234 The Pnenomena of Morphogenesis 



process here is a common means of reproduction. In Sphoerocarpos, 

 Rickett (1920) found that regeneration occurs from single cells (or some- 

 times groups of adjacent cells) from almost anywhere on the thallus. At 

 first the mass of cells is globular, cylindrical, or ribbon-like but it soon 

 develops into a typical thallus much as does a germinating spore. 



In a study of vegetative reproduction in Metzgeria, Evans (1910) ob- 

 served that certain cells on the thallus dedifferentiate and that each then 

 grows into a gemma from which a new plant arises. The distribution of 

 these regenerative cells is not a random one, however. A robust thallus 

 produces no gemmae, and they are fewer in plants that bear sex organs. 

 If the apical region is very active, there are no gemmae near it. If a piece 

 of thallus is isolated, however, gemmae arise in it abundantly. Evidently 

 there are factors in this plant that tend to inhibit regeneration by its cells. 



Plantlets are frequently produced from single cells in the leaves of 

 Jungermanniales, and here they often, though not always, develop much 

 as spores do. It is sometimes difficult to tell whether they come from ordi- 

 nary vegetative cells or from ones that are predisposed to produce them. 

 Fulford ( 1944, 1954 ) has described many cases of reproductive regen- 

 eration in these plants. 



Many mosses also regenerate readily. The early work here has been 

 reviewed by Heald (1898). Protonemata and, from these, new plants arise 

 on the stem of some mosses but rarelv from the leaves unless the latter 



J 



are detached (Gemmell, 1953). Here they grow chiefly from the surface 

 cells of the midrib. 



Morphogenetically, the most significant aspect of moss regeneration is 

 that under appropriate conditions protonemata develop not only from 

 the gametophyte but from sporophyte tissue, both seta and capsule, and 

 thus are diploid. From these diploid gametophytes, tetraploid sporophytes 

 can be produced. This possibility was first discovered by the Marchal 

 brothers (1907-1911) and opened up a wide field for exploration. Its 

 genetic and physiological aspects have been explored by F. von Wettstein 

 (1924) and his students (p. 437). In several cases (as by Springer, 1935, 

 with Phascwn ) sporogonia have been observed to develop directly and 

 apogamously from diploid gametophytes without a sexual process. Still 

 more remarkable, Bauer (1956) observed that diploid protonemata of 

 another moss, Georgia pellucida, under certain conditions form buds 

 which do not develop into leafy gametophytes, as ordinarily happens in 

 such cases, but produce sporogonia directly. Spores in these develop 

 rarely, but when they do they germinate into normal haploid protonemata. 



Regeneration of diploid gametophytes from sporophytes of Anthoceros 

 was accomplished by Rink (1935) through cutting away portions of the 

 sporophyte. Here the diploid thalli are smaller and more irregular in 

 shape than the haploid ones. 



