Word Growth Puzzle

Our friend, Mr. PetDander, gave me a word puzzle yesterday that you all might enjoy. Here it is...

What is the longest word in the English language which you can form starting with a base single-letter word (e.g. A, I, or O) and adding one letter at a time without rearrangement, where each additional letter creates a new legal word? Internal additions are allowed.

Here is an example of a valid 7 letter solution:

A --> AN --> ANT --> PANT --> PLANT --> PLANTS --> PLANETS

So, what is the longest valid solution?

I'll post the answer later on if no one gets it. I was unable to find any really long answers, so I cheated and ended up writing a program that grew words from A, I, O seeds. Turns out that there is a single word that bests all others. Interesting.