Why Do We Need Categories

Nature itself neither needs categorization nor has any knowledge of categories. However, for the scientist, categories are essential for classifying natural characters in their diversity, for defining their range and for placing them in systematic order. Nevertheless, categories are artificial and always delimited by an individual or collective convention, mostly not by nature.

In addition to the theoretical concept, categorization always depends on the manner in which a character is perceived: i.e. on the visibility of a character, and/ or their specific value. Categorization also greatly depends on the technical equipment and method(s) used, as well as on the subjective interpretation of character(s)1. Thus, categorization of features is difficult to standardize. A well known example is pollen size2. However, depending on the preparation method(s), the pollen sample may show pollen grains of one and the same plant species fitting into more than one size category (pollen size categories: see "Pollen Morphology"). Moreover, sometimes the size of pollen grains is found just at the boundary between two adjacent pollen size categories. Placing the pollen grain in one of the size categories therefore depends entirely on the material, the preparation method(s) and the observer's evaluation.

Characterization of pollen ornamentation is even more complex, modifications of basic ornamentation characters or combinations of different characters usually

1 To be successful in characterization consider the following hints: be familiar with good microscope practice. The microscope, LM or EM, should be in good order. Primary magnification should be adequately high, but any enlarging of details beyond a beneficial magnification is counterproductive. For high magnifications in SEM, the best point resolution should be achieved. Quality of sample preparation is an all-too-often underrated item.

2 The importance for dimension measurements is acknowledged but there is no need for decimal places, since dimensions vary considerably according to different treatments, as already shown by REITSMA (1969).

giving rise to a seamless transition between neighboring characters or to a combination of characters.

Seamless transitions between related but clearly defined features exist, e.g., in gemmate pollen and its "neighbor" clavate pollen. Both types of ornamentation are very variable in shape and size and rather rare in their typical form.

Combination of ornamenting characters is very common. Often, the ornamentation is composed of two or more characters, such as reticulate and foveolate, or a combination of echinate and perforate (for examples see Illustrated Glossary). From the observer's viewpoint it is desirable to name the ornamentation characters in a defined order: in the case of two or more combined characters, the most eye-catching, prominent character (the "leading term") should be mentioned first.

For example, in Aristolochia, the pollen grain surface bears very prominent verrucae

Combination of ornamenting characters

Aristolochia arbórea Aristolochiaceae inaperturate, spheroidal verrucate, perforate surface detail Verrucae and perforations

Combination of ornamenting characters

Aristolochia arbórea Aristolochiaceae inaperturate, spheroidal verrucate, perforate surface detail Verrucae and perforations

Combination of ornamenting characters

(the "leading term") combined with a great number of small perforations. Such ornamentation therefore should be called verrucate, perforate.

Sometimes it is debatable which feature represents the "leading term". As a sample, in Caryophyllaceae, there are numerous, more-or-less regularly arranged microechini and perforations. In some taxa the micro-echini are more prominent (microechinate, perforate), in others the perforations (perforate, microechinate). There are also taxa, where the two features are on a par (micro-echinate and perforate). Micrographs elucidate the actual situation at a glance.

Combination of ornamenting characters

Sfellaria media Caryophyllaceae microechinate, perforate

Saponaria officinalis Caryophyllaceae microechinate and perforate

Silene succulenfa Caryophyllaceae perforate, microechinate

Sfellaria media Caryophyllaceae microechinate, perforate

Saponaria officinalis Caryophyllaceae microechinate and perforate

Silene succulenfa Caryophyllaceae perforate, microechinate

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