An Orchidist's Miscellany

SPECIES 

 We often read and talk about orchid 'species', and it is appropriate to look at this designation.

It is to be noted that a 'species' notation given to a similar group of plants is an artificial designation, representing one person's view. Different workers can, and do, have differing opinions as to what constitutes a valid species, such differences of opinion often making life difficult for orchid growers. All plant populations naturally show some variation, and where to draw a line and say 'here is one species', or 'here are two', can be most difficult. Because orchids are desirable, there is often a tendency for relatively small differences to be recognised by different species names, such differences in other less desirable plant families often being considered as acceptable variations within a larger, more variable species.

The general criteria on which decisions as to whether specific rank is appropriate are that the population of plants studied must:

(a) be capable of interbreeding freely amongst themselves, and,
(b) be isolated from other 'species' populations that exist.

These distinctions are good in theory but it is often difficult to prove in the field that these criteria exist, hence the problems that can often arise.

Obviously interbreeding within the population is critical. While some variation in their characteristics and features will exist, there must be certain stable features which all members show. Obviously the characteristics distinctive to the species must be able to be maintained, and therefore there must be some barriers preventing the introduction of other genetic material. A number of mechanisms have evolved to ensure this Isolation.

In most plants other than orchids, the 'Isolating mechanisms' involve hybrid inviability through chromosome, or physical incompatibility. Orchids have, however, developed a different primary system, which involves the pollinating system itself. Chromosome incompatibility is not a major factor in orchids, as evidenced by the large number of intergeneric hybrids which have been created by man. The main isolating mechanisms evolved in orchids are:-

1 . The species may grow in different areas;
2. Reproductive controls may exist:

(a) The species may grow in different habitats.
(b) The plants may flower at different times of the year or day.
(c) They may be pollinated by different insects.
(d) The plants may grow over different altitudinal limits.
(e) There may be certain mechanical or structural features of the flower preventing cross hybridisation.
(f) Some insects may show different behavioural responses to different species in the same area.
(g) There may be chemical antagonism preventing pollination.
(h) There may be different insect attractants between similar species, i.e. scent, colour, nectar may differ.

Botanists sometimes talk of 'sub-species'. This also involves an interbreeding race of individuals which show one or more genetic variants from the typical species. Whereas be- tween species there should be a lack of intergrades (plants showing some characteristics of the two populations) a sub-species will show a full range of intergrades. It will .not have developed the necessary distinctive characteristics or isolating mechanisms required to preserve the individual characteristics to continue as a 'species' on its own, although it may eventually reach this status.

When you see names being changed, these notes may help you understand what may have caused these changes. Incidentally, the word 'species' is both singular and plural.


Site established 9th May 1998