238 THE ANATOMY OF WOODY PLANTS 



THE DOCTRINE OF CONSERVATIVE ORGANS 



This doctrine has received a great impetus from the study of 

 Mesozoic conifers, but was first put forward, naturally enough, in 

 connection with comparisons between the older existing gymno- 

 sperms and their Paleozoic ancestors. The leaf first came into 

 prominence in relation to the hypothesis of conservative organs. 

 It has been known for many years, particularly since the investiga- 

 tions of Mettenius, that the foliar organs of the Cycadales present 

 remarkable features of anatomical structure. Here the fibro- 

 vascular bundles of the leaves are distinguished by the presence of 

 centripetal or cryptogamic wood, a detail of organization conspicu- 

 ously absent in the stem of the genera of the living Cycadales. In 

 the Cycadofilicales of the Paleozoic the bundles of the stem were 

 always characterized by the presence of centripetal xylem and 

 sometimes by concentric organization as well. The Cycado- 

 filicales are by common and competent consent regarded as the 

 ancestral types from which the living cycads have been derived. 

 The clear and universal presence of centripetal wood in the foliar 

 fibrovascular bundles of living genera of the Cycadales is good 

 evidence at once of the relationship of these gymnosperms to the 

 Cycadofilicales and of the validity of the doctrine of conservative 

 organs so far as it applies to the anatomy of leaves. Many other 

 illustrations of the prevalence of this principle might be supplied 

 in foliar organs, but the one described above will serve appropri- 

 ately to elucidate the situation and is particularly apposite in the 

 present connection because it is probably the first case to be cited 

 in evolutionary anatomy. 



The foliar organs are not, however, the only parts of the adult 

 plant which present illustrations of the principle of conservative 

 organs. The stem in that region devoted to the function of repro- 

 duction has also figured strikingly in this connection. To Scott 

 belongs the credit of having drawn attention to the fact that the 

 peduncle or base of the cone in certain Cycadales furnishes a clear 

 example of the persistence of an ancestral structure in the 

 reproductive axis which has quite disappeared in the ordinary 

 vegetative stem. In Stangeria, Zamia, and other genera of the 

 Cycadales he noted that the fibrovascular strands of the axes 



