Comparison of main stems of young hemlock and cow parsley plants

This picture was taken in May. On the left is a young hemlock plant (Conium maculatum). On the right is a young cow parsley plant (Anthriscus sylvestris). The main stem and the leaf stems of hemlock are circular, smooth, pale green with reddish purples spots and blotches. The main stem of cow parsley is also circular but has longitudinal grooves.