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Tuesday 2nd February 2016

Senna to Sleep

Over the past few weeks many of the trees have started to flower.  The increasingly dry conditions stimulate blooming at this time of the year but because most of the floors are 100 foot up in the forest canopy then the only time the visitor has to see them is when flying over the forest or when the spent blooms fall to the ground and lie littering the forest floor amongst the dry leaves.  However there are some smaller trees, shrubs and herbaceous plants that grow beneath the canopy or in more open conditions.  Here you can get to see the floral display at a closer proximity.

One of those small trees generally found growing along sunny paths and roads is the Saragundi, (Senna reticulata).  Its bright yellow flowers tinged with a hint of orange stand upright like fat golden candles giving the whole of the tree crown an overall fiery glow against the green background of the forest or azure blue of the sky.  Stand beneath the tree and you will hear the constant buzz of bees attracted by its flowery display as they come to take nectar and pollen.

Despite its showy appearance Saragundi is not a welcome part of the native flora as far as farmers are concerned.  It is a rapidly growing weedy tree that will quickly invade and take over open areas such as pasture land and nor is it easy to eradicate.  It is little wonder that the local gardening team around here hates them with a vengeance and cut them down as soon as they germinate.

Should you see a Saragundi later in the day, more towards dusk, you will notice that the leaves start to droop and the plant goes to “sleep.”  This is a feature common to many plants in the Fabacaea family, (legumes), and it is known as a nastic response.  The base of each leaflet has a fleshy elbow called a pulvinus.  During the course of the day the rhythmic flow of potassium ions causes water to either enter or exit the cells of the pulvinus.  In the morning the large water holding cells become full of water making the pulvinus turgid which holds the leaflet out straight.  At dusk water exits the cells making the pulvinus flacid and so the leaflet folds down appearing as if to go to sleep.  It is not known what evolutionary benefit this nyctinasty conveys on the plant but it most certainly looks like some of the forest trees are dozing off for the evening.

A Rod of Gold

Growing along the currently dusty roadsides or lining the forest edge abutting the gardens are weedy long-stemmed plants that have a crown of yellow flowers atop.  These are the Jackass Bitters, (Nuerolaena lobata).  It belongs to the Aster family which includes the daisies and dandelions.  It is one of the most species rich families of plants on the planet rivaled only by that of the orchids.

The flowers heads are composites; each individual head is a group of flowers, the greater display of many flowers together provide a more attractive visual target to potential pollinators.

There is a lot of folk medicine currently based on the supposed curative properties of Jackass Bitters but none that has any scientific backing.  However its anti diabetic qualities are being scrutinized in a laboratory situation with regards to its ability to exert some control over blood sugar levels.

Canna Look at Your Lily

Tucked under the shady conditions beneath the taller vegetation where the garden meets the forest edge are a low growing plant with yellow flowers.  These are the Canna Lilies, (Canna x generalis).  It is mostly a hybridized form cultivated to bring a splash of color to leafy green flower borders.  It is not obvious from which natural species of Canna that this variety has been hybridized but it is possible that it may be a Neotropical native Indian Shot, (Canna indica).  You will seeing this variety growing freely in many garden situations around the country.

Going Bats for Garlic

Going for a walk through the forest this time of year may certainly cause a stimulation of your olfactory senses.  There are many scents and odors that seem vaguely familiar but you cannot quite put bring to mind what it is simply because it out of context.  Currently walking through the forest there are areas where you experience the definite smell of chives, or is it onions, no wait a minute it is garlic.  What would garlic be doing growing in a tropical rain forest situation?

Here and there along the forest trail there are patches of yellow flowers littering the forest floor en masse.  At this point the aroma of garlic is very strong.  The flowers lie for some distance concentrically around the base of a large tree which given its appearance looks more like a tree from higher temperate forests.  The bark is rough and deeply fissured, closely resembling an Oak Tree than a smooth bark tropical tree.  This is the Ajo or Garlic Tree, (Caryocar costaricense), so named because its flowers give off a scent reminiscent of garlic.

There is very little wind in the Pacific lowland forests of Costa Rica so the plants have to rely on animal agents for pollination and seed dispersal.  Bats are very important pollinators and seed dispersers but bats prefer musky smells rather that sweet smells which is why there are some very strange odors in the forest.  It is the scent of garlic that attracts the bats in so the Garlic Tree is a bat pollinated tree.

Seeing Things in a Different Light

On the hot sunny days of the dry season many butterflies can be seen flying around the garden areas.  There are several species that are noticeable due to their striking yellow coloration.  These could be the “butter” flies.  These are the sulphurs and they are not easy to tell apart species wise unless you have them in the hand.

Sulphur butterflies belong in the Subfamily: Coliadinae of the family: Pieridae.  To the human eye the butterflies all have yellow coloration to a greater or lesser extent.  Some may appear to be more creamy-yellow, others lemon yellow and yet others orangey-yellow but without exception – yellow, hence the name sulphur.  But that is not how they look to other butterflies.

Butterflies, unlike humans see light at the ultraviolet wavelength of the electromagnetic spectrum.  The dorsal surface of the male sulphurs wings are colored with a yellow pigment.  This re-enforces a reflective ultra violet component that covers most of the dorsal wing surface and appears to sexually attractive to females which only have a small amount of ultra violet reflectance on their wings in comparison.  The ultra violet patterning is used by the males in courtship displays.  To human eyes the butterfly appears as a dancing honey colored piece of confetti.  The females however see something different.  Larger males with bright reflective radiance appear to be more attractive to larger females.  In the world of the sulphurs flashy show offs get the girls.

Philip Davison is a biologist, writer and photographer based in Costa Rica.