13 March 2021

The Deadly Dewdrops of the Drosera

Look closely, but be wary of touching.  Those beautiful glistening drops of dew at the end of the plants you can see in these photographs are not quite what they seem.  In fact, rather than being dew, that jewel of the earth, these gleaming globules are in fact mucilage. Muci-what?

Mucilage is a thick, extremely sticky matter which is produced by most of the plants on the planet: in fact it is oozed out by some microorganisms too.  Mostly it helps the plant to store water and germinate its seeds and is even used as a kind of emergency food store in some plants. The Drosera, however, has a much darker reason for producing mucilage.

You guessed it. The leaves of the drosera, the largest genera of carnivorous plants with over 194 species in its family, are studded with mucilage with dinner in mind.  Once an insect alights upon this gummy substance it cannot break free. Without their freedom to find food they die and then the drosera supplements its diet with the minerals found within the insect’s body. We will elaborate upon that process in a while, but first back to the quarry.

The insect will die of one of two things.  Either it will exhaust itself and simply give up and expire of its own accord.  Or they will ‘drown’.  In most cases the mucilage overwhelms them first.  It envelops them and blocks up the insect’s spiracles (an opening on their exoskeleton which allows air to reach its trachea).  Either way death will mostly occur a maximum of fifteen minutes after first contact.

Despite its sinister nature (for a plant, at least) the common name of the drosera is sundew.  Its botanical name is from Greek – drosos – which means dew or dewdrops.  It is a beautiful name for a plant which, one has to admit, does have it in spades itself particularly when examined by way of macro photography as in the examples here.

Mostly perennial the plant forms upright rosettes which can vary in length from the tiny (around 1 cm) to almost a meter in height, depending on which of the 164 species you are looking at. Some species of sundew have adapted to climb and as such can reach up to 3 meters in length. Although they look somewhat delicate they are surprisingly sturdy and can reach an age of over 50 years.

As the genera became specialized, over millennia, to take up nutrients in the form of the gooey remains of insects one of the species, the pygmy sundew, abandoned earth-bound nitrates altogether.  It has, somewhere along the way, lost or abandoned the enzyme, nitrate reductase, it would need to take nutrition from the soil. Most retain the ability to take nutrients from the soil, however.

Drosera have glandular tentacles which are crowned by the mucilage directly on top of and covering their laminae (the flat parts of their leaves). There are two glands involved in the capture, conversion and digestion of insects.  The first is penduncular, which secretes the mucilage in the first instance.  It is also thought that it exudes a scent that is attractive to the insects and which draws them to their fate.

This first enzyme will also turn the unwitting insect in to soup.  The second set, sessile glands, absorb the soup in to the plant and extends it life.  This is only after the plant has secreted several enzymes which dissolve the prey – only once it is turned to soup can it be absorbed through the leaf and help stimulate growth.

So far, so triffid.  Yet can the sundew actually move, like its science fiction counterparts? The answer is yes – the tentacles move when contact is made with prey that it can digest.  The tentacles will curve and twist in what looks to the human eye as an attempt to encourage the insect to move to the point where there are the maximum of stalked glands.

This scientific name of this reaction is called thigmonasty and – in plant terms – the drosera has a spectacularly fast response.  Some species bend their tentacles within a tenth of a second of contact.  Most species of drosera, however, take a few seconds.  One or two have an additional feature – once the prey is caught and dead they will curl a leaf completely around the prone insect.

The flowers of the drosera are almost always at a much greater height than the tentacles which grab its insect prey.  This was once thought to happen so that pollinators do not become the plant’s next dinner accidentally.  However, it is now believed that the sundew has evolved this feature simply because a higher flower will attract more potential pollinators.  Those insects which pollinate it are as welcome at its table as those which do not.

The genera Drosera is more widespread than you might imagine.  They can be found growing from Alaska to New Zealand, though Australia is home to more than fifty percent of the species. However, although they are distributed around the world, they are not generally considered a cosmopolitan (that is worldwide) species because so many areas have no evidence of drosera ever having colonised them.

They need very specific conditions in which to thrive and that is why they have not made it in to our homes in the same way as the Venus Fly Trap, although some of the hardier species are often sold alongside it.  For the greater part, the real beauty of the drosera is to be found only in its native habitat.