Figure 2: Increase of Azolla biomass of Milan strain, expressed as f.w. RGR (Relative Growth Rate) ( )/( ), calculated every seven days from Mar. 22nd to Apr. 28th 2002. Every period of which RGR is calculated, is reported on -axis. The extreme of the date intervals is included. The bars represent the standard deviation. Fern biomass increased linearly until 19 April, followed by a more rapid growth until 28 April.