275 
Evans .— Vegetative Reproduction in Metzgeria. 
average gemma is represented in Fig. 2, E. It consists of a flat strap¬ 
shaped thallus about eight cells broad and narrowing somewhat towards 
the base. At the rounded apex the single apical cell, still active in cutting 
off segments, can be distinguished. Such a gemma is about o*6 mm. long 
and 0-25 mm. wide. Along the margin are scattered hairs, each about 
twice as long as one of the ordinary cells of the gemma and almost always 
hooked at the apex. The hairs are often strictly marginal, but in some 
cases are slightly displaced to one surface. The gemma shows no signs 
of dorsiventrality; even when the hairs are displaced they appear on 
either surface indiscriminately. There are also no signs of cell-differentia¬ 
tion except for the marginal hairs. 
Fig. 2. M.uttcigera. A-d. Gemmae in various stages of development. X300. e. A gemma 
at time of separation, x 80. 
The germination of a gemma is a comparatively simple process, 
although its course is liable to be affected by external changes. The 
apical cell of the gemma begins to function at once as the apical cell of the 
young thallus. In fact it is quite impossible to discern where the gemma 
merges into the young plant to which it gives rise. As development 
proceeds the young thallus gradually becomes broader and the new hairs 
formed tend to be straight instead of hooked and to act as rhizoids. Before 
long the median cells of the thallus become arranged in two longitudinal 
rows and begin to divide by walls parallel to the surface, thus giving rise 
to a rudimentary costa. With the appearance of the costa the evidences 
of dorsiventrality become more distinct. The marginal hairs, for example, 
when displaced from the margin, always appear on the lower, or postical, 
surface; the costa itself begins to give off hairs, most of which function as 
