194 BLECHNOID FERNS [CH. 



sorbifolia (L.) J. Sm. (Christ, Farnkr. p. 40, Fig. 96), and it is a regular 

 feature of the dendroid genus Sadleria. A sinuous or dentate margin else- 

 where may be held to represent a further suppressed branching in simply- 

 pinnate species. Such facts are intelligible enough according to the hypo- 

 thetical origin of the Blechnoids from Onocleoid Ferns, and finally from some 

 Cyatheaceous source : for in these families varying degrees of pinnation are 

 exemplified, down to a simple entire blade, as in Cyathea sinuata (Vol. II, 



Fig- 557)- 



An Acrostichoid development may be traced by easy steps from the 

 Blechnoid coenosorus. It involves in simple cases merely a widening and 

 flattening of the receptacle, while the indusioid margin still covers it, as it is 

 seen to do in B. attemiatuin (Fig. 696), and in B. penna-Diarina. It is but 

 a step from these to Stenochlaena (Fig. 699), in which the extended 

 receptacle is concave, actually following the concavity of the curved wing 

 of the pinna. But at ripeness the curve is everted, giving so clearly an 

 Acrostichoid appearance that the genus was included by Sir W, Hooker 

 under the old comprehensive heading of Aa'osticJmm. In all of these, which 

 have the narrow Lomarioid pinnae, the old marginal flap is maintained. 

 But in Brainea the start was made from a full Eu-Blechnoid state, with a 

 well-developed flange. Here, however, the indusial flap is absent, and 

 developmental study shows that it is not even initiated (Figs. 701, 702). 

 The distal region of the pinnae of Brainea may often reveal the coenosoral 

 origin, with its narrow spread of sporangial development: but lower down 

 the sporangia extend outwards from the commissure so as to cover a con- 

 siderable area of the flange, or even the whole of it. It thus appears that an 

 Acrostichoid state has been initiated more than once, and with differences 

 ot detail in the Blechnoid Ferns. Further, it may be remarked how in Brainea 

 the progression has been theconverse of that in Acrostichunt praestantissimum 

 (Chap. XXXVIII, Figs. 621, 622). The spread of the fertile area in the latter 

 is from the margin inwards, a fact naturally related to its Pteroid origin with 

 marginal coenosorus ; but in Braijiea it extends from within outwards, 

 starting from the superficial coenosorus seated near to the mid-rib. 



A comparative study of the leaves of Ferns at large leads to the recogni- 

 tion of a progressive integration of the blade. Originally derived by branching 

 of narrow parts, these became either webbed or widened: or more commonly 

 both changes may have worked coincidently. The venation, naturally open 

 in the first instance, was liable to undergo anastomosis, forming first a 

 coarse network, and finally in many families a finer reticulation. The final 

 result of such progressions often resulted in types having an entire blade, 

 and usually with a reticulate venation. An attractive morphological problem 

 relating to the origin of these will be to decide what part lateral extension 

 and what part webbing has taken in producing the broad blade as it is. 



