There are about 25,000 species of orchids and more species are discovered and described every year. Orchids are one of the most evolved families with the largest number of species in the entire plant kingdom. They occur on all continents except the polar circles. They live from the lowest altitudes of the plains to the edge of the high mountains. Orchids are classified into three main groups: they are terrestrial when they grow on the ground like most plants; they are epiphytes when they grow on tree branches; they are lithophytes when they grow on rocks.
Orchid growth occurs in two forms, monopodial and the sympodial. The monopodial orchids are plants without a rhizome (modified stem) and pseudobulbs, which grow from a single vegetative apex. All the Vandeae (Vanda), for example, have this structure. The more or less long and thickened stem, erect or procumbent, may be provided with leaves, arranged more or less closely together, along the entire length of the stem or only towards the apex. In the latter case, the lower ones have usually fallen off, leaving their dry basal part around the stem. This group also includes the genus Vanilla, whose lianoid stem can easily reach several tens of meters.
Some species, usually of medium or small size, are leafless and the roots have a photosynthetic function. Typical examples are those of the genera Microcoelia and Chiloschista. The formation of offshoots from the base of the same stem in monopodial orchids takes place through the development of axillary buds which give rise to new individuals.
In several species, the roots, which are usually quite large, round or flattened, are also formed on the aerial part of the stem. The sympodial orchids grow from numerous vegetative tips, more or less close together, arranged on a rhizome which often has many branches. The rhizome is a creeping stem, in some cases underground, which produces other stems, improperly called secondary stems, more or less erect, in some cases practically non-existent as in the species of Cypripedioideae. These secondary stems may enlarge in size to form reserve organs called pseudobulbs. In the subtribe Pleurothallidinae, these structures are always absent.
In the sympodial orchids the leaves may be formed either at the base or at the apex or along all the pseudobulbs, and are either deciduous or persistent. Roots are formed from the rhizome. In the genus Dendrobium, for example, under special conditions, shoots may be produced at the nodes of the pseudobulbs, at the base of which new roots grow.
In some species of the genus Maxillaria the pseudobulbs grow from a secondary stem which is provided with leaves along its entire length. In the genus Scaphyglottis, for example, new pseudobulbs may form at the apex of old ones, and so on. However, this structure is rare.
The pseudobulbs, which vary in shape even within the same genus, can be very small, up to 1-2 mm in diameter, as in Bulbophyllum globuliforme or Bulbophyllum minutissimum, or reach up to 5 metres, as in the species considered giants: Grammatophyllum speciosum and Grammatophyllum papuanum. The rhizome between one pseudobulb and the next may almost disappear, so that these organs are joined together, as for example in Cymbidium and Stanhopea; in other cases it may be quite long, and then the plants, having the possibility of growing freely, form very extended clumps, as for example in several species of Cattleya and Bulbophyllum.
Orchid Care Guide
Orchid care instructions and culture tips
Vanda Care Guide
Vanda orchids care instructions