532 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



racters of two or more species, come up spontaneously in culti- 

 vation, it has been conjectured that they are truly hybrids. It 

 is obvious that in the case of such minute bodies as the arche- 

 gonia and antheridia, it is very difficult to make experiments. 

 Hybridization, if it take place at all, must be between the 

 prothallus of one species and that, of another ; and if experi- 

 ments are to be convincing, they must be directed to seedlings 

 soon after the prothallus has begun to expand. 



8. PoLYPODiACE^, Presl. 



Sori globose, naked, seated at the apex of a vein or veinlet 

 on the disc of the frond, rarely medial. Sporangia on long 

 peduncles. Receptacle punctiform. 



602. This is one of the largest of all the tribes, the study of 

 which requires much care, as the indusia in Aspicliacece are so 

 deciduous that a mistake is easily made.* As regards affinity, 

 however, it is quite certain that the presence or absence of an 

 indusium does not deserve all the importance which has been 

 assigned to it. The tribe is distinguished from Grammi- 

 tacece by the globose sori, but this, again, is a character which 

 must not be too strictly limited. The tribe is divisible into 

 three sections, allied, respectively, to Lecano2)teris, Polypo- 

 dium, and Struthiopteris. 



603. The first section is very small. It is characterised by 

 the marginal sori being seated on the hypertrophied teeth. 

 Lecanopteris has anastomosing veinlets. The sori are pro- 

 duced on exserted concave indusiiform marginal lobes, which 

 are reflexed like the shields of Nephroma. In Galymmodon 

 the veins are simple, and a part of the tooth of the frond is 

 turned over the sorus. The few species are tropical. 



604?. The second section abounds in species. In these the 

 margin is even, and is not turned over. The sori are seated at 

 the tips of the veins. In Goniopteris the pinnate costa-like 

 vein bears short pinnules, each of which suj)ports a round 

 apical sorus, so that the sori form double lines between each 

 pair of pinnee. The sporangia, moreover, are hispid, and the 

 spores rough instead of smooth. Stenosemia is sometimes 



* Amhlia, 'Presl.= PhaneropMebia, for example, is an Aspidiad wliicli 

 has lost its involucre. 



