Published online by Cambridge University Press: 05 June 2012
Plant life forms and biological spectra
Variation in vascular plant and bryophyte life form
In addition to taxonomic classifications which endeavour to place closely related species in the same family, botanists have for centuries attempted to distinguish particular life forms, any one of which may be adopted by quite unrelated species. The simplest of these is the distinction between woody and herbaceous plants. Raunkiaer (1934) developed the most widely known scientific description of life forms, and then used it to initiate the use of biological spectra to compare different floras. The main feature of this ecologically valuable system is the position of the vegetative perennating buds or persistent stem apices during the cold winter or dry summer forming the unfavourable season of the year. The main life forms shown in Fig. 3.1 form a sequence showing successively greater protection from desiccation, indicating the position of the vegetative buds when the plant is dormant.
It was assumed that the flowering plants evolved when the climate was more uniformly hot and moist than it is now, and that the most primitive life form is represented by the phanerophytes which still dominate tropical vegetation. These large terrestrial plants can grow continually forming stems, often with naked buds, projecting high into the air. Those whose buds are protected from cold or desiccation by bud scales are considered to be more highly evolved.