Fundamental features of modern inflorescence morphology

Abstract

Great diversity is found in inflorescences of angiosperms. According to Troll this is due to the variation of two types only: the polytelic and the monotelic type.

In the monotelic inflorescence, the apex of the inflorescence axis commonly ends with a terminal flower. This also applies to all the floral branches below the terminal flower. All of these branches, whether branched or not, proved to be homologous elements, and they are all referred to by the term ‘paracladia’ because these branches repeat the structure of the main axis of the flowering system.

Accordingly, their ramifications are called paracladia of the 2nd to nth order. Since by the presence of the paracladia the number of flowers in the flowering system is increased, they may be called ‘enriching branches’. Consequently the whole area which produces the enriching branches may be designated as an ‘enriching field’. In the lower part of the flowering shoot this zone is commonly preceded by a ‘field of inhibition’ within which the development of paracladia is inhibited more or less abruptly. The same zonation can be recognized in the individual paracladia if these are not reduced in any way. In perennials, the axillary buds at the base of the whole stem do not develop within the same season, but will give rise to the innovation shoots at the beginning of the following season. Therefore this area has to be distinguished as a ‘field of innovation’.

The polytelic type of inflorescence probably has been derived repeatedly from the monotelic during the

evolution of angiosperms by reduction of the terminal flower and specialization of the paracladia of the monotelic system. The distal elements are reduced to single lateral flowers or lateral cymes (partial florescences) which constitute elements of an apical system composed of lateral flowers. Instead of ending in a single flower, the floral axis thus terminates in a multi-flowered so-called polytelic ‘florescence’. The lower lateral branches repeat the structure of the main stem by producing (indefinite) florescences themselves and therefore may be termedparacladia (of the polytelic system). As in monotelic inflorescences the area of paracladia — the ‘enriching field’ — can be preceded by a ‘field of inhibition’ and, in perennial plants, by a ‘field of innovation’.

Though generally we may confirm that the inflorescences of all species investigated (about 20 000 from nearly all angiosperm families) proved to be variations of one of the two fundamental types, it sometimes needs morphological experience to apply Troll’s system to woody plants of tropical regions. Difficulties may derive from prolepsis and syllepsis of the innovation shoots, prolification of the inflorescence axis and, abundantly, by the deficiency of clearly marked limits between sprouts growing in different seasons. In each of such cases, however, the homologous flower-bearing-elements can be identified by comparing their positions within the whole system of ramification.