Epiphytic Orchids

These usually grow on tree trunks. Generally they do no harm to the host plant, being dependant upon them only for support.

Since they do not have the advantage of a regular supply of water absorbed from the soil, epiphytic orchids live under more or less xerophytic conditions and have corresponding adaptations such as thick leathery leaves, which are very oftern succulent. But the greatest adaptation of the epiphyte to its aerial existence is the development of that unique tissue called velamen in its roots . As in normal soil plants , the roots of epiphytes also serve the double purpose of anchorage and absorption . The roots that bind the epiphyte to the host serve this purpose admirably adhering to the bark of the tree with a high degree of tenacity . But a large number of roots hang freely in the air . It is in these roots that the velamen tissue reaches its maximum development . Instead of the piliferous layer of the monocot root, here there are several layers of dead cells with angular thickenings to prevent collapse. This velamen tissue has cells filled with air in the dry condition, which gives the root the characteristic grey colour . A layer of thick walled cells just inside this , prevents water from escaping from the water conducting elements contained in the interior of the roots . But there are occasional thin walled cells which allow water from outside to pass into the conducting tissues . The cortex which is just inside the velamen , are of thin walled parenchymatous cells with plenty of chlorophyll which give the green colour to the roots when elements of alternate xylem and phloem, inside which is the pith of parenchymatous cells.

The velamen tissue is supposed to act like a natural blotting paper absorbing moisture from the atmosphere .But the exact nature of its function is still a matter of controversy . The fact that the root dries out as quickly as it soaks up water, has given rise to the opinion that it can, on no account , be considered as a place to store water. Dycus and Knudson (1957) are of opinion that even though the velamen can absorb moisture from the atmosphere , they are incapable of passing it on to the interior of the root. Shushan (1959) attributes its main function to one of protecting underlying tissues.

The velamen root, when it comes in contact with the host tree, is capable of functioning as an ordinary root, developing root hairs and absorbing whatever food materials the dead bark tissue may offer , along with the excreta of birds and animals which has been accidentally deposited on the bark . The growing tree will always have fresh supplies of dead bark peeling off from the outside which is broken down by the mycorrhiza residing inside the orchid roots ,and whatever nutrients are thus released are made available to the orchid . Also the blue green algae which colonise on the bark of trees fix nitrogen which also becomes available to the orchid .

Instead of taking up residence on tree trunks , orchids may also occupy rock surfaces as lithophytes .

The epiphytic orchids are both of the sympodial type and the monopodial type. The sympodial types have stems of determinate growth . The creeping rhizome produces upright stems or pseudobulbs each year which is topped by one or more leaves. These produce flowers terminally or laterally . Next year’s growth is taken up by a new shoot produced from the side of the old pseudobulbs . In genera like Dendrobium , Cattleya etc., the rhizome is very reduced while in Bulbophyllum , they are long and trailing . The active growth of the plant is initiated and completed under favourable climatic conditions , the plant taking rest during drought seasons. These types of orchids attain their maximum distribution in the subtropical regions , where there is a prolonged dry season alternating with the wet period.

The pseudobulbs among sympodial forms show a great range of variation in shape and size. They may be small , round and globular as in several species of Bulbophyllum , depressed and discoid as in species of Eria and Porpax , angular and fusiform as in some species of Coelogyne , oblong and thumb- shaped as in Eria musorensis , or elongated and club-shapped as in Dendrobium heterocarpum , D. aqueum , Eria pseudoclavicaulis etc. Usually the pseudobulb is smooth but sometimes they are grooved or quite artistically wrinkled as in Coelogyne nervosa .

The monopodial orchids have indeterminate terminal growth ,and inflorescences are almost always lateral. They flourish in the equatorial regions where conditions for growth like humidity , temperature etc., are always in the optimum condition and growth can continue uninterrupted throughout the year . They have no pseudobulbs and food materials , when necessary , are stored in leaves as for eg., in terete and semiterete Vandas Luisia , Diplocentrum etc.