At the fifth internode, where a cambial ring was just formed, pressure was applied to the twig surface by girdling it with a rubber tube. This blocked also aeration of the surface. In the presence of the compact pith within, the pressure produced thicker wood with a low frequency of pores, in comparison to normal wood at that region. When the pith was replaced by vaseline the pressure, without a resistance from the centre, produced a very thin wood with long vessel elements and more abundant rays. Blocking of aeration from outside, without pressure, produced more abundant vessels. Aeration from the pith region produced a narrower wood zone. Absence of pith (replaced by vaseline) resulted in a wider zone of wood with narrower and shorter vessel members. © 1976 Oxford University Press.