200 
F. Cavers. 
these there is an interesting form, M. saccata, which bears curious 
water-sacs. Each sac is formed by the margin of the wing becoming 
in certain places rolled over towards the underside and then by 
active superficial growth converted into an ovoid structure with its 
opening directed towards the lower side of the mid-rib; from the 
convex outer surface of the sac rhizoids frequently grow out. 
A third line of differentiation of the thallus is seen in Umbra- 
ciiluni. In the allied genus Podomitrium , e.g. P. ( HymenopJiytum) 
phyllanthus (Fig. 30), ordinary apical branching occurs, but numerous 
branches arise from the mid-rib on the underside of the thallus. 
The basal portion of each branch is nearly cylindrical, but the distal 
portion develops on either side a broad wing, which frequently 
narrows again and becomes obsolete at the end of the branch ; the 
rhizoids spring chiefly from the stalk-like basal portion. In Umbra- 
culum, e.g., U . ( Hymeiiophytuin) Jlabellatum, however, the creeping, 
wingless rhizome-like basal portion, to which the rhizoids are 
confined, gives off erect branches which are at first cylindrical, but 
towards the top become winged and then undergo repeated 
dichotomy to form a fan-like assimilating branch-system (Fig. 31). 
Goebel (26) has shown that when this highly differentiated plant is 
grown in moist air and feeble light the fan-like shoots may produce 
rhizoids and their growing-points may grow out to form wingless 
runners or rhizomes. The axis or mid-rib in Podomitrium and 
Uuibraculuiu is traversed by a strand of narrow elongated cells with 
thick walls which bear slit-like pits. The axial bundle of a ventral 
branch is not continuous with that of the parent-axis, but is 
separated from it by a zone of thin-walled cells, hence these ventral 
branches readily become detached. 
In all three genera the sexual organs are borne on special short 
branches. In Aneura these arise from the margin of the thallus 
and are merely lateral branches whose growth in length is arrested 
by the formation of the antheridia or archegonia, but in the other 
genera these organs are confined to short ventral branches arising 
from the sides of the mid-rib. The antheridia are either sunk in 
cavities in the tissue of the male branch, or are covered by the 
inrolled margins of the branch, or protected in both these ways. In 
Podomitrium , the male branch has a cylindrical stalk and an 
expanded distal antheridium-bearing portion (Fig. 30, II.); in Metz- 
geria, its margins are rolled up so as to form a sac ; in Umbraculum, 
the male branch is reduced to a sessile cushion-like outgrowth of 
the mid-rib (Fig. 31). There is thus a gradual reduction in the 
