558 



NATURE 



[October 8, 1903 



tion of the plant-world. In speaking of the plants of the 

 Devonian and Lower Carboniferous or Culm phase, it is not 

 assumed that the specimens entombed in the snow-covered 

 cliffs of Bear Island were actually contemporaneous with 

 those found in rocks of the same geological period in the 

 Southern hemisphere. The Bear Island rocks are, in the 

 language which Huxley taught us to use, homotaxial with 

 certain Devonian plant-bearing strata in other parts of the 

 world ; they occupy the same relative position in the geo- 

 logical series. 



Homotaxy by no means implies contemporaneity ; indeed, 

 the late Edward Forbes maintained that similarity of organic 

 contents of distant formations should be accepted as prima 

 facie evidence of a difference in age. 



What do we know as to the composition of the floras that 

 flourished in the later stages of the Devonian and in the latter 

 part of the Carboniferous era? The following list, which is 

 by no means exhaustive, represents some of the more impor- 

 tant generic types which may be very briefly described : — 



1. Equisetales. 



Archaeocalamites. 



2. Sphenophyllales. 



Sphenophyllum. 

 Cheirostrobus. 

 IPseudobornia ?^ 



3. Lycopodiales. 



Lepidodendron. 

 Bothrodendron. 



4. FiLlCALES. 



Archaeopteris. 

 Adiantites. 



Rhodea. 



Cardiopteris. 



Todeopsis. 



Cephalotheca. 



Rhacopteris. 



5. Cycadofilices. 



Calamopitys. 



Heterangium. 



Lyginodendron. 



6. Gymnosperm-^. 



(Cordaitales). 



Cordaites. 

 Pitys. 



In Archaeocalamites we have the oldest example of an un- 

 doubted Equisetaceous genus. The structure of its com- 

 paratively thick and woody stem is practically identical with 

 that of our common British type of Calamites, one of the 

 most abundant of the Coal period genera, while the strobilus 

 differed in no essential feature from that of a modern Horse- 

 tail. The genus Cheirostrobus, founded in 1897 by Dr. 

 D. H. Scott on a single specimen of a petrified cone dis- 

 covered in the rich volcanic beds of Lower Carboniferous age 

 at Pettycur on the shores of the Firth of Forth, affords a 

 striking illustration of a Palreozoic plant exhibiting a struc- 

 ture far more complex than that of any known type among 

 existing Vascular Cryptogams. As Scott clearly shows in 

 his admirable memoir, Cheirostrobus is a synthetic or com- 

 pound genus, one of the numerous extinct types brought to 

 light by the anatomical investigation of fossil plants, from 

 which we have learnt more about the inter-relations of exist- 

 ing classes than we could ever hope to discover from the 

 examination of recent species. 



In this Scotch cone, about 3-5 cm. in diameter, we recog- 

 nise Equisetaceous and Lycopodinous characters combined 

 with morphological features typical of the extinct genus 

 Sphenophyllum. Some specimens of vegetative stems de- 

 scribed by Nathorst from Bear Island under the name 

 Pseudobornia — characterised by their whorled leaves with 

 fimbriate blades borne on nodal regions separated by long 

 internodes — may, as Scott has suggested, represent the 

 branches of the tree of which Cheirostrobus was the cone. 

 Both Devonian and Culm rocks have furnished many ex- 

 amples of Lycopodinous plants. The genus Bothrodendron, 

 closely allied in habit to Lepidodendron, has been recorded 

 from Bear Island, Ireland, and Australia, and the cuticles of 

 a Lower Carboniferous species form the greater portion of 

 the so-called paper-coal of Tula in Russia. Lepidodendron 

 itself had already attained to the size of a forest tree, with 

 anatomical features precisely similar to those of the succeed- 

 ing Coal period species. 



Our knowledge of the ferns is not very extensive. The 

 genus Archjeopteris from Ireland, Belgium, Bear Island, and 

 North America has always been regarded as a fern, but we 

 must admit the impossibility of accurately determining its 

 systematic position until we possess a fuller knowledge of 

 the reproductive organs and of its anatomical structure. 



NO. 1771, VOL. 68] 



Similarly the genera Rhacopteris, Adiantites, and Rhodea, 

 with other characteristic members of the Lower Carbon- 

 iferous vegetation, may be provisionally retained among the 

 oldest known ferns. The genus Cardiopteris — a plant with 

 large oblong or orbicular pinnules borne in two rows on a 

 stout rachis — is known only in a sterile condition, and it is 

 quite as likely that its reproductive organs may have been of 

 the Gymnospermous as of the Filicinean type. 



Renault has described under the name Todeopsis some 

 petrified sporangia which appear to be practically identical 

 with those of existing Osniundaceee, and a new Devonian 

 genus Cephalotheca has been instituted by Nathorst for fer- 

 tile specimens of a strange type of plant which he refers to 

 the Marattiacea;. Of much greater importance than the 

 sterile fern-like fronds, which cannot be assigned with con- 

 fidence to a definite position, are the petrified remains of 

 stems and leaves of such plants as Heterangium, Lyginoden- 

 dron, Calamopitys, and others which demonstrate the ex- 

 istence of a class of synthetic genera combining Filicinean 

 and Cycadean characters. These plants are of exceptional 

 interest as showing beyond doubt that Ferns and Cycads 

 trace their descent from a common ancestry. Some of the 

 supposed ferns from Lower Carboniferous rocks are known 

 to have been fronds borne on stems with the structure of 

 cycads, and we have good reason for believing that some at 

 least of the gymnospermous seeds of Palaeozoic age are those 

 of plants of which the outward form was more fern-like than 

 cycadean. The announcement made a few months ago by 

 Prof. Oliver and Dr. Scott that they had obtained good evi- 

 dence as to the connection of the gymnospermous seed known 

 a-5 Lagenostoma with the genus Lyginodendron is one of the 

 most important contributions to botarry published in recent 

 years ; if, as I firmly believe, the evidence adduced is con- 

 vincing, it gives satisfactory confirmation to suspicions that 

 previous discoveries led us to entertain. The fact demon- 

 strated is this : the genus Lyginodendron, a plant known to 

 have existed during the greater part of the Carboniferous 

 epoch, possessed a stem of which the primary structure was 

 almost identical with that which characterises some recent 

 species of Osmundaceae, while the secondary wood produced 

 by the activity of a cambium is hardly distinguishable from 

 the corresponding tissue in the stem of a recent cycad. The 

 fronds were those of a fern, both in the anatomy of their 

 vascular tissue and in their external form ; so far, therefore, 

 as the vegetative characters are concerned, we have a com- 

 bination of ferns and cycads. We still lack complete know- 

 ledge of the nature of the reproductive organs, but it seems 

 clear that Lyginodendron bore seeds constructed on the Gym- 

 nospermous plan, but characterised by an architectural com- 

 plexity far beyond that represented in the seeds of any 

 modern Conifer or Cycad. 



In such genera of Gymnosperms as Cordaites, Pitys, and 

 others, we have examples of forest trees possessing wood 

 almost identical with that of existing species of Araucaria, 

 but distinguished by certain peculiarities which point to a 

 relationship with members of the Cycadofilices, and suggest 

 that Conifers as well as Cycads may have sprung from a 

 filicinean stock. 



These waifs and strays from the vegetation of an era in- 

 credibly remote, when strange amphibians were lords of the 

 animal world, afford, as Newberry expresses it, " fascinating 

 glimpses of the head of the column of terrestrial vegetation 

 that has marched across the earth's stage during the dif- 

 ferent geological ages." 



Two facts stand out prominently as the result of a general 

 survey of what are practically the oldest records of plant-life. 

 One is the abundance of types which cannot be accommo- 

 dated in our existing classification founded solely on living 

 plants. 



The Devonian and Lower Carboniferous plants lead us 

 away from the present along converging lines of evolution to 

 a remote stage in the history of life ; they bring us face to 

 face with proofs of common origins, which enable us to re- 

 cognise community of descent in existing groups between 

 which a direct alliance is either dimly suggested or absolutely 

 unsuspected if we confine our investigations to modern forms. 

 We recognise, moreover, in such a plant as Archaeocalamites 

 an ancestor from which we may derive in a direct line the 

 existing members of the Equisetales. In other types, by far 

 the greater number, we see striking examples of Nature's 

 many failures, which, after reaching an extraordinary com- 



