Evans .— Vegetative Reproduction in Met2geria . 295 
The first gemmae formed are marginal in position. Sometimes they 
appear on branches which are just beginning to show signs of specialization, 
and under these circumstances the branch may recover, as it were, and 
continue its growth normally. In most cases, however, the formation of 
gemmae does not begin until the modification of the branch is well under 
way. They become more numerous as the growth of the branch continues, 
and before long arise not only from the alar cells, but from the cortical 
cells as well. When the thallus ceases to develop wings, any surface cell 
seems to have the power of producing a gemma. When a gemma is to be 
formed, whatever its position, the projecting cell becomes at once the 
mother-cell of the gemma (Fig. 16, d). At the time of separation the 
gemmae are small circular to oblong plates of cells about 0*12 mm. wide 
(Fig. 16, E and f). They taper more or less distinctly towards the basal 
end and show a rounded apex with a single apical cell. Except for the 
scattered rudiments of marginal hairs they are quite undifferentiated. They 
also show no distinct signs of dorsiventrality, although they are sometimes 
slightly concave. It will be seen that these gemmae are among the simplest 
that have been described. 
The germination of a gemma, when following a normal course, shows 
few peculiar features. The marginal rudiments of hairs become elongated 
and assume the function of rhizoids. The apical region then grows out 
into a thallus which gradually becomes broader. Although consisting at 
first of a single layer of cells it soon produces marginal and surface hairs. 
The latter are restricted to the surface turned towards the substratum, 
which at the same time becomes concave, so that the dorsi-ventrality 
of the young plant is established at an early stage. The marginal hairs 
usually occur singly, but occasionally appear in pairs as differentiation 
proceeds. They may be truly marginal or slightly displaced to the 
postical surface, a condition which tends to become less and less frequent. 
With the differentiation of the costa, which takes place in the usual way, 
the postical hairs become more numerous along its course and less 
numerous on the postical surface of the wings. Forking occasionally 
takes place before the costa has made its appearance. 
The germination, however, often follows an aberrant course. This is 
sometimes due to reversion, by means of which the thallus becomes 
narrower or less differentiated as it advances in length. But it is much 
more frequently due to the development of new gemmae and to the 
modifications associated with their production. The new gemmae may 
arise on the original gemma itself or on the young thallus in any stage 
of its differentiation. When the production of gemmae is abundant the 
growth of the gemmiparous axis soon comes to an end, and this may 
take place while the young plant still consists of a single layer of cells. 
If the costa is already differentiated before the new gemmae appear the 
