Cortical lacunas in older regions of rice roots are formed by the radial unification of intercellular spaces which enlarge as the reflection of radial contractions of cortical cells. This means that no lacuna is lysigenous. The degree of contraction of cortical cells in rice roots is variable in accordance with the nutrient employed for the culture of rice seedlings : Potassium and pure water accelerate the contraction, but inorganic nitrogenous nutrient, especially NH4-N, inhibits it. According to paper electrophoresis, older regions of roots have always less proteins per fresh weight than younger regions. These results show that the less the cytoplasmic proteins in older cortical cells, the longer the distal young parts of the roots, and that the decrease of cytoplasm in cortical cells is one of the promotive factors for the histogenesis of lacunas. The elongation of distal part of the root is associated with the increase of root length and mostly with the branching in the root. Elongation and branching are as well accelerated by the sugars in the top or in the medium as potassium absorbed in seedlings, but inhibited by ammonious nitrogen. Excretion of soluble nitrogenous substances from cortical cells induced by IAA is an assumable cause of the formation of lacunas.
