A study has been made of the content of GC pairs and 5-methylcytosine in the total DNA of generative (inflorescences, flowers, flower buds) and vegetative (shoots, leaves, vegetative buds) organs in pea, tobacco and apple due to flowering gradient. In all the plants studied, the content of 5-methylcytosine in the DNA of generative organs in high as that in the DNA of vegetative organs. The DNA from various organs of one and the same plant hardly differs by the GC content. The differences in the amount of 5-methylcytosine are probably indictive of various levels of DNA methylation. Thus tissue (cell) DNA of both plants and animals differs in the content of 5-methylcytosine. Besides, methylation of genome in them changes upon flowering, i.e. we revealed the gradient of the degree of methylation of plant genome which correlates positively with the known flowering gradient. Methylation of plant genome regulated by phytohormone is possibly associated with cell differentiation and may be considered as being one of the mechanisms of regulation of transcription.