Preformation, the initiation and early development of organs one or more growing seasons before maturity and function, is ubiquitous among alpine plants. This pattern of development is believed to allow plants to survive the short growing seasons found in alpine environments but few data have been presented to support this belief. The development of Acomastylis rossii (alpine avens) was studied in a moist meadow and a fellfield on Niwot Ridge (a Colorado alpine tundra site) to characterize the relationship between preformation and environment in alpine tundra. Moist meadow sites endure a longer snowpack and thus have a shorter growing season than adjacent fellfield sites. If preformation provides an advantage in shorter growing seasons, then a greater extent of preformation is expected in the moist meadow site. A. rossii plants in both communities have an extended period of preformation. Three years are required for each leaf and inflorescence to progress from initiation through structural and functional maturity. As a consequence, three cohorts of organs, initiated in successive years, are borne simultaneously on an individual plant. Plants extend an initial flush of leaves shortly after snowmelt after which no additional leaves are matured until the next growing season. Preformation proceeds below ground through the remainder of the growing season and into the period after aboveground structures senesce. Routine abortion of axilary inflorescences and vegetative shoots is observed. It appears that neither the extent nor the duration of preformation varies with environment in A. rossii. It is likely that variation in mature structure among sites is due to variation in abortion of axillary shoots and variation in the extent of organ elongation and expansion during the season of maturation.

Key words: Acomastylis rossii, Alpine tundra, architecture, development, preformation, Roseaceae