Fig. 9.—After Vöchting. A, A¹, A². Pieces of thallus of Lunularia communis regenerating at the apical end. B. Piece of thallus cut in two in the middle line. B¹. Same split at side of middle. C. An oblique piece extending to middle line. C¹, C². Oblique pieces not extending to middle line. D. Fruiting stalk stuck into sand, producing new thallus above sand. D¹. Same laid horizontally regenerating near base. E. Same with fruiting head cut off. Regenerating at base. E¹. Twisted piece regenerating at two points. F. Piece of ray of head regenerating near base. F¹. Same with distal end of ray cut off. Also regenerating at base.
In the lower plants, such as the mosses, the liverworts, the moulds, and the unicellular forms, regeneration also takes place. Vöchting has shown that pieces from any part of the thallus of a liverwort[8] produce new plants. If a cross-piece is cut off, there appears a small outgrowth from the middle of the anterior cut-edge, as shown in [Fig. 9], A, A², that gradually enlarges to form a new thallus. It will be seen from the figures that the whole anterior edge does not grow forward, but a new thallus arises from a group of cells at, or near, the anterior edge. These cells are the least-differentiated cells in the piece, and have softer cell walls than have the other cells.
Fig. 10.—After Pringsheim. A. A piece of seta of sporophore of Hypnum cupressiforme, sending out protonema-threads. B. Longitudinal section of a piece of the seta of sporophore of Bryum cæspitosum. C. Piece of same of Hypnum cupressiforme. Moss-plant arising from new protonema. D. Piece of same of Hypnum serpens with protonema and moss-plant arising from it.
Pringsheim has shown that if a piece of the stalk of the sporangium of certain mosses is cut off, it produces at its ends thread-like outgrowths which are like the protonema-stage of the moss, and from this protonema new moss-plants may arise ([Fig. 10], A, B, C, D).
Braefeld has obtained a somewhat similar result in one of the moulds, in which a piece of the sporangium stalk gives rise to a mycelium from which new sporangia may be produced.
REGENERATION IN EMBRYOS AND EGGS
Regeneration takes place not only in adult organisms, but also in embryos, and larvæ of many animals. It is often stated that the power of regeneration is more highly developed in embryos than in adults, but the facts that can be advanced in support of this view are not numerous. One of the few cases of this sort known to us is that of the leg of the frog, that does not regenerate, while the leg of the tadpole is capable of regenerating.