Distribution patterns, frequency, size, shape, and surface texture of silica grains in the secondary xylem of neotropical taxa, especially from Surinam were studied extensively. Over 2000 samples were examined. Silica grains occur in about 300 species (32 families and about 90 genera). The grains proved to be present in many taxa so far considered as non-siliceous. They are most frequently found in the parenchymatous tissues (in 80% of siliceous material studied, grains were present in the ray cells). Their distribution is very constant, especially when they are restricted to one type of tissue, although some exceptions exist. The diagnostic value of the silica grains, often neglected, appears to be very high. The shape of the grains is usually variable though, in a few cases it is very characteristic. Various types of surface structures are recognized. Size of the grains is reported. The great variation in grain size greatly reduces its importance as a diagnostic tool.