Is a leaf all that it seems to be -- just a leaf? It turns out that fungal endophytes - fungi that live inside plants without harming them - are common and to be found in almost all leaves of tropical forest plants. It seems that the more we learn about organisms and their interactions, the more the idea of the 'free living organism' turns out to be a big lie! Not only are all organisms dependent upon others in obvious ways such as for food and nutrient cycling, but virtually all macroscopic creatures seem to have microscopic organisms living in them too (and frequently even the microscopic ones are hosts to yet smaller ones.)
We commonly understand fungi to adopt a saprobiontic lifestyle, acting as decomposers in the soil, in wood, in leaf litter, or perhaps as pathogens and parasites such as rusts and smuts. But the diversity of these lifestyles may be matched by the diversity of fungal endophytes, which live in the leaf tissues of plants without showing any symptoms or substantially impairing host productivity. They offer benefits to their hosts' fitness, such as preventing pathogenic infection, and possibly making leaves less palatable to herbivores. At the same time the leaves offer a substrate for the fungi to grow upon and provide food substances for their growth and reproduction. Most of these endophytes are transmitted horizontally rather than vertically, though the most well-studied case, that of grass endophytic fungi, are vertically transmitted.
Check out the webpage of Betsy Arnold from the University of Arizona. Her lab does a lot of work on fungal endophyte diversity and interactions with their host plants. Their diversity is still barely understood, especially in the tropics, and culture methods are relatively simple. This could be a pretty cool project for those of you living in places where the leaves aren't all falling off the trees!