DISTRIBUTION AND RELATIONSHIPS OF THE CYCADEOIDS 1 67 
The general subject of later tracheidal structures as bearing on the 
^ origin of modern stem types is too broad to take up in any detail. But a 
few observations may be made. There are a number of facts accessible 
especially in the great work of Solereder, going to show that no one process 
can account for the origin of vessels. Possibly they have at times arisen 
by direct evolution very anciently in unknown and upland Arctic floras, 
and later secondarily from both pitted and scalariform tracheids. Perhaps, 
as Jeffrey contends, scalariform wood can even result mainly from pit fusion. 
But it will not do to call only pitted wood ancient, and the scalariform types 
the more modern. The remarkable Carboniferous Lyginopteris has the 
large-celled, many-pitted wood, but either the contemporaneous relatives 
or the ancestral types of the quasi-ferns may and must have had the scalari- 
form wood. The peduncular wood of cycads and cvcadeoids alike is 
scalariform. 
This much may be safely said : In the pines a high degree of ray speciali- 
zation is geologically recent. Also in the dicotyls the course of ray change 
must be coordinated with recent development of storage tissues. Such 
structures may be subtracted in order to glimpse or to hypothesize ante- 
cedent dicotyl wood in the Jurassic. If then the pit wood of Drimys and 
Trochodendron with its suppressed growth rings, and the scalariform wood 
of Trochodendron and Tetracentron, have any signficance at all, the in- 
escapable conclusion is that both cycadeoid and cycad wood is old and 
near the type basal to many modern forms. It is indeed delusive to read 
this history in terms of Gnetum alone. 
Dicotyls and Gnetaleans 
Analogies, rather than relationships, between the cycadeoids and the 
dicotyls and gnetaleans may be quite conveniently discussed as under a 
single topic. For here the gymnosperm border line is crossed, and all the 
near relationships cease. The older view of dicotyl derivation through 
early conifers and gnetaleans is now opposed to the newer view of near 
cycadeoid derivation, in part coupled with suggestions of an extreme 
parallelism amongst both gymnosperms and angiosperms. But following 
various recent and thorough studies of the gnetaleans, the idea that they 
indicate the real angiosperm precursors is even accentuated by some. 
Lignier and Tison say the gnetaleans are merely aberrant angiosperms which 
retain early gymnosperm features and lead toward the amentifers. And 
Hallier even suggests they are reduced dicotyls like Loranthus and the 
Myxodendraceae. 
E. W. Berry is the most recent to follow and emphasize the Lignier and 
Tison view, so far as relates to descent. He says that the primitiveness of 
. . . Evolution has not consisted in the production of new types of protoplasmic structure 
or cellular organization, but in the development of constantly greater specialization and 
division of labor between larger and larger groups of cells." 
