PYROPHYTES AND XEROPHYTES

Given the seemingly ever more frequent fluctuations in weather, looking at plants which have adapted to the most extreme of weather circumstances is more pertinent than ever. These more unusual plants may give us more insight into what future evolutionary routes plants experiencing climate stress may take and perhaps we can learn what adaptations plants have made to better suit their environment (whether that be deserts or deep sea) and what we, as plant biologists, can do to preserve them!

What are pyrophytes?

Pyrophytes are plants that are really really good at living in areas prone to fires. The word pyrophyte in fact comes from the Greek “pyro” meaning ‘fire’ and ‘phyte’ meaning plant! There are two main types of pyrophyte: passive and active. Passive pyrophytes are those that have adapted to resist the damaging effects of fire, whereas active pyrophytes have adapted to fulfil an ecological niche, where their reproductive cycles are reliant on fire. In other words, fire is good for them. In some cases, these plants actually encourage the spread of fire by producing inflammable oils on their leaves. An example of this is the Eucalyptus of Australia – these bad boys benefit from inflammable oils that spread wildfires throughout their ecosystem; this prevents invasive species from entering their habitat and clears the ground so seed dispersal is more efficient.

But, what are the specific adaptations these plants have to fire prone conditions?

Plants often have thick and fire-retardant bark, made up of dead tissue that protects the inner workings of the xylem and phloem as well as living tissue from fire damage. Alternatively, or sometimes alongside this, plants will have moist tissues that prevent dehydration, and tall crowns, so that the majority of their leaves are unaffected by small scale, or low-lying fires. Some plants even have epicormic buds. These are buds protected beneath the bark or even deeper that only come out to sprout when the plant has detected it is safe to do so.

Lignotubers are woody and swollen root crowns that store buds and starch to provide buds with sufficient energy when germination starts. Some plants also display serotoiny. This is when a resin that seals shut the fruit or cone melts when heated, releasing the seeds contained within. In some plants, there is further protection whereby both heat and moisture are required to release the seeds. Seeds are dispersed only after the fire dies down and there is sufficient moisture in the atmosphere for successful germination. In active pyrophytes especially, prolific flowering after a fire has occurred is common.

Xerophytes

Xerophytes and pyrophytes are often terms that can be applied to the same plant, but not always. Xerophytes are plants that have adapted to live in conditions with very little liquid water. Think of the Mojave, Gobi or Sahara.

Succulents are probably the most well-known xerophytes. Cacti are often what we call vestigial. What this means is that parts of the leaf will have no function in these plants but would in others. However, it would be silly to say that all parts of xerophytes have no function! So, let’s look at what adaptations they do have. Can you spot any similarities or differences with pyrophytes?

 Many xerophytes, such as cacti and succulents, store water in their tissues. Their thick, fleshy stems or leaves act as reservoirs, allowing them to retain moisture during periods of drought. These plants can access stored water when the external environment is dry. Often times, xerophytes have small or reduced leaves, or in some cases, no leaves at all!   With smaller leaves, these plants reduce water loss through transpiration. Some xerophytes, like cacti, have modified their leaves into spines, further limiting water loss and protecting themselves from herbivores. Many xerophytes possess a waxy cuticle or coating on their leaves and stems, which acts as a barrier to reduce water loss. This waxy layer also helps to reflect sunlight, keeping the plant cooler during intense heat.

 To access water deep below the soil surface, these guys also have extensive and deep root systems. These roots can reach underground water reserves that other plants cannot access, allowing the xerophytes to survive long dry periods.

 Some xerophytes, particularly succulents, use Crassulacean Acid Metabolism (CAM) to carry out photosynthesis. This process allows them to open their stomata (pores for gas exchange) at night instead of during the day, reducing water loss. At night, they take in carbon dioxide, which is stored and used for photosynthesis during the day when the stomata remain closed to prevent water loss via the transpiration stream.

 Xerophytes may have thickened, leathery, or waxy leaves, which help reduce evaporation. Some plants, like aloe and agave, have leaves with a spongy tissue that stores water, enabling them to survive harsh conditions.

So, what do you think? Can you think of any other examples of cool adaptations or specific plant species we should talk about? Let us know!