Are fungi plants?

An interested reader writes, "Are mushrooms plants?" Mushrooms are fungi, and fungi are not plants, although this is a bit more of a problem that you might imagine. First off, biologists have known that fungi were something quite apart from plants for a long time, but what happened was that in terms of teaching and academic departments, biology was divided up into zoology, the study of animals, and botany got everything else, and that not only included the fungi, but bacteria as well. In modern day biology, the old taxonomic concept of kingdoms is not faring so well, nor for that matter is the whole taxonomic hierarchy. However, let me stay on task. Fungi do form a single lineage, a group that shares a common ancestry, and this lineage conforms to what many mean when they refer to Fungi as a kingdom. The same may be said for animals, and interestingly enough, fungi and animals are each others closest relatives. That is, they share a common ancestry too somewhere back among their unicellular ancestors. See how similar these two pink-orange critters are?

So as bacteria and fungi were recognized as distinct groups, the old catch-all plant kingdom got smaller, and even more so when about 45 years ago, even all the algae were removed (and placed within a grab-bag kingdom of protists) leaving behind nothing but the land plants. No question land plants form a single lineage, but they share a common ancestry with a small group of green algae, and then a more general common ancestry with the rest of the green algae. So where do you draw the kingdom line? This is part of the reason why the concept just isn't all that useful anymore. And there are even more green organisms out there that are only very distantly related to plants, things like the chlorarachniophytes, little spider like green amoebae.