It is well known that floral characters have been, and still are, those most used in the classification of angiosperms. At one time elementary botanical courses were laden with innumerable, often complicated (and not infrequently redundant) terms which had been invented to describe a vast array of floral variation (Fig. 3.1). Every conceivable facet of the variation of the infloresc ence, bracts, receptacle, hypanthium, calyx, corolla, nectaries, disk, stamens, carpels and ovules has been found by taxonomists to be valuable in the classification of one or other group of angiosperms. In the diagnostic keys to taxa in Floras, floral characters are those most used, and they figure equally prominently in the descriptions of the taxa which follow the keys. Examples could be given of the use of such features at all taxonomic levels. This reliance
70 Structural information
PERIANTH and ANDROECIAL POSITION
PLACENTAL POSITION
Axile Laruinate Marginal Parietal Pendulous
INFLORESCENCE TYPES Simple C Y M ES C O R YM B S Compound DICHASIA Determinate Indeterminate
HEADS PANICLE RACEM E SPIKE U M B E LS
Fig. 3.1 Terminology of floral structure as exemplified by inflorescence types, and by perianth, androecium and placenta position, reproduced from Radford ef a/.335
on the flower is remarkable when one considers that for most of the time the majority of angiosperms lack any flowers at all!
From this vast pool of experience certain generalizations can be made with regard to the characters found to be most useful in certain taxa, or the level of conservativeness of certain characters. For example, in the Ranunculaceae the bracts, sepals and petals are particularly important, in the Asteraceae and
Reproductive and vegetative characters 71
Poaceae the inflorescence, bracts and general flower-type, in the Fabaceae the stamens and carpels, in the Scrophulariaceae and Lamiaceae the corolla and stamens, and so on. There are, however, very large numbers of exceptions to these generalizations, and too much reliance on the latter would result in important errors or oversights.
Although the broad pattern of the structure of fruits and seeds is in general a less reliable taxonomic criterion (superficially similar fruits or seeds sometimes being found in only remotely related groups), they are often of supreme importance. In such large families as the Brassicaceae and Apiaceae, for example, the fruits provide the most useful characters of all, and they are scarcely less important in the Rosaceae. In such families the number of special terms used to describe the variation of the fruits is, as expected, high. Beginners in field botany soon learn that care must be taken when collecting members of the Brassicaceae, for example, to include ripe fruits with the specimen in order to guarantee identification by means of a Flora.
Similarly, seeds are of particular significance in families such as the Caryophyllaceae. In this family it is not simply that seeds are useful in delimiting tribes and genera, etc., but that, in many cases, seed-structure produces taxonomic criteria at all levels of the hierarchy. The genera Silene,
Dianthus, Petrorhagia, Stellaria, Spergularia and Spergula illustrate this well.
Whereas seed-structure usually differs interspecifically in these genera, in certain cases it varies at an infraspecific level (e.g. Spergula arvensis) or even within an individual (e.g. Spergularia media). The four European subspecies of Montia fontana (Portulacaceae) are distinguished almost solely on seed morphology460 (Fig. 3.2). Seed characters are particularly investigated by botanists interested in fossil (especially Quaternary) floras, as the seeds are often the only parts remaining. The use of seed characters, from gross morphological to subcellular levels, is today rapidly increasing.17
In lower plants flowers are lacking, and taxonomists have always had to rely on other characters. In algae and bryophytes, particularly, reproductive organs are often rather rare or shortlived, and most classifications have in the past been based upon vegetative structure. By diligent searching and careful recording over the past two centuries we have now amassed a great deal of information on the reproductive features of these groups, and have therefore obtained many new taxonomic criteria. In some cases these data have thrown a different light on the relationships of lower plants, but in the main they have caused taxonomic adjustments rather than revolutions. In bryophytes, for example, the sex organs are very conservative (see Chapter 8), and in the case of the liverworts (hepatics) this applies to the sporophytes as well. In mosses the sporophytes are less conservative and have provided many extra charac ters which have led to some notable changes in the classification. Neverthe less, the identification of bryophytes is still carried out using mainly vegetative characters, for obvious reasons. In the algae the reproductive organs provide many taxonomic characters, but these largely parallel the delimitation of taxa indicated by the vegetative structure and chemistry. In lower plants as a whole, in fact, the distinction between vegetative and reproductive structures is blurred; the sporophyte of a bryophyte, or the gametophyte of a pter- idophyte, for example, could be regarded as either vegetative or reproduc tive.
72 Structural information
07b mm
> ... ...-.1
Fig. 3.2 Scanning electron micrographs of seeds of the four subspecies of Montia
fontana: A, subsp. fontana; B, subsp. variabilis; C, subsp. amporitana; D, subsp. chondrosperma. Photographs by A. N. Scott.
In the higher plants vegetative characters are often looked upon as risky evidence, because there are many cases where superficially similar morpholo gical features are found in quite unrelated plants. Similar growth-habits (e.g. cactus-like plants, broom-like plants, trees) or leaf-shapes (e.g. palmately- lobed, compound pinnate) are good examples. The similarity in the leaf- shape of various maples (Acer) and planes (Platanus), which belong to very remote families, are well known; in Britain the name sycamore is applied to a species of maple, whereas in America it refers to a plane-tree, and the sycamore of the bible is yet another plant (a fig) with a similar leaf-shape. Thus vegetative characters have been used rather reluctantly, and particularly in groups where reproductive characters are unhelpful in classification. The grasses (Poaceae) are in this category. In the elms ( Ulmus) the flower and fruit are of little taxonomic value, and the shape of the leaves provides the most important taxonomic features. In oaks (Quercus) and birches (Betula) the leaf characters are again the most important ones. Within the family Ranunculaceae the genera Aquilegia and Thalictrum differ by many impor-