12 OUR NATIVE FERNS AND THEIR ALLIES. 



are ovate, surrounded at the apex by a complete annulus, and 

 open by a longitudinal slit (Fig. 4). In the Osmundace^e or 

 flowering ferns the sporangia are larger, globose, and naked, with 

 the mere trace of a transverse annulus, and open longitudinally. 

 The various methods of fructification can be best understood 

 by describing the peculiarities of the various genera in regular 

 succession and noting the variations occurring in the sections 

 or sub-genera. By this means we will arrive at a better under- 

 standing of the principles of fern classification as discussed in a 

 future chapter. As the subject of venation is closely connected 

 with that of fructification, it will be treated in the same connec- 

 tion. 



29. Acrostichum. — In this genus the sporangia are spread 

 in a stratum over the under surface of the upper pinnae in our 

 solitary species, but in some exotics they cover portions of the 

 upper surface as well. There is no indusium. 



30. Polypodium (Fig. i). — This genus contains the larg- 

 est number of existing ferns, and though all the species agree 

 in the roundish naked sori, the venation is widely different in 

 the various sections, which are chiefly formed on the character 

 of the veins. Four of the five sections are represented in our 

 nine species. 



In § EuPOLVPODiUM the veins are free, yet are occasionally 

 known to unite,* thus indicating a tendency to vary toward the 

 next section. The sori are generally found at the end of a free 

 veinlet. 



In § GONIOPHLEBIUM the veins unite near the margin, form- 

 ing large areolae, each containing a single free veinlet which 

 bears the sorus at its end. A tendency to variation is seen in 

 P. polypodioides, whose veins are free, as well as in P. Califor- 

 nicuDi in which they are often partly free. 



In § Phlebodium the veins form ample areolae in a row 

 next the midvein, and frequently in one or more secondary rows, 

 each bearing a single sorus at the junction of two or more vein- 

 lets. A large number, however, bear the sori at the end of a 

 single veinlet. From the fertile areolae to the margin the veins 

 anastomose more copiously. 



* Catalogue of the Davenport Herbarium, p. 8. 



