transmitted light. 

In this case the spores are hardly transparent.

THE HORSETAIL IN DAILY LIFE

• Historic application: The German trivial name of the horsetail is “Zinnkraut”, related to the fact that horsetail extract is used for polishing “Zinn” (tin, pewter) materials like mugs. Silicic acid is the component responsible for a unique abrasive effect.

• Modern use in pharmacology: Multiple effects in pharmacology. People talk about medical applications on several parts of the human body. Even pest control in agriculture is mentioned.

CONCLUSIONS

A group of plants with a long history, interesting recent representatives, nice morphological features ready to be discovered with a microscope.