Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation?

In steam distillation, the organic liquid starts to boil when the sum of vapour pressure due to the organic liquid $\left(p_{1}\right)$ and the vapour pressure due to water $\left(p_{2}\right)$ becomes equal to atmospheric pressure $(p)$, that is, $p=p_{1}+p_{2}$

Since $p_{1}