214 



THE SPORE-PRODUCING ORGANS 



[CH. 



gradate Fern which is known to retain the median dehiscence. There the 

 annulus is incompletely indurated : it merely opens the distal end of the 

 sporangium, and the spores are shaken out (Fig. 206). But in the Hymeno- 

 phyllaceae, as in other Gradatae, there is oblique lateral dehiscence. The 

 sporangia have a definite orientation, being placed relatively to one another 

 as seen in Fig. 207, so that the distal surface of each annulus has freedom 

 to alter its form independently of the adjoining sporangia. 



Fig. 206. Mature boius of Loxsoina Cuii- 

 ninghamii with the receptacle elongated 

 so as to raise the sporangia above the 

 cup-shaped indusium, where they open 

 by a median dehiscence. (After von 

 Goebel.) (X25.) 



Fig. 207. Diagram ilhistrating the 

 relative position of the sporangia 

 on the receptacle in the Hymeno- 

 phyllaceae. It was constructed 

 from Prantl's section of a mature 

 sporangium of Trichomanes spe- 

 cie siwi. 



The Mixed Sorus 



It has been found as the result of examination of the remainder of the 

 Filicales, which constitute the great majority of living genera and species, 

 that the sorus is of the type called "mixed." Here the sporangia of different 

 ages are aggregated together without any definite sequence. In development 

 younger sporangia are interpolated promiscuously between those already 

 present. There is no definite orientation of the sporangia, and their stalks 

 are usually long. The annulus is almost always vertical, and it is interrupted 

 at the insertion of the stalk. The number of spores per sporangium very 

 rarely exceeds 64, while lower numbers are frequent. There may be very 

 great differences in the number, position, and individuality of the sori, and 

 in the presence or absence of an indusium. It is upon these characters that 

 the classification of the Mixtae has principally been founded. 



