30 



DAVALLIOID FERNS 



[CH. 



F'g- 598- Development of the sorus of Odontosoria retusa. 

 A=a young stage where the margin is flattened, B = a. later stage 

 with the indusial flaps advanced, C=a still later stage with 

 the oldest sporangium central on the receptacle, while other 

 sporangia and hairs originate laterally, indicating a gradate 

 succession, ?< = upper indusium, /= lower. A section along the 

 line a, b shows that the first sporangia are constantly median. 

 {A, B, X 250; C, X 100.) 



the sorus is actually marginal, and is at first gradate: but a mixed condition 

 appears later, though it is never far advanced (Fig. 598, C). The sporangia 

 accord structurally with this condition of the sorus, for they have a slightly 

 oblique annulus consisting of a continuous sequence of cells that extends 

 uninterruptedly past the insertion of the stalk (Fig. 599). These facts show 

 that the sorus of O. retusa is not far removed from the type of the marginal 

 Dicksonioids, though it is liable to lateral fusion into coenosori and shows 

 a transition to the mixed state. 



I'ig- 599- Base of a sporangium of Odontosoria 

 retusa showing the oblique annulus with its 

 induration stopping short of the insertion of 

 the stalk. (XT25.) 



