An interesting case of physiological regeneration has been described by Balbiani in a unicellular form, stentor. From time to time a new peristome appears along the side, moves forward and replaces the old peristome, that is absorbed as the new one comes into position. In other infusoria the peristome may be absorbed before encystment, and a new one appears when the animal emerges from the cyst. Schuberg states that when division takes place in bursaria the new peristome develops on the aboral piece in the same way as after encystment; and Gruber observed that, when an aboral piece of an infusorian is cut off, a new peristome develops in the same way as after normal division of the animal. These observations indicate that the process of physiological regeneration may follow the same course and probably involves the same factors as the process of restorative regeneration. Tubularia absorbs its old hydranth-heads if placed in an aquarium, and regenerates new ones. It may even absorb the hydranth while growing in an aquarium, as Dalyell has shown, and presumably, therefore, also under natural conditions. After each regeneration the new stalk behind the head increases in length. In plants, in which there is a continuous apical growth, new parts are being always added at the end of the stem, and old parts are continually dying, as seen in palms. Most trees and shrubs in temperate climates lose their leaves once a year and produce new ones in the spring. Since the new leaves develop from the new shoots at the end of the stem and branches, the old ones can, only in a general way, be said to be renewed.