We have collected a large body of NIR ( H band ) , UV ( 2000 Å ) and H \alpha measurements of late-type galaxies .
These are used , jointly with spectral evolutionary synthesis models , to study the initial mass function ( IMF ) in the mass range m > 2 { M \odot } .
For spirals ( Sa-Sd ) , Magellanic irregulars ( Im ) and blue compact dwarfs ( BCD ) , our determination is consistent with a Salpeter IMF with an upper mass cutoff M _ { up } \sim 80 { M \odot } .
The history of star formation and the amount of total gas ( per unit mass ) of galaxies are found to depend primarily on their total masses ( as traced by the H band luminosities ) and only secondarily on morphological type .
The present star formation activity of massive spirals is up to 100 times smaller than that average over their lifetime , while in low mass galaxies it is comparable to or higher than that at earlier epochs .
Dwarf galaxies have presently larger gas reservoirs per unit mass than massive spirals .
The efficiency in transforming gas into stars and the time scale for gas depletion ( \sim 10 Gyrs ) are independent of the luminosity and/or of the morphological type .
These evidences are consistent with the idea that galaxies are coeval systems , that they evolved as closed-boxes forming stars following a simple , universal star formation law whose characteristic time scale is small ( \tau \sim 1 Gyr ) in massive spirals and large ( \tau > 10 Gyr ) in low mass galaxies .
A similar conclusion was drawn by Gavazzi & Scodeggio ( 1996 ) to explain the colour-magnitude relation of late-type galaxies .
The consequences of this interpretation on the evolution of the star formation rate and of the gas density per comoving volume of the Universe with look-back time are discussed .