62 BASIS FOR SYSTEMATIC TREATMENT OF FERNS [CH. 



fication, priority being given to the propagative organs. Though there may 

 be less reason for this difference of valuation in Ferns than in some other 

 cases, the interest in their propagative organs certainly predominates. These 

 appear in Dryopteris on the under surface of certain adult leaves, and are 

 grouped in "sori" disposed in a single linear series on either side of the mid- 

 rib of the pinna or pinnule, being seated on the secondary veins (Fig. 2). 

 But this position of the sorus is not constant in Ferns. It is true that in 

 many of them it is on the lower surface of the sporophyll, but in others, and 

 especially in those which are of archaic type, it may be borne close to, or 

 actually on the margin of the leaf The extent, individuality, and relation 

 of the sori to the venation and to the margin of the leaf are very variable. 

 It will then be found that the position and relations of the sorus are features 

 useful for comparison. 



VII. The indusium, which is so marked a feature in Dryopteris, is not con- 

 stant for Ferns at large (Fig. 14). There is reason to believe that all primitive 

 sori were without it: also that indusial protections of several distinct sorts 

 have originated in the course of evolution along distinct phyletic lines. Thus 

 indusial protections will afford another useful criterion for comparison. 



VIII. The structure of the sporangium above described for Dryopteris 

 (Figs. 14, 1 7) represents the type common for relatively advanced Ferns, though 

 not for those of the most primitive, nor yet for those of the most advanced 

 types. All the details of the sporangium are liable to vary from one type 

 to another. It will be shown on the one hand that the sporangium is more 

 massive in relatively primitive Ferns, but more attenuated in those which 

 are relatively advanced. It will be found that the oi'igin of the sporangium, 

 the details of its segmentation, the length and thickness of the stalk, the form 

 of the head, and the position and struct2ire of the annulus all offer poiiits of 

 comparative value. 



IX. Closely related with these features is also the numerical spore-output. 

 In archaic Ferns, where the sporangia are large, the numbers are as a rule 

 high compared with those of more modern Ferns such as Dryopteris. 

 Lastly, the form of the spores and the character of their walls also present 

 features that have their value in comparison. The rugged surface and dark 

 colour of the spores of Dryopteris are examples showing how pronounced 

 such characters may sometimes be. 



X. Each spore on germination may produce aprothallus. In Dryopteris 

 it is flattened, with no distinction of axis and appendages, and with un- 

 differentiated structure. But this shape is not constant in the Ferns: some- 

 times it is filamentous, sometimes massive, and in the latter case it may even 

 develop underground, and contain little or no chlorophyll, being nourished 

 saprophytically. Thus the form and physiology of the prothallus may provide 



