 Another material that is rather difficult for plants and is essential to them is nitrogen. Nitrogen of course is present in almost all proteins, amino acids and a lot of enzymes and so on as well. Now the problem for plants is they're surrounded by nitrogen, roughly 80% of the atmosphere is nitrogen. But they can't use it. It's only a very limited set of plants, mostly the legumes that use bacteria in their roots to actually grab atmospheric nitrogen. Most plants rely on ammonia or nitrate that they get mostly from the soil. What do they do with it? Well, of course Rubisco, which is the commonest enzyme in the world, which captures the CO2 at the beginning of the process of photosynthesis, is their major sink for nitrogen. So typically if you look at the living parts of a plant, say the leaves, they may contain about let's say 5% of nitrogen. Some plants, particularly C4 plants often have less nitrogen than the C3 plants, like for example sunflower. Other parts of the plant, the wood for example, have almost no nitrogen in them. So if you cut down a eucalyptus and you take a sample of the wood from the center of the trunk, you will find almost literally no nitrogen. They have captured the nitrogen that was originally in there when the wood was made and they've moved it to places where it's actually going to be useful to them.