DEVELOPMENT OF THE ENDODERMIS AND PHI LAYER OF APPLE ROOTS

Suberin lamellae and a tertiary cellulose wall in endodermal cells are deposited much closer to the tip of apple roots than of annual roots. Casparian strips and lignified thickenings differentiate in the anticlinal walls of all endodermal and phi layer cells respectively, 4-5 mm from the root tip. 16 mm from the root tip and only in the endodermis opposite the phloem poles, suberin lamellae are laid down on the inner surface of the cell walls, followed 35 mm from the root tip by an additional cellulosic layer. Coincidentally with this last development, the suberin and cellulose layers detach from the outer tangential walls and the cytoplasm fragments. 85 mm from the root tip the xylem pole endodermis (50% of the endodermis) develops similarly, but does not collapse. 100-150 mm from the root tip, the surface colour of the root changes from white to brown, a phellogen develops from the pericycle and sloughing of the cortex begins. A few secondary xylem elements are visible at this stage. Plasmodesmata traverse the suberin and cellulose layers of the endodermis, but their greater frequency in the outer tangential and radial walls of the phi layer when compared with the endodermis suggests that this layer may regulate the inflow of water and nutrients to the stele. © 1979 Springer-Verlag.