  We make some remarks on earlier works on $R-$bisectoriality in $L^p$ of
perturbed first order differential operators by Hyt\"onen, McIntosh and Portal.
They have shown that this is equivalent to bounded holomorphic functional
calculus in $L^p$ for $p$ in any open interval when suitable hypotheses are
made. Hyt\"onen and McIntosh then showed that $R$-bisectoriality in $L^p$ at
one value of $p$ can be extrapolated in a neighborhood of $p$. We give a
different proof of this extrapolation and observe that the first proof has
impact on the splitting of the space by the kernel and range.
