Early researchers concluded that the winter-annual flora was richest in the California deserts and became relatively poorer towards the east, whereas summer-annuals were richest in the deserts of western Texas and their richness progressively declined in to eastern California. US/IBP Desert Biome and subsequent studies support these trends and add much relevant information. Winter-annuals have the C3 photosynthetic pathway, whereas most summer-annuals have the C4 pathway. While the Sonoran and Chihuahuan deserts have a few C3 'summer' annuals (actually 'winter' annuals which can persist into the summer), the Mojave Desert lacks C3 summer-annuals. The Great Basin Desert has only a few prevalent winter or summer-annuals (mostly introduced species). Studies of the micro-distribution of desert-annuals have shown their close association with perennial shrubs. Some shrubs produce substances that are reportedly toxic to some annuals; the role of such toxins in structuring annual plant communities is unclear. The importance of nutrients associated with shrub canopies on the micro-distribution of many desert-annuals is becoming evident through LTER and other studies.