Plant species are so numerous and varied that every time a rule is proved it isn’t long, relatively speaking, before the rule is disproved.
Much of what we understood to be ‘the basics’ of science was based on research done decades, or even centuries ago. Modern microscopes have allowed us to confirm much of this research by observing processes happening on a molecular level, but almost as often these long held truths have proved to be not quite false, but rather not the whole truth. In the same vein, scientific concepts are often simplified for educational purposes- which can lead to some people making leaps in logic that don’t stand up to scrutiny. It’s important to remember that science rarely deals in absolute truths, but rather it attempts to be less wrong than the work upon which it builds.