I will have a go!

To be grounded in the idea of species, where better to start than with something familiar, something that by any other name would smell as sweet.

This is obviously from a book about biology. I imagine that it is trying to explain about categorising plant. The idea that they can be grouped after species - for example Roses. Something that by any other name would smell as sweet is a reference to Romeo and Juliet.

What �s in a name? that which we call a rose
      By any other name would smell as sweet;

botanists tell us there are over 100 species, yet members of some species show more variation than samples that are declared to be separate species.

Botanists have identified over 100 different types (let �s just say Roses here as I think that it will make it easier), however within some of these categories the roses don �t always look alike, indeed they can look more like roses from other categories.

When we got our hands and green thumbs on them, defining species became an even more thorny issue

When we took a closer look at them it became even harder to put them into categories.

Green thumbs is a reference to people who are good at getting things to grow - they have green fingers.

A thorny issue is something that is difficult/tricky

I hope this helps!

Cindy