INTRODUCTION TO CRYPTOGAMIC BOTANY. 513 



the indusium.* Nor, again, is the position of the sori always 

 decisive. We have seen how it varies in Poly podium rugu- 

 losum. In Hke manner, as the frond gets broader, the indusia, 

 in other Ferns, retire from the margin, and thus Darea, as 

 exemphfied by Darea 'prolijica in Norfolk Island, becomes 

 Aspleniu'm.-f The venation, though evidently of great 

 importance, from its close connection with the fruit, is too 

 subject to variation, to afford incontestible general characters, 

 however valuable it may be in particular instances. The same 

 venation occurs in groups of very different aflEinities, as in 

 Leptogramma and Asplcnium, Goniofteris and Steno-^ 

 gramma, Nephrodium and 3Ieniscium, to take examples in- 

 dicated by Dr. Hooker. It will not, therefore, do for the dis- 

 tinction of larg-er groups of genera. Dr. Hooker has figured, 

 in his paper on the Vegetation of the Carboniferous Period as 

 compared with that of the present day,J two pinnules of 

 Diplazium, Malaharictim, in which the venation is totally 

 different. In the one the veinlets meet ; in the other they 

 are perfectly distinct. Without fruit, they might be referred to 

 Callipteris and Digrammaria. He has also sliown, on the 

 other hand, that in Cyclopdtis Presliana and C. semicordata 

 the only sure distinction consists in the fact that, while in both 

 the unbranched veins which spring from the costa have a 

 sorus at their tip, the right-hand veinlet of the branched veins, 

 which alternate with the unbranched, in the one case bears a 

 sorus at its tip- -in the other, in its centre. 



574. As regards the different forms of Rhizoma, I doubt very 

 much whether any particular type of formation attends the 

 rejection or retention of the stipes. § This does not seem to me 



* If Polypodium anomalum be really a state of Polystichuon vestiticm, 

 as intimated above, it would be most important to ascertain the early 

 condition of the sori. 



f This is well illustrated by A. bulbifemm, as cultivated at Kew, where 

 different pai'ts of the same frond exhibit extreme ditfereuces of de- 

 velopment. 



+ Memoirs of the Geological Survey of Great Britain, vol. 2, part 2, 

 p. 387. 



§ Those Ferns in which the stipites arc articulated with the rhizoma. 

 as Angiopteris evecia, are called by Mr. J. Smith Eremobr>/a; those in 



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