370 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



their productive swellings in the same way as do the bract whorls to the parts 

 of the strobilus situated above them. In this connection Fagerlind amplified the 

 telome theory in the following way. The stem is a columnar syntelome composed 

 of radial, parallel telomes or telome sympodia. Appendages arranged in whorls 

 or spirals are formed by simultaneous and successive bifurcations respectively. 

 The inner of the resulting telomes build up a new columnar syntelome, while the 

 remainder form an outer collarlike syntelome, which may later split into several 

 parts. The leaf pairs, the bract whorls, and the envelopes of the male and female 

 flowers in Gnetuni are more or less deeply split collarlike synetelomes, while the 

 shoot apex, the productive swelling, the nucellus, and the primordium of the 

 stamen are columnar syntelomes. 



Eames's views of the probable origin and interrelationships of the chlamydo- 

 sperms differ radically from those of Lavier-George (1935). In her opinion the 

 line of division runs between on the one hand Welwitschia and Ephedra, which 

 both show affinity to the gymnosperms in foliar and caulinar features, and on 

 the other, Gnetum, which resembles the dicotyledons in the same respects. The 

 presence in all three genera of a medullosean stem structure in course of re- 

 gression, and of centripetal xylem, indicates that the chlamydosperms derive 

 wholly from the base of the pteridosperm stock. These and other divergences of 

 opinion are reflected in the taxonomic treatment of the chlamydosperms. An 

 older type of arrangement is that of Gaussen (1944-1952), who regards them 

 as forming a single class and order composed of three families. Other investiga- 

 tors have two orders (Pulle, 1937, 1950; Arnold, 1948), while a third group 

 (Skottsberg, 1940; Takhtadjan, 1950; Lawrence, 1951; and Eames, 1952) con- 

 siders each of the families to deserve ordinal rank : Ephedrales, Welwitschiales, 

 and Gnetales. Rothmaler (1951, 1951-1952) even went so far as to raise these 

 to the rank of classes, of which that of the Ephedropsida was referred to the 

 stachyosperms and the remaining two to the chlamydosperms. 



Classification of the Gymnosperms 



Danser (1950) emphasized that the modern conception of systematics as the 

 theory of the classification of life cycles, rather than of objects, necessitates con- 

 sideration of various matters otherwise often regarded as belonging to other 

 sciences, as long as these are useful for classification purposes. The present re- 

 view is worked out from this point of view. Owing mainly to the progress in 

 paleobotany and comparative morphology in the widest sense, the last decades 

 have seen great progress in our knowledge of the actual chronological succession, 

 the evolutionary history, and the interrelationships of the several gymnosperm 

 groups. This has not failed to affect their phylogenetic or vertical classification. 

 The pteridosperms and the cordaites are both groups of very great antiquity. 

 The cycads, ginkgoes, conifers, and probably the taxads, are also fairly old, while 

 the bennettites, Pentoxylaceae, Peltaspermaceae, Corystopermaceae, and Cayto- 

 niaceae are still unknown from paleozoic times. The chlamydosperms have not 

 yet been recognized with certainty except in tertiary strata. Looked upon as a 

 whole, the gymnosperm division of the cormophytes evolved progressively dur- 

 ing the carboniferous, Permian, Triassic, and Jurassic periods, while from the 

 Cretaceous onwards it has been characterized by relative stability. 



Of late years it has been realized to an ever-increasing extent that the clas- 



