Small spores and large spores

A student measured five spores from each of four pteridophytes under a microscope. The widths are made up for this lab, but they are of the right size. You will find which plants are heterosporous, and draw the largest spore of each.

The idea. A homosporous plant has spores of about the same size. A heterosporous plant has small microspores and much larger megaspores. Here, call a plant heterosporous when its largest spore is more than $5$ times as wide as its smallest.

Your tasks.

1. In size_ratio, return the width of the largest spore divided by the width of the smallest.
2. Set heterosporous to a list of the names of the heterosporous plants, in the order of the dictionary.
3. Set big to the number of spores of Plant B that are wider than $100$ µm.