Abstract: The theoretical proof of Monsi formula must suppose that the distribution of leaf area in a stand is exactly obeyed on Poission distribution law.Using methematical methods,this paper shows the biological meanings of parameters in Poission distribution.It is indicated that Monsi formula is a special form of Poission distribution.And hence,the promise of Monsi formula has to be that plant stand possesses a property of spatial Poission distribution.Most of the stand in the nature doesn't possess this property,so the theoretical value doesn't meet the measured value of solar direct radiation,which shows a definite deviation.The quantity of this deviation maybe possess some significations for studying thermal and dynamic states in plant stand.

Key words: Semi-desert algal crust, Species composition, Distribution