Abstract

It has been found in the study of Dendrobium kwashotens Hay. that the various root issues arise in regular seguence from the root apical meristem. The detailed anatomical and histological analysis shows that the adhesive roots, i.e., the roots attached to a solid substrate, are more adapted for the water absorption, since they have root hairs, smaller cells in the velamen and many thin walled passage cells in exodermis. On the contrary, the aerial roots, i.e., the roots developrd freely in the air, possess no root hairs and have many thick walled evodermal passage cells which are covered by lignified by lignified and suberized secondary walls. The exodermis consists of long and short(passage) cells; they regularly interpose each other. The exodermis is suggested to be the main barrier of the water path in orchid roots and the structure of exodermal cells are considered to play an important role in regulation of water absorption. The exodermal passage cells are compared to the potential root hairs in the root epidermis of angiosperms. The orderly arrangement of endodermal cells in relation to the distribution of protoxylem is described. The sequence of initiation, differentiation and maturation of various tissues in the vascularcylinder is also studied.