Spring Directs Spore Dispersal in Low Humidity

horsetail cones grow a the top of the stems against a blurry green background

Biological Strategy

Horsetail family

AskNature Team

Image: Kirill Ignatyev / Flickr / Some rights reserved

Sense Atmospheric Conditions

For some living systems, the ability to detect changes in atmospheric conditions can be very valuable. By predicting changes in regional weather or in very localized conditions, living systems can avoid or take advantage of those changes. Since such adjustments can be very subtle, living systems must be able to detect miniscule variations in moisture, barometric pressure, ions in the air, and other environmental cues. Many insects and birds, for example, can predict oncoming storms and take cover before their lives are put at risk.

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Disperse Seeds

Plants disperse their seeds using a variety of mechanisms, including wind, rain, and attachment to animals. Seed dispersal can be important if plants growing too close together will compete for resources, such as nutrients and sunlight. Since plants are literally rooted in place, seed dispersal is also an important mechanism for expanding their range or reaching new environments.

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Ferns

Class Polypodiopsida (“many footed appearance”): Ferns, horsetails, whisk ferns

Ferns are a class of plants that reproduce without flowers or seeds, but by spores. With over 10,500 known species, ferns are the second most abundant group of vascular plants after angiosperms (flowering plants). Though they can be found in a variety of habitats, ferns prefer moist, humid environments. Ferns first evolved in the Carboniferous Period about 360 million years ago. Today, ferns are grown for food, medicine, ornamental landscaping, and hold importance in culture and folklore. Several species can also remove pollutants from the atmosphere.

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The elaters of horsetail plants aid spore dispersal by coiling and uncoiling in response to humidity.

“Besides their distinctive stems and leaves, horsetails and calamites bear spores in cones produced at the tips of the stems or branches. Each cone is composed of many tightly fitting, polygonal scales attached to a central axis (Figure 66). On the inner surface of the scale are several oblong yellowish sporangia filled with green, photosynthetic spores. When the spores are ripe, the central axis of the cone elongates, separating the scales and exposing the sporangia to the air. Upon drying, the sporangia split lengthwise and release the spores, allowing them to be carried away by air currents. The spores are helped on their journey by four strap-like structures called elaters that catch the wind. The elaters coil and uncoil in response to changes in humidity. When the air is dry they extend outward and create wind resistance so that the spores float (Figure 67). When the air is humid the elaters coil around the spore so that buoyancy decreases and the spore drops–with luck onto moist soil where it can germinate. Elaters occur only in horsetails and calamites and are evidence of the close relationship between these plants.” (Moran 2004:115)

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