Evans .— Vegetative Reproduction in Metzgeria. 301 
such as those described for M. uncigera. In all these cases the 
inhibitory influence of the apical region is either destroyed completely 
or else diminished to such an extent that the thallus cells are able to 
overcome it and develop gemmae. In other words, some or all of the 
food available for growth is distributed among the cells capable of 
forming gemmae. 
A similar connexion between the limitation of growth in the gemmi- 
parous shoot and the production of gemmae may also be observed in many 
of the leafy Hepaticae. It is especially marked in species with unicellular 
or bicellular gemmae, such as Odontoschisma denudatum , Sphenolobus 
HellerianiiS) and Calypogeia Trickomanis. In these species the gemmi- 
parous shoots curve upwards until they are erect instead of prostrate, their 
leaves diminish more and more in size, and the shoot as a whole becomes 
more and more nearly radial. The gemmae are at first limited to the leaf- 
margins, but eventually, when growth in length has come to an end, the 
whole apex of the shoot becomes a mass of gemmae. In species with 
discoid gemmae the connexion is rarely so marked, but in Cyclolejeunea 
convexistipa the gemmae are frequently borne on short branches with 
reduced and specialized leaves. The modifications exhibited by these 
gemmiparous shoots with limited growth are comparable with those seen 
in Metzgeria fruticulosa. 
Just why the normal activities of the apical region are lessened in these 
cases and finally brought to an end is by no means clear. In some instances 
the result is perhaps due to poor nutrition, bringing about an enfeeblement 
of the whole plant, but this cannot be the effective cause in all cases, 
because a limitation of growth often takes place in plants which are robust. 
Under these circumstances the plant is probably able to control the apical 
growth, perhaps by diverting the currents of food to other regions. 
Apparently something of the same sort takes place in such species as 
M. dichotoma , where the growth of the gemmiparous branch continues for 
an indefinite period. The power of the plant to regulate the distribution 
of the nutritive materials, and thus to weaken or destroy the inhibitory 
influence exerted by the apical region upon the cells capable of producing 
gemmae, may be considered a specific characteristic. 
In certain species of Metzgeria , notably in M. fruticidosa, the strength 
of the inhibitory influence seems to increase as the plant grows more 
differentiated. In other words, a plant which is passing through an em¬ 
bryonic or juvenile stage is more likely to produce gemmae, and to have its 
growth brought to an end, than a plant with well-differentiated wings and 
costa. The same tendency is also strongly marked in M. furcata, especially 
in the variety ulvula , and may be observed in a less degree in several of the 
other species. As the thallus of M. fureata becomes more and more 
differentiated, the apical growth gains so strong a supremacy that its 
