Researchers holding plant roots during a study into the relationship between plants and fungi. Most plant roots are associated with soil fungi in what are known as mycorrhizal networks. One of the benefits of these networks is that the fungi can connect neighbouring plants and facilitate the exchange of organic molecules. To investigate this process, these researchers injected carbon 13 in various forms into the tree roots. Carbon 13 is an isotope of carbon which only makes up around 1 percent of natural carbon, meaning it can be traced in experiments.