201 
moment of development. The possibility that there might be two distinct forms is 
quite precluded by the fact that transitional forms are everywhere met with, and 
by their accordance in all other respects. Specimens are sometimes found which 
are partly cushion-shaped, partly inflated and bearing sori in both parts of the 
frond. It cannot be doubted that the inflated specimens arise under certain con- 
ditions which are usually only realised on the margins of the Zostera-leaves. It 
= 
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IT 
Fig. 120. 
Rhododermis Georgi, vertical sections of the inflated form. A, sterile plant. B, plant with sorus with unripe spor- 
angia. C, part of sorus with ripe and emptied sporangia, the latter becmoing filled with new sporangia from the 
stalk-cells. A, B 200:1; C 350:1. 
might be supposed that the causa efficiens must be sought in the movements of 
the water which are much greater at the margins than on the faces of the undu- 
lating leaves. It must be left to experimental studies to decide this and to deter- 
mine whether it is the friction against the water, or the better conditions for nutrition 
caused by the stronger movements which induce the increased growth of the inner 
cells of the frond. 
The sorus occupies the central part of the frond. Usually there is only one, 
but sometimes two (or more?) are met with, which perhaps fuse together. As men- 
tioned above, the sorus may sometimes be sunk in a groove. The paraphyses are 
D.K.D. Vidensk. Selsk. Skr, 7. Række., naturvidensk. og mathem. Afd. VII, 2. 26 
