The opacity of etymology

The word disseminate is a familiar one. It appears hundreds of times on my hard drive and in well over 20 email messages I've read or written in the last five years. But until today, I had never seen the seeds in the word.

We often use a plant metaphor to talk about words. Morphology is a branch of linguistics just as plant morphology is a branch of biology. Both sciences talk of roots and stems, but in linguistics, seeds aren't part of the metaphor.

The root of disseminate though is semin or semen, from the Latin word meaning seed. Dissemination is the spreading of seeds. Semin is also the root of the word seminary, literally a seed plot, but now metaphorically used to mean a place to train priests. This is also where seminar comes from. I hadn't connected up these words either.

Disseminate appears in a passage that my level-8 class is studying. It's a passage that I've been over many times with other classes, and I have had to explain the word before. But never before have I made this connection.

How could this be?

On the flip side of this are cases of people seeing connections where there are none. Consider the regular flare ups about the word niggardly based on a mistaken perception that it's based on a racial slur. There are so many opportunities for false positives. The string semen, for instance shows up in a variety of words such as basement and horsemen, and nobody would ever think it referred to seeds.

How does this noticing thing work?