162 FILICES. 



chymatous cell-groups, which sometimes enclose gum-passages, and the 

 differences in the form of the transverse sections of these groups have 

 been employed for distinguishing species. These sclerenchymatous strands 

 which are sometimes much crowded together never, as far as I know, 

 touch immediately on the epidermis, being everywhere separated from 

 it by parenchyma ; but it is true that the tissue-layers outside the 

 sclerenchyma-zone are very rarely preserved. The individual vascular 

 bundle shows a very characteristic structure, being undoubtedly collateral 

 with the xylem towards the centre. This part of the bundle, con- 

 sisting of a variable number of broad scalariform tracheides, shows in all 

 cases, where there is any means of deciding this rather difficult question, 

 the narrow elements of the protoxylem on the side towards the phloem 

 of the bundle, as it is in Cycadeae ; but the xylem next the phloem 

 which is present in Cycadeae (the bois centrifuge of French authors) is 

 entirely wanting. The phloem-portion is almost always destroyed, and in 

 its place is a broad vacant space. But in bundles in which it is preserved, as 

 is the case in a specimen before me from Grand' Croix which belongs to 

 the collection at Strassburg (Fig. 14, B], it is entirely composed of delicate 

 thin-walled elements of which I can say nothing further, as I know them 

 only in the transverse section. The whole bundle is surrounded by a sheath 

 of small-celled parenchyma, the elements of which, where they border on 

 the xylem, have their membranes thickened to a variable extent and are 

 changed into elongated sclerenchyma-cells. Now when Renault endeavours 

 to prove that the remains which we are considering are of the nature of 

 ferns, he relies chiefly on the following circumstances. First of all it is 

 certain that they were branched. This might have been concluded at once 

 from the very great variations in the diameter, which if referred to one 

 rhachis would have led necessarily to the assumption that the leaf was one 

 of prodigious length. Renault l however has figured a superabundance of 

 branched specimens. The leaf must therefore have been of considerable 

 size and repeatedly pinnate, and this is the case, as we know, in no recent 

 Cycad except Bowenia. Then it is in the highest degree remarkable that 

 in the pebbles of Grand' Croix Myeloxylon is almost always associated with 

 detached and well-preserved pinnae of Alethopteris, that some fragments 

 contain nothing else, a fact which is confirmed by the testimony of Renault 

 and Grand' Eury 2 , and which I can vouch for unreservedly from exami- 

 nation of specimens at my disposal. Lastly, Renault has observed the 

 structure characteristic of Myeloxylon in the very prominent median nerve 

 on the under side of pinnae which belong by common consent and un- 

 doubtedly to Alethopteris, and has figured it 3 also, but unfortunately on 

 so small a scale that his drawings alone would not remove every doubt. 



1 Renault (2), t. 28, f. 10. Grand' Eury (1). :! Renault (2), vol. iii, t. 27, f. ia. 



