In the early stage of development, the sinkers of Pecteilis radiata are typical lateral buds arising at the axils of the fourth and fifth leaves on one monopodium. In summer the hypopodium of the lateral bud is elongated extraordinarily by a rib meristem and is hardly distinguishable from the absorbing roots except by its white colour and the absence of root hairs. Then a mass meristem appears within the periblem near the second node, and later it develops into a tuber. The full-grown sinker consists of three parts-the stalk, the neck and the tuber. The stalk corresponds to the hypopodium of a lateral shoot and is a typical cauline axis; the tuber, the distal part of the sinker, is a single adventitious root of the mesogenous origin. The calyptrogen is not so active that a root pocket derived from the periblem of the mother axis covers the tuber apex firmly as if it were a real root cap, but in the older tuber the root pocket is peeled off completely and the tuber apex is also covered with the rhizoderm having root hairs. The neck is represented by the connecting part between the stalk and the tuber, and morphological peculiarities are observed in this part alone. The neck has a central cavity, at the bottom of which the terminal bud of the lateral shoot is situated. The ontogeny shows that the neck is derived from the parts between the first and the third nodes of a lateral shoot by the characteristic growth of the parts. The rhizoderm is, however, differentiated not only at the part of typical root nature but also at the part of the neck, and then the epidermis of the neck is replaced by the rhizoderm after it is peeled off. Therefore, the neck may be said to be a sort of the periclinal chimera consisting of the stem and root.