646 GENERAL COMPARISON OF THE FILICALES 
expansion before the sudden jerk. But some space is necessary for the 
straightening of the annulus before its quick recovery: the free space 
available is found obliquely upwards, towards the apex of the receptacle. 
In that direction the annulus is free to straighten itself out, dehiscence 
taking place at one side, near to the attachment of the stalk: it can then 
execute without obstacle the sudden jerk by 
which the spores are scattered (Fig. 350). 
The dehiscence by a lateral transverse slit, 
worked by a vertical annulus, is the prevalent 
type of the Mixtae. The ripe sporangia usually 
have long stalks, and show no regularity of 
orientation. The vertical annulus with trans- 
verse dehiscence is a mechanical arrangement 
which makes use of the free space immediately 
above the surface of the sorus for the straighten- 
ing of the annulus prior to the jerk of ejection: 
a bias to either side is quite unnecessary, and 
may be a positive disadvantage. As the young 
sporangium grows in a mixed sorus, for instance 
of a Polypodium, its stalk elongates, carrying the 
head vertically upwards from the receptacle: it 
is thus lifted above the crowd of younger spor- 
ee ie ee relative  angia, and the space directly above it is free 
receptacle in the Hymenophyll- for the movement of ejection. The vertical 
aceae. It was constructed from 
Prantl’s section of a mature spor- annulus thus satisfies the conditions of the 
angium of Trichomanes Speciosunt. 
Fic. 350. 
mixed sorus. 
It has already been shown how the different types of annulus charac- 
teristic of the three types of sorus pass phyletically one into another; and 
it is now seen that therg are biological reasons for this in the exigencies 
of the mechanism of dehiscence; in fact, the details of the method of 
dehiscence in the more specialised Ferns appear to have been determined 
by the mutual relations of the sporangia. 
ANATOMY. 
It has been shown from comparison of the external characters of Ferns 
that they were probably in the first instance strobiloid types, with a radial 
construction of the shoot, and that their present condition was probably 
attained by advance from a smaller-leaved state to megaphylly: with this 
went frequent assumption of a dorsiventral development. This matter must 
now be considered from the point of view of comparative anatomy, and 
especially of the vascular system. If the Fern-shoot were primitively 
strobiloid and radial, we should expect the fact to be reflected in the 
vascular construction of those Ferns which are held on comparative or on 
palaeontological evidence to be primitive; and also that it would be 
