Orchidaceae
Introduction

The first section of the introduction is a compilation of direct quotes from Van der Pijl & Dodson (1966), Dodson & Gillespie (1967) and Hartley & Smith (1983).

The majority of orchid genera in the tropics lead a rather uniform vegetative life as epiphytes, especially when growing in humid forests, with a minimum of competition and a uniform macro environment. The biotic environment for flowers however, provides pluriformity even there. In fact, the primary characteristics that distinguish the orchids as a group are found in the flower.

Orchids are commonly organized in an inflorescence. The flowers usually open successively, each attracting visitors on its own and being visited independently, thereby spreading the total flowering over a long period. Although bees appear to be the most frequent pollinators of the orchids, a number of other insects (moths, butterflies, flies, mosquitoes) and a few specialized birds also function as such.

In most orchid flowers the stamens and the style are completely joined into a single organ, the column. A labellum offers a landing place or provides other advantages, such as positioning the insect for the deposition of pollinia or clumps of pollen on its dorsal surface (back or head). The combination of column and labellum provided a foundation for all manner of specializations regarding the attraction of specific pollinators, including form, colour and patterns. The majority of the orchid species produces small flowers that are not necessarily colourful, although in general, sensitivity of pollinators to certain colours is important in attraction.

Insects are differentially attracted by odours as well, which appears to be the oldest means of attraction. It works at distance and often at night. In beetle flowers, in many fly-flowers, and in some bee-flowers the sense of smell of the visitors is practically the only one involved. Flowers often deceive or attract visitors by producing putrid aminoid substances occurring in decaying organisms. In some instances odours are produced which insects perceive and distinguish but humans do not.

Orchid seeds are generally extremely small, the largest is about 14 µg, and within them the embryo is little differentiated.

(1) Pijl, van der, L, Dodson, CH (1966) Orchid Flowers, their pollination and evolution. University of Miami Press 214 p.

(2) Dodson, CH, Gillespie, RJ (1967) The biology of the orchids. The Mid America Orchid Congress Inc 158 p.

(3) Hartley, JL, Smith, SE (1983) Mycorrhizal symbiosis. Academic Press 483 p.

 

Epiphytic orchids

The second section of the introduction is a compilation of direct quotes from Withner (1974) and Hartley & Smith (1983).

Orchids are frequently encountered as epiphytes, although they may be found growing epilithically (on stone) or terrestrially under certain conditions. The total environment of moisture, temperature, exposure, nutrients, light and available space must act together in determining the microhabitat where any particular orchid species may be found, and, indeed, where any particular tree serving as phorophyte (host) may be found. It is apparent that the less favourable a site, the more specific is the phorophyte-epiphyte relationship, since the orchid becomes very dependent on the ameliorating characteristics of the tree, especially its bark. The bark characteristics of influence may be both its physical structure and its chemical composition.

Roots are adapted to life in the open air in two ways; they may contain chlorophyll so that they can make their own food, and secondly they may be protected against excessive evaporation by a special tissue on their surface - the velamen. This tissue consists of dead and therefore empty cells that can contain either water or air. The very first runoff from the bark contains the highest concentration of dissolved minerals, and this runoff may be trapped by the empty velamen cells.

Orchids are all mycorrhizal: living throughout life in association with fungi. The dependence of orchids on mycorrhiza for seed germination and seedling establishment has been a major factor in habitat determination since the very beginning of the family. Any possible exception to this would be in environments rich in sugar and amino-nitrogen leachates from plant material (bark-moss-liverwort), but it might also be possible that bacteria and yeasts on site may utilize carbohydrates more rapidly than orchid seeds.

The orchid-fungal relationship, although very broadly 'symbiotic', may vary greatly. One of the primary functions of mycorrhizal association is the provision of carbon compounds to the seedling during its development, which may be very prolonged, even in years, before green leaves are formed. Some observations show that adult terrestrial orchids with mycorrhizal roots must continue to gain carbon through them.

Another feature of orchid species can be the presence of pseudobulbs. Pseudobulbs serve as foodstores and can supply water to leaves and roots in times of drought. To epiphytes the distribution of rain is much more critical than its amount, since they will be saturated after a few minutes of rain; continued precipitation will merely run off. High atmospheric humidity is thus much more important to the exposed epiphyte than to terrestrials.

Finally, many orchids are able to absorb CO2 at night and use it for photosynthesis during the day. This would allow daylight closure, or semi-closure, of stomata for water conservation without disrupting photosynthetic activity, and so would be an important modification, especially in epiphytes.

(1) Withner, CJ (ed) (1974) The Orchids; scientific studies. Wiley-interscience 604 p.

(2) Hartley, JL, Smith, SE (1983) Mycorrhizal symbiosis. Academic Press 483 p.


next