Giant ragweed, Ambrosia trifida L., was grown over a period of 36 days in a glasshouse at two levels of soil fertility. Growth was investigated both by the conventional (whole-plant) techniques of growth analysis and by these same techniques applied to data obtained on the growth of roots, stems and each of the successive leaves.

Results showed that the superior performance of the fertilized plants was due to their higher leaf-area ratio rather than to an enhanced unit leaf rate. Here, the rates of dry-matter commitment to leaves at nodes 2, 3 and 4, expressed per unit of whole plant dry wt, were initially around 0-04 g g−1 day−1 -about twice the values obtained for the same leaves in the unfertilized plants.

There was a heavy early commitment to root growth especially in the unfertilized plants. Stem production was moderately high throughout in both treatments. The large response to fertilizer application would be expected of the species since it colonizes arable land and flood-plains both of which experience nutrient pulses. This behaviour, together with the flexibility of material allocation contribute to the high success of the species.

The study provided the opportunity to assess the techniques of growth analysis when applied at the sub-organismal level. The value of this approach was discussed in relation to the parallel activity of leaf demography.