Transpiration of plants cultivated in greenhouses was analysed in the paper on the bases of laws, which describe natural convection utilising similarity numbers. Experimental works were carried out in greenhouse. Transpiration of growing in there tomatoes one measured utilising lisimetric method. It was found that standard correlation equations described incorrectly transpiration process. That is why other correlation equation was found (relationship among: Sherwood's, Archimedes's, Schmidt's numbers). Comparison was done between measured transpiration and calculated value of it on the base of above mentioned equation. In graphic way has been shown relation between mass penetration coefficient and measured values of microclimate parameters inside of greenhouse. It was found that to describe transpiration process there is enough to know the basic values of microclimate (humidity and environment temperature as well as leaves one).