A comparison of the primary production of the leaf of corn, a C specie, and sugar beet, a C specie, is made. Models adapted to these two kinds of plants are developped. The main differences appear to be related to photorespiration and to the CO
of a corn crop all along the season is described. The net assimilation rate is nearly constant at low irradiances and variable at high irradiances. A first period of high net assimilation rate is located around flowering and a second one is observed when
the growth period of a corn crop are principally analysed as a function of two climatic variables: the global radiation and the CO exchange coefficient. The analysis is also used to calculate from climatic data the daily photosynthesis and the total