Rooting For Grassroots

My senior told me, if you are referring to the root of the grass, then it's grassroot, and if there are many roots, it's grassroots.

However if you are using the term in reference to people, then it must never be grassroot. It should be grassroots.



Hence, he has the support of the grassroots; or if you want to use the term without the letter s, then it must be grassroot leaders or grassroot members. Hence, he has the support of grassroot members.

I think he is both right and wrong. He is wrong about the grassroot being the reference for the root of the grass. I think if I was indeed referring to the root, shouldn't I be writing it as grass root or grass roots, with the spacing in between instead?

The fact that the term is spelled as one word itself is already an evidence that I was not referring to the actual root. No?

Note to self: must research some answer on this. Until and unless I get the definite answer, it would be best that I heed the advice of my senior.