Although the five cacao viruses studied produce different external symptoms in infected plants, they affect internal anatomy similarly. Symptoms on leaves occur only if these are still developing when they become infected, and the viruses seem to produce their effects usually by preventing differentiation of the cells. The tissues of chlorotic areas of infected leaves are undifferentiated and similar in structure to young unexpanded leaves. In stem and root swellings xylem and phloem are both increased, but they occur in the same proportions as in normal secondary thickening. The anatomical effects of infection seem insufficient alone to account for the death of cacao trees, but they may well be complementary to the serious necrosis of the root system which results from virus infections.