406 VEGETATION OF THE CARBONIFEROUS PERIOD 



Fructification. — The sporules (fruit or seeds) of the ferns are invari- 

 ably produced on the under-side of the frond, and as that is all but 

 universally the concave surface, and generally presents other inequalities 

 beyond those caused by the organs of reproduction, it becomes firmly 

 attached to the rock during petrifaction, and very rarely exposed to 

 view. Hence, when the frond is coriaceous, it cannot even be deter- 

 mined whether the plant was in fruit or not, except in such rare case 

 as that of Pecopteris lonchitica, to which I have already alluded. When, 

 however, the texture of the frond is membranous, the position of the 

 sori (or clusters of spores) is indicated by a prominence on the upper 

 and exposed surface. This stamping through is usually the only appa- 

 rent sign of fertility in fossil ferns, and is conspicuous in several British 

 species (as Pecopteris obtusifolid) ; also in three American species, indi- 

 cated by Mr. Bunbury,* who has lately drawn attention to the circum- 

 stance. Lindley and Hutton were, I believe, the first to point this out 

 as the probable cause why the fructification of fossil ferns should be very 

 seldom apparent, even supposing fruiting specimens more frequent than 

 they really are.j Goeppert % again finds that in producing artificially 

 fossilized ferns, the under surface invariably remains attached to the 

 matrix, which separates from the upper. 



It is, unfortunately, impracticable to class the most perfect specimens 

 of living ferns without the aid of characters derived from their sori. 

 Much more is this the case with the imperfect ones of the fossil. The 

 former indeed are almost universally rejected from the Herbarium, 

 many species being in every other respect identical. 



In illustration of this fact two wood-cuts have been prepared, each 

 displaying how great a difference may occur in fructification unaccom- 

 panied by any disparity in the outline, form, surface, texture, or vena- 

 tion of the frond. 



The first (Fig. 4) represents a portion of a frond whose outline and 

 venation answer either for a species of Leptogramma (A) or of Asple- 

 nium (B), genera which are precisely alike in those points ; but if the 

 fructification be regarded, it will be seen, that the sori of the portion, 

 marked (B) and enclosed in the black line, are covered with a protecting 

 scale (involucre or indusium) as is more distinctly shown in the magni- 

 fied portion, whilst those of the rest of the figure are naked. Now this 

 absence of an involucre in Leptogramma is not accidental in that fern, 

 but a constant character, separating both it and many allied species 

 (all equally wanting the involucre) from Asplenium, and a similar group 

 which possess that organ. 



* Fossil Flora sub Neuropteris undulata, t. 83. 



t " Quarterly Journal of London Geolog. Soc," vol. ii. p. 85. 



t " Systenia Filicum Fossilium," p. 293. 



