The paper deals with the study of morphological differentiation of flower buds in 10 varieties of apple-trees in relation to the growth of summer shoots and the earliness of varieties. The data obtained indicate that in the warmer regions of Slovakia the critical season of transformation of flower buds from vegetative to generative forms takes place, on the average, between July 20 and 29, and, at the same time, the variety Yellow Transparent deviates from the average four week-earlier. According to the author we can speak neither of a positive relation between the onset of morphological differentiation and the active growth of summer shoots nor of the relation between the earliness of the variety in maturing and the onset of differentiation. The morphological differentiation may be classified into five phases: 1st phase — the formation of the growing point primordium, 2nd phase — the formation of the cup on sepal primordia, 3rd phase — the formation of petal primordium, 4th phase — the formation of carpel and stamen primordia, 6th phase — completion of the differentiation through substantial enlargement of flower organs before the onset of winter.