India, due to its tropical location and topographical diversity, associated with favourable climatic conditions, has a moderately rich and diverse orchid flora. The monsoon conditions with annual alteration of hot humid and cool dry seasons in the montane moist deciduous and semi-evergreen forests in India has given shelter to orchids with diverse habits of terrestrial, saprophytic and epiphytic forms. The terrestrial and epiphytic forms are represented in the ratio . The epiphytic forms are better represented in areas of moderately high altitude and high rainfall.
The terrestrial orchids are subjected to seasonal changes in temperature, light, water and fungal activity. They occur in most of the varying climatic regions of India and are adapted to growth in a correspondingly wide range of conditions. They are therefore more diverse than their epiphytic counterparts, which are geographically restricted and less diverse in form. Orchids are autotrophic and never lead a parasitic life. Their lithophytic way of living contradicts the notion of those who feel that the ephytic orchids are all parasites. Life forms of the different orchids are discussed below.
The terrestrial orchids in India have in general an aerial, leafy and flowering shoot, along with an underground shoot, specialised for conservation of food and water as well as for vegetative propagation. New growth starts from the base of the sub-terranean stem. The basal parts of the shoots are thus connected in series, forming a sympodium. All the terrestrial orchids of India (save the climbing Vanilla) exhibit sympodial form of growth.
Generally two different kinds of terrestrial orchids are to be found in India. Those growing in evergreen or semi-evergreen forests with moist soil under shaded condition are “evergreen”. But most of the terrestrials appear during the rainy and autumn seasons only. During the dry winter and summer the plants disintegrate and disappear above the ground again to be seen in the growing season. The growth habits of these appear to be seasonal. But actually when the leafy shoots die, the plants undergo a period of dormancy during the dry season. They perennate, that is, remain alive through their subterranean organs in which food and water is conserved. All such orchids are thus ‘perennial’.
The terrestrial orchids in India exhibit a lot of diversity. They are erect or creeping; grow individually or in tufts; bear normal aerial shoots, thickened or not at the base; or lack an aerial stem and exist with a solitary leaf only. They can be broadly of two forms. In the first, like the amoebophytes, the leafy and flowering shoots appear one at a time alternately in a growing season, each separated by a dormant period. The leafy shoot consists of a solitary leaf and there is no aerial stem in this form. In Pachystoma pubescens growing in the meadows, the fleshy subterranean rhizome produces first a floral shoot. A single grass-like long leaf is later produced from a well- separated bud from the rhizome. In Nervilia the globose corm produces a bud which gives rise to two closely situated shoot-buds. The floral shoot develops first later followed by the foliar one, which consists of a solitary cordate plicate leaf with a long petiole or a sub-orbicular short-petioled leaf remaining above or lying on the ground respectively.
In majority of cases the floral and the foliar shoots appear simultaneously, that is, the plant flowers in leafy condition. During flowering the foliar shoot may be young in many species like Eulophia explanata, E. spectabilis, Geodorum recurvum, etc. or it might have developed well. In some cases it may even start wilting while in flowers as in Eulophia gram inea, Zeuxine affinis, etc.
The different terrestril forms are discussed below.
Phaius, and Tainia etc., the corm is superterranean and the term pseudobulb is commonly used forit. This group is with characteristically caespitose or tufted evergreen plants with large plicate leaves inhabiting semi-evergreen or evergreen forests by the streamside. The inflorescence develops from a basal or upper node of the pseudobulb and the new growth is born from the base of the plant of the previous season.
ii) In several species of Dienia (D. ophrydis), Liparis (L. nervosa, L. paradoxa etc.), and Seidenfia (S. rheedii, S. versicolor etc.), the entire stem is superterranean and is thickened to form a plurinodal pseudobulb with thin herbaceous venose leaves and a terminal inflorescence. In Crepidium mackinnoni, Liparis deflexa etc., the pseudobulb is entirely subterranean with thin prostrate leaves.
iv) In several genera like Goodyera, Hetaeria, Myrmechis, Zeuxine etc., the plants have creeping stem. It consists of a basal horizontal leafless part and an erect apical leafy and flowering stem. A few thick and spongy roots covered with short and soft hairs develop at the nodes of the basal stem. The leaves are thin, soft and herbaceous. New growth starts from an axillary sub-apical bud while the erect shoot drops down forming an additional segment of the basal stem.
v) In Spiranthes sinensis, Cryptostylis arachnites, the rhizome is very short and erect. It produces buds, which develop in to rosettes of thin leaves above with an inflorescence at the centre. The storage of food here is taken up by a fascicle of tuberous roots.
There are no aquatic orchids although some are swamp inhabiting. Spiranthes sinensis always grows in swamps or occasionally on moss-covered rocks on edge of perennial streams. Zeuxine strateumatica, the grass-orchid, is found usually in waterlogged condition on the edge of rice fields. The plant has a very short aerial life span, barely of two months, between middle of January to middle March. For the rest of the year it disappears from above the ground after flowering and fruiting. Obviously the plant perennates underground with its slender nodose rhizome and the tuberous roots with mycorrhizal symbiotic association.
The saprophytes (sapros: rotten; phyta: plants), unlike the common plants, are non-green in colour. In absence of chloroplast, the green pigments contained in the leaves, these plants cannot prepare their own food — mainly sugar and starch. Therfore they derive nutrients from decayed organic substances of vegetable or animal origin available in the substrate. In the undisturbed primary forests, the decaying leaf litters constitute the topsoil, which is very light and rich in nutrients. Some orchids afford to live there as saprophytes. They lead mostly a subterranean life and emerge above the ground usually before flowering time when a shoot is sent up to bear the flowers.
Saprohytes are not capable of developing root hairs and depend upon mycorrhizal fungi present inthe roots for their existence. They posses in their roots greater amounts of mycorrhiza than the non-saprophytic orchids. This is because they fully dpend on the fungi for their nutrition requirement. It is no more a relationship of symbiosis, but a matter of the orchid living off the fungus. The mycorrhiza commonly associated with the saprophytic orchids is Hymenomycetes.