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- 

 angia, and the space directly above it is free 

 for the movement of ejection. The vertical 

 annulus thus satisfies the conditions of the 

 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 there 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. 



FIG. 350. 



Diagram illustrating the relative 

 position of the sporangia on the 

 receptacle in the Hymenophyll- 

 aceae. It was constructed from 

 Fraud's section of a mature spor- 

 angium of Triclwmanes Sfeciositm. 



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 dorsivcntral 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 



