Orchid Roots: Botanical Sponges

You can crudely divide orchids into two groups: ground orchids, rooted in the soil - like Pleione species, for example - and epiphytic orchids like the one below, that often grow on the branches of trees in tropical forests. The dangling roots of the epiphytic types have a dual role, sometimes anchoring the plant and always acting as storage vessels for water that they absorb from mist and sudden tropical downpours. If you cut a section through one of these roots (above) you can see their unique structure, that allows them to absorb and store water. The bright yellow ring of thick-walled cells at the bottom of the image above is the plant's internal plumbing - the pipes (xylem vessels) that conduct water from the roots to the leaves and flowers. Beyond that the thin-walled blue cells are the packing cells that are alive and contain some chloroplasts, like the leaves. Beyond that, sheathing the root and separated by a distinct layer of mostly hexagonal cells is an outer sheath of dead cells called the velamen layer, and their role is to soak up water as the roots dangle in the air, high above the forest floor. They are, in effect, a sponge.

The recommended way to water tropical orchids, like Vanda species for example, is simply to stand them in water for a few minutes each day, so their root velamen layer fills up with water, then simply hang the plants up with their roots dangling in space.

You can see here what happens when you water an orchid root here. When it's dry (above) the dead velamen layer cells reflect light and the whole root looks silvery-white. Make them wet for just a few seconds (below) and those dead cells fill up with water, become translucent and reveal the green photosynthetic tissue within.

Travelling light

This strange object, magnified one hundred times under the microscope, is a single seed of a common spotted orchid Dactylorhiza fuchsii. The lower photo shows a couple of the orchid’s seed capsules, with the dust-like seed laying on the paper below.

Unlike seeds of oak and horse chestnut, which send their seeds out into the world with a large food store surrounding the embryo, orchids have a much more minimalist approach to equipping their embryos for future survival. The orchid embryo – inside the darker object in the centre of the seed in the top photo – has no food store and is housed in a fragile papery coat, just one cell thick. The whole seed is no larger than a speck of dust and is so light that it can be swept up by air currents and wafted long distances – orchid seed could easily be blown across the English Channel, for example. So, unlike heavy seeds with a large food that are unlikey to disperse very far from the parent plant, orchid seeds are great travellers heading for random destinations and this accounts for their tendency to suddenly appear in unlikely places – lawns, roadside verges, industrial spoil tips, to name but a few. A large orchid flower spike will produce tens of thousands of these minute seeds, but only a tiny fraction will ever achieve the next critical step in the life cycle – landing on soil that contains the essential mycorrhizal fungus that will link up with the germinating seed and provide the embryo with the nutrients that it lacks until the seedling is large enough to produce leaves and survive on its own. After that the orchid's roots returns the favour by supplying its partner fungus with nutrients for the rest of the orchid's life. Early growth of the orchid seedling is slow and its leaves passes unnoticed - until it's large enough to produce a spectacular flower spike........and to read about the next step in the life cycle - pollination of the flowers - take a look at  http://cabinetofcuriosities-greenfingers.blogspot.com/search/label/orchids