10 BRITISH EOCENE FLORA. 



Phanerogam has become in its turn developed from the Gymnosperm.^ To the present, 

 however, the remains of former forests which have been found scattered, with wide intervals, 

 through stratified rocks, have scarcely rendered it possible to trace the lines through 

 which the evolution of any of the existing groups of plants has taken place. Still the 

 known antiquity of completely differentiated Acrogens and Gyranosperms is such that we 

 can only suppose the common ancestors of the spore-producing Lycopod and seed- 

 producing Gymnosperm to be locked up in the form of graphite and other carbons in the 

 estimated 70,000 feet of sedimentary strata comprised between the Laurentian and 

 Devonian formations. Yet in a general way the development of the vegetable kingdom 

 may be divided, as long since pointed out by Adolphe Brongniart, into three great periods — 

 that of the Acrogens, of the Gymnosperms, and of the Angiosperms. In the oldest sedi- 

 mentary rocks there are, as is well known, few recognisable plant remains ; but in the 

 Silurians, impressions ascribed to Cryptogams, and wood of a very anomalous and primitive 

 character, are occasionally met with. In some of the Devonian rocks wood with the 

 typical coniferous structure abruptly appears ; but in other localities of the same age 

 wood has been found with a structure tending to show that the Coniferae were less 

 completely differentiated. Angiosperms appear much later. 



In 1831 Witham pointed out that the higher and more complex organisation of 

 Coniferae existed in the Carboniferous strata of Edinburgh and Newcastle. This was at 

 one time believed to possess an Araucarian structure, and more recently to be nearer the 

 Pines ; but there is a complete absence of true cones in the Carboniferous ; and the fruits, 

 such as Trigonocarpus and Nosgyeratlda, have been thought by Hooker and Saporta to be 

 allied to Ginkgo. If the frequent reference of the earliest known Conifers to such types 

 as Araucaria and Ginkgo should be confirmed by future discoveries, it would be a matter 

 of considerable interest to find that the forms, which at the present day still closely 

 approach in their foliage the Club-mosses and Ferns, are the older types. Ginkgo 

 presents in its mode of seeding, according to Saporta and Marion, a nearer approach to 

 Cryptogams than to any existing Phanerogam. 



From the base of the Permian the Gymnosperms are more defined. Voltzia, 

 ^ The publication of that important •work 'L'Evolution des Phan^rogames ' has been delayed owing 

 to the desire of its authors to vindicate the determinations of Algae in ' L'Evolution des Cryptogames,' 

 and a comprehensive memoir on " Les Algues Fossiles " has appeared. In the meantime the Marquis de 

 Saporta has favoured me with a note (29th November, 1882), in which he informs me that the parts 

 treating of the Vrogymnosperms, Gymnosperms, and Metagymnosperms or GnetacecB are completed : " Nous 

 nous basons pour tracer le mouvement evolutif des Phanerogames sur I'etude aualytique de la structure 

 des tiges ou plan caulinaire et aussi sur celle des parties intimes et essentielles de I'ovule, de maniere 

 \ faire voir comment le passage organique a pu s'operer sans effort et graduellement des types crypto- 

 gamiques de I'ordre le plus eleve vers les Phanerogames proprement dites. II y a la une serie de 

 nuances dues au deplacement ou a la distribution relatives des diverses zones de tissus composant la 

 tige, et aussi une serie de modifications conduisant de la macrospore a I'ovule et du prothalle inclus 

 vers le sac embryonnaire et les corpuscles, qui permettent de saisir le sens et la direction du mouvement 

 evolutif d'oil sont a la fin sorties les vraies Phanerogames, d'abord uniquement gymnospermiques." 



