298 Evans .— Vegetative Reproduction in Metzgeria. 
wards develops, so that the gemmae here, as in Metzgeria , show certain 
of the characteristics of embryonic plants. 
Thalloid gemmae are now known in nine genera of the leafy Junger- 
manniales, and have been carefully studied in Radiria , Cololejeunea , Metzge- 
riopsis , and Cyclolejeunea} They usually arise on the leaves themselves, but 
are occasionally borne on the margins of other gemmae or on young plants 
which are still in the thalloid condition. In all the known cases the gemmae 
are but one cell thick throughout. Each one takes its origin in a single cell, 
just as in Metzgeria , and in all cases that have been described the cell first 
projects and then divides into two cells by a wall perpendicular to its long 
axis. The outer cell thus formed represents the mother-cell of the future 
gemma, while the inner cell may be regarded as a poorly defined stalk. 
The conditions are essentially the same as in M. imcigera and M. crassipilis. 
The separation of the gemma is schizolytic, and is brought about by a 
splitting of the walls between its basal cells and the stalk-cell. Although 
the gemmae in all of these leafy genera are very similar to those described 
for Metzgeria , the resemblance is especially close in the tropical genus 
Cyclolejeunea , of which about six species are at present known. In this 
genus there are two types of gemmae, one with a single apical cell (which 
does not always persist until maturity) and the other with two. The first 
type is apparently the more frequent, the second being known in the single 
species C. angrdistipa. Gemmae of the first type are essentially like those 
of Metzgeria crassipilis , M. Imearis , &c., while those of the second type are 
duplicated in AT. dichotoma. The resemblance is made still more striking 
by the presence of hairs, formed by the elongation of small cells. These 
hairs sometimes function as rhizoids, but often fail to do so, the true rhizoids 
being independently produced. 
According to Lindberg (’ 75 ) the marked similarity between the various 
thalloid gemmae indicates a true genetic relationship, and in his classification 
of the Hepaticae he separates Metzgeria from the other thallose Junger- 
manniales and includes it in his group Anomogamae, to which he refers 
also Frullania , Lejeunea (in its broad sense), Radrda , Porella , and Pleurozia. 
His views on the subject, however, have not met with much favour, most 
writers preferring to retain Metzgeria in the group to which the other 
thallose genera are referred. It should be remembered in this connexion 
that the species producing thalloid gemmae all grow in places where it is 
difficult for young plants to gain a foothold, such, for example, as rocks, 
the bark of trees, or the surface of living leaves. The thalloid form is 
especially effective in enabling the gemmae (as well as the young plants 
developing from spores) to hold themselves in place. It seems perfectly 
reasonable, therefore, to associate the similarity in the gemmae with the 
similarity in environment. 
1 Cf. Goebel (’ 87 ), Schiffner (’ 93 ), and Evans (’ 04 ). 
