The formation of discrete 'tablets' of hydrated silica in the bulliform cells of the leaf blade was followed over a 16-day period in three species of the Gramineae representing different habitats. Seedlings of Oryza sativa (rice) and Cynodon dactylon (Bermuda Grass) were cultured under growth-cabinet conditions at levels of 50 and 500 ppm dissolved silica (SiO2) in the nutrient solution. In addition, bulliform deposition was studied in mature tiller leaves of Sieglingia decumbens (Heath Grass). Attached leaves, as well as those excised from the culm, were used.Initial stages of tablet formation were observed by the 2-day harvest in the central and basal zones of the fully expanded seedling blades. Deposition did not occur at a stage when bulliform turgor changes might influence blade evolvement. At the 16-day harvest, deposition was heaviest in the tip zone, and decreased progressively towards the base of the blade. In addition, proportionately higher tablet counts (P = 0.05) generally were absent from the leaves grown at the higher silica level. This indicated a limited availability of deposition sites.These results are discussed in relation to (i) cellular maturation; (ii) internal leaf anatomy; (iii) leaf expansion; (iv) a basipetal senescence gradient within the leaf blade. Certain of these are considered to be possible limiting factors to silica deposition in the grass leaf. © 1969 OXFORD UNIVERSITY PRESS.
