C PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [vol. lxxix, 



some wood from rocks stated to be of Middle Devonian age in 

 Ohio, which is undoubtedly of the Gyrnnospermous type ; but 

 Dr. Scott informed me that Dr. H. M. Ami assigns the Ohio beds 

 to an Upper Devonian horizon. In 1895 Count H. Solms-Laubach 

 described a small piece of petrified stem from the Lenne Sbales of 

 Grafrath, of Middle Devonian age, showing radially disposed wood- 

 elements and strips of medullary-ray tissue, which, he suggested, 

 might perhaps belong, either to a plant allied to the Lyginopteris 

 famibj, or to a member of the Calamarieae. The specimen is clearly 

 a fragment of a stem capable of secondary growth in thickness- 

 but we cannot determine its precise affinity. 



We have seen that the older Devonian floras included certain 

 plants which were trees in stature, and there are indications that 

 some of the terrestrial species had solved the problem of secondary 

 increase in girth. One of the outstanding features in the archi- 

 tectural plan of Palaeozoic vegetation is the widespread occurrence 

 of the arborescent habit. On several lines of evolution the method 

 of adding to the diameter of stems and branches by means of an 

 ever-young cambium-cylinder was adopted at an early period in the 

 history of the vegetable kingdom. Ability to increase the 

 number of branches and the area of the foliage, which is an 

 attribute only of plants that can also meet the consequent rise in 

 the demand for water and manufactured food by supplying 

 additional means of transport, is a conspicuous feature of several 

 different families in the Palaeozoic floras. While some of these 

 plants belonged to lines of evolution which cannot be directly 

 connected with any surviving forms, others, which possessed this 

 capacity of unlimited expansion, belong to classes the modern 

 representatives of which are herbaceous in habit, and a limit is set 

 at an early age to further increase of the tissues concerned with 

 water- and food-conduction. 



Until some well-defined type of Gymnospermous plant is dis- 

 covered, I prefer to think of the older Devonian floras as chiefly 

 composed of relatively simple representatives of the Lycopod and 

 Filicinean phyla, plants which were adapted to conditions not very 

 far removed from an aqueous habitat. Some may have grown in 

 water, while others flourished in swamps where the composition of 

 the soil rendered essential economy in transpiration, a circumstance 

 which was reflected in the general absence of thin and well- 

 developed leaves. In the succeeding Upper Devonian period a 



