4 1909} SINNOTT—MESARCH STRUCTURE IN LYCOPODIUM 139 
_ of the cycads with the Sigillariae on the basis of a common possession 
of this character. He carefully distinguished the irregularly arranged — 
_ centripetal wood from the radially disposed centrifugal wood of the 
_ sigillarian bundle, and was the first to describe the structure of the 
_ leaf-trace in this genus. The elements in the leaf are also formed in 
_ two directions, those on the outside often being radially arranged. This 
_ structure making the comparison with the cycads still closer, RENAULT 
comes to the conclusion (3, p. 281) that “les Cycadées actuelles qui 
4 possédent dans la structure du cordon foliaire cette analogie si frap- 
9 pante avec certaines plantes houilléres, n’en sont que les représentants 
_ amoindris et en voie de décadence.”’ 
_ _Renautrt has applied the term diploxylic to bundles possessing 
q centrifugal as well as centripetal wood, and has grouped together 
_ Sigillaria and Poroxylon under the head DreLoxyiéEs. Count 
4 Sotms-LAUBACH (4, p. 263), however, called attention to the fact that 
_ where all the centrifugal wood is secondary, the original primary 
_ bundle, which thus has its oldest elements on the very outside, is of a 
_ different type from one where there is primary wood on the outside, 
_ as well as on the inside, of the protoxylem. For the former type of 
_ primary bundle he suggested the name exarch, and for the latter type 
_ Mesarch, terms which have since been in current use with those mean- 
_ ings. RENAULT’s diploxylic implies the presence of secondary wood. 
__ On the characteristic possession of one of these two main types of 
q primary bundle, modern writers have often divided living and fossil 
g vascular cryptogams into two great groups—the Fern series generally, 
3 as typically mesarch, and the Lycopodiales, fossil and living, as 
__ typically exarch. 
___ This general rule, however, does not hold at all closely. Among 
’ living ferns, there are numerous instances of exarch structure, for 
a example in the stem of Trichomanes scandens, of Lygodium dicho- 
_tomum, and of Loxoma Cunninghamii. The same holds true of 
fossil ferns and their allies. Zygopteris shows both internal and ex- 
_ ternal protoxylem, while Megaloxylon is clearly exarch. 
_ Among the typically exarch forms, on the other hand, RENAULT 
(3) has figured mesarch leaf-traces in the Sigillariae. They are also 
found in certain species of Lepidodendron. Numerous cases of such 
_ development also occur in the modern relatives of this group. In 
