The fine structure of primary, secondary, and tertiary stages of Zea endodermal cell development was investigated. The casparian strip formed in situ in the anticlinal walls and remained at a fixed point relative to the endodermis‐pericycle boundary. The only protoplasmic structure that had a constant spatial association with the developing strip was the plasmalemma. Plasmodesmata appeared to be more numerous on the tangential walls than on radial walls; only rarely were they located in the casparian strip. The suberized lamella developed on inner and outer tangential walls before it appeared on the radial walls. No cytoplasmic organelles were found to have any particular spatial association with this layer. The suberized lamella was about 0.04 μm thick except near plasmodesmata and along the adaxial margin of the casparian strip, where it was thicker. Occasionally it failed to form along the abaxial margin of the strip. The adherent affinity between plasmalemma and casparian strip was lost after the strip was covered by suberized lamella. The secondary wall became asymmetrically thickened by differential deposition of successive lamellae. A thin layer of secondary wall material extended across the floor of each pit. Pit cavities often contained mitochondria, and plasmodesmata were restricted to the pits. The plasmodesmata were constricted where they entered the thin layer of secondary wall material and where they penetrated the suberized lamella. The various stages of cell development tended to be asynchronous. No passage cells were observed. Endodermal cell development in Zea closely resembles that described for barley.

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-ceIl'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.

No abstract

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.