SIGILLARIEAE. 257 



minology, had he called for instance the leaf-strand of Cycadeae mesarch, 

 and that of Isoetes exarch in opposition to the normal form, we might 

 perhaps have been spared this misconception. It is evident from what has 

 been said above that to employ the current phraseology and speak of a 

 diploxylous bundle has no meaning, except when we are dealing with 

 a certainly collateral trace-strand. For this reason I have been able to 

 employ the word without hesitation when speaking of Cordaiteae, and it 

 may also be used here for the leaf-trace which passes through the cortex of 

 Sigillariae. But we must think twice before we go beyond this, and say with 

 van Tieghem 1 : 'this double wood is also found in the leaves of Sigillariae, 

 but here we are going back to the rule, for the stem too has a double wood, 

 the interior primary and centripetal, the exterior secondary and centrifugal.' 

 It is true that this is also going back to Mettenius 2 , but he only knew 

 Sigillaria from Brongniart's description. The objection is that in this mode 

 of conceiving the matter a sufficiently sharp distinction is not drawn between 

 primary wood and secondary additional growth. If the latter supervenes, 

 it certainly unites immediately with the outer portion of the xylem. But 

 whether in a given stem of the species in question having secondary growth 

 the original structure of the primary bundle followed the type of Isoetes or 

 that of Cycadeae cannot be determined, unless we are in a position to study 

 the history of the development of the specimen. Van Tieghem appears to 

 underrate the importance of this distinction, or else he could not possibly say 3 : 

 ' The leaf of Sigillariae derives its ligneous elements at once from the angles 

 of the primary wood and from the inner margin of the superposed secondary 

 wood' ; for this leaves it uncertain whether the latter elements connect with 

 the outer strand of the primary wood or with that of the secondary, or 

 partly with the one and partly with the other. Moreover the above 

 sentence determines a priori the extremely doubtful question of the 

 anatomy of Sigillariae, for it assumes the presence of a medullary cylinder 

 surrounded by isolated leaf-trace-strands. And yet we have seen how 

 strongly the stages in the differentiation of the primary structure point 

 to the conception, that the central parenchyma with the surrounding ring 

 of bundles is a single central strand partly developed in the form of 

 parenchyma, after the analogy of Lepidodendron Harcourtii. The strand 

 would in that case be concentric in structure and could not come into 

 consideration here ; the diploxylous leaf- traces would be attached to its 

 periphery. Van Tieghem it is true would not consider this an important 

 distinction, for he considers every central strand of the kind to be formed by 

 the coalescence of collateral leaf-traces, as was observed above on p. 253- 

 To discuss in detail the mutual relations of the two distinct types which 



1 van Tieghem, Traite de hot. 1884, p. 812. 2 van Tieghem, Traite de bot. 1884, p. 582. 



3 van Tieghem, Traite de bot. 1884, p. 1307. 



