Both existing and new taxa need to be characterized, that is, described with respect both to the salient features by which they can be recognized and distinguished from similar taxa, and to the features they share with other taxa deemed to be related.
Linnaeus' nomen specificum legitimum, the diagnostic phrase of 12 or fewer words with which to "...distinguish this species from all others of the same genus speedily, safely and pleasantly" (Critica botanica No. 288). This is the phrase name, for which the specific epithet in the binomial is, in effect, a shorthand. Consider the following example from Species Plantarum (Linnaeus 1753).
Modern descriptions, in monographs and floras can be much more detailed, especially when new taxa are being described for the first time. The terminology encountered already in keying exercises is just a small subset of that required for accurate descriptions of the variation in structure that is exhibited by plants.
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Both in characterizing the course of plant evolution and in characterizing individual taxa (cf. Judd et al. 2002, chapters 3 and 4), microscopic internal and external features of the plant body can be vitally important.
Colonization of land surfaces resulted in increasingly specialized structures associated with acquiring, distributing, and retaining liquid water. Some of this specialization has also involved accomodation for gas exchange. At the same time reproductive structures became increasingly independent of liquid water as a means of gamete transport. Consider the contrasts between Bryophytes and Tracheophytes, or those between gymnosperms and angiosperms.
Just within the flowering plants, anatomical data has been of great importance on a number of hierarchical levels. Some of these will become apparent in the monographs that you review. One example of great ecological significance is the repeated evolution in different families (but most notably the grasses) of a variety of structural (and physiological) adaptations for carrying out photosynthesis under conditions of elevated temperature and limited water availability.
The Angiosperm Phylogeny website provides a good example of the way in which traditional phytography as well as anatomical and biochemical data can be integrated into a comprehensive analysis of systematic relationships.
Davis, P. H. & V. H. Heywood (1965). Principles of Angiosperm taxonomy. Edinburgh, Oliver & Boyd.
Judd et al. (2002), Ch. 4.
Lawrence, G. H. M. (1951). Taxonomy of vascular plants. New York, Macmillan.
Porter, C. L. (1967). Taxonomy of flowering plants. San Francisco, W. H. Freeman.
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