This paper proposes a thermodynamic model of biological development. Several key thoughts are presented: 1) in view of thermodynamics, biological development processes irreversibly
2) in view of thermodynamics and molecular biology, positive autoregulation, or self-regulation, of transcription factors is the only way to ensure irreversibility of a thermodynamic process of biology
3) change in the autoregulation of transcription factors can irreversibly result in alterations in the physiological state
) a physiological state is a system of signaling networks
5) a cell and its physiological state can be identified by the pattern of its transcription factors. 6) from points aforementioned, we can analyze some thermodynamic properties of biological development by knowledge of molecular biology and biochemistry. The possible mechanisms of plant vernalization are also proposed.