CORDAITEAE. 107 



account of Renault's statements has not given them correctly, inasmuch as 

 in the explanation of the figures he terms the outer strand of the primary 

 wood the ' phloem,' and must therefore suppose that the elements in ques- 

 tion are fibre-cells of the bast. But I have satisfied myself by examination 

 of Renault's J original preparations that his account of the matter is perfectly 

 correct. With regard to the rest of the leaf-tissue, the epidermis on both 

 sides of the leaf is formed of cells bounded by straight lines and approxi- 

 mately rectangular ; on the under side are rows of stomata lying parallel to 

 the surface of the leaf, but there are none on the upper. In some forms the 

 cells of the upper side are developed into pointed papillose processes, as in 

 Cordaites tenuistriatus 2 . The parenchyma of the leaf may be uniform 3 , but 

 it is usually differentiated into three layers, a loose irregular tissue on the 

 under side, a tissue more or less distinctly resembling palisade-tissue on the 

 upper side, and an intermediate 'transfusion-tissue' occupying the space 

 between the bundles ; this middle tissue is lacunose in character with broad 

 intercellular spaces, and is formed of cells lying across the leaf. There are 

 also the mechanical elements in a variety of forms. Where these are slightly 

 developed and the entire parenchyma of the leaf is also uniform (C. crassus 4 ), 

 they appear as single sub-epidermal fibres answering to the vascular 

 bundles on both sides of the leaf, but not reaching as far as the bundle- 

 sheath. Besides these there is on the under side between every two bundles 

 a narrow much- projecting strand of fibres, which has nothing to correspond 

 to it on the upper side. In other forms the fibre-strands unite from above 

 and below with the sheath of the vascular bundles, forming the well-known 

 customary longitudinal trabeculae. Lastly, in C. angulostriatus (Fig. 7, i) 

 these are connected together on both sides by a continuous sub-epidermal 

 layer of fibres, which swells up into one or more rib-like projections between 

 the bundles, and this perhaps explains* the appearance in the impressions 

 of the fine intermediate nerves peculiar to some Cordaitae. There must 

 certainly be a connection between the greater or lesser degree of prominence 

 of the nervation in the impressions of the leaves, and the fact that in some 

 forms the thin leaf-surface over the nerves appears thickened into a knot 

 in the transverse section, while in others nothing of the kind can be ob- 

 served. That with all this the structure of the leaves is essentially resistant 

 has been already remarked by Schenk 5 ; we see by this case the great 

 antiquity of these anatomical phenomena of adaptation to external con- 

 ditions ; we shall find them reappearing on different occasions as we 

 proceed with our subject, and we may conclude on the whole that those 

 external conditions, which we see determine this adaptation at the present 

 day, prevailed as early as the period of the Coal-measures. 



1 Renault (1), t. 16. 2 Renault (1), t. 16, f. 2. 3 Renault (1), t. 16, f. 7. ' Renault 



(1), t. 16, f. 7. 5 Zittel (1). 



