Structural changes in phi thickenings during primary and secondary growth in roots. 1. Apple (Pyrus malus) Rosaceae

摘要

Whole roots and freehand cross sections of apple roots, cleared and stained with a root extract ofChelidonium majusL., revealed the presence of a uniseriate layer of phi thickenings. The layer, externally adjacent to the endodermis, consisted of cells with a heavy deposition of lignin in the thickened midportions of their transverse and radial walls. The thickenings were completely developed within 3ndash;5ensp;mm from the root tip. Lateral root primordia emerged through the phi layer by means of openings in the layer resulting from the removal of entire phi thickenings from the cortical cells. The layer was not continuous from the main to the lateral roots unlike the Casparian band of the endodermis. During secondary growth, the middle lamellae of the cells of the phi layer split, disrupting the regular appearance of the layer. Ultimately, the epidermis and cortex were sloughed off as the periderm, arising in the pericycle, developed to form the most external tissue of the mature root. However, remnants of the phi thickenings were persistent in many areas of the mature root.