OBSERVATIONS ON THE PHI-THICKENINGS AND CASPARIAN STRIPS IN PELARGONIUM ROOTS

An ultrastructural examination of hypodermal cell walls failed to demonstrate the presence of casparian strips as has been reported in the literature. Rather, these cells have supportive phi-thickenings which differ significantly from casparian strips. Adjacent phi-thickenings are convex-convex and stratified in transection, with an uneven surface like the rest of the phi-cell's wall. They may be unbranched or branched and in the latter case, especially, they may appear on tangential as well as radial and transverse walls. They are lignified early in development and are associated with microtubules which lie parallel to the long axis of the thickening. Further, the plasmalemma exhibits no special adherence to the thickened wall region. In each of these and other characteristics phi-thickenings contrast sharply with casparian strips.