THE FLORA OF THE COAL FORMATION. 4G9 



plants. Tlicy must have been perennial ; and the resemblance of 

 their trunks to those of Cycads, together with their hard and narrow 

 leaves, would lead us to infer that their growth must have been very 

 slow. A similar inference may be drawn from the evidences of very 

 slow and regular expansion presented by the lower parts of their 

 stems. On the other hand, the distance, of a foot or more, which 

 often intervenes between the transverse rows of scars, marking pos- 

 sibly annual fructification, would indicate a more rapid rate of 

 growth. Further, it may be inferred, from the structure of their 

 roots and of their thick inner bark, that these, as in Cycads, were 

 receptacles for great quantities of starch, and that the lives of these 

 plants presented alternations of starch-accumulation and of expen- 

 diture of this in the production of leaves, wood, and abundant inflo- 

 rescence. Tliey would thus, perhaps for several years, grow very 

 slowly, and then put forth a great mass of fructification, after which 

 perhaps many of the individuals Avould die, or again remain for a 

 long time in an inactive state. This view would, I think, very well 

 harmonize with the structure of these plants, and also with the mode 

 of their entombment in the coal. 



From the manner of the association of Calamites with erect Sigil- 

 ktrice, I infer that the former were, of all the plants of the Coal for- 

 mation, those of most rapid dissemination and growth. They appear 

 to have first taken possession of emerging banks of sand and mud, to 

 have promoted the accumulation of sediment on inundated areas, and 

 to have protected the exposed margins of the forests of Sigillarice. 



In applying any conclusions as to the rate of growth of Carbon- 

 iferous plants to the accumulation of coal, we must take into account 

 the probable rate of decay of vegetable iiiatter. >Vhen we consider 

 the probable wetness of the soils on which the plants which produced 

 the coal grew, the density of the forests, and the possible excess of 

 carbonic acid in the atmosphere of these swamps, we must be prepared 

 to admit that, notwithstanding the warmth and humidity, the condi- 

 tions must have been favourable to the preservation of vegetable 

 matter. Still the hollow cylinders of bark, the little fragments of 

 decayed wood in the form of mineral charcoal, and the detached 

 vascular bundles of ferns, testify to an enormous amount of decay, 

 and show that, however great the accumulation of coal, it represents 

 only a fraction of the vegetable matter which was actually produced. 

 It has been estimated that it would require eight feet of compact 

 vegetable matter to produce one foot of coal ; but if we reckon 

 the whole vegetable matter actually produced in the process, I should 

 suppose that five times that amount would be fiir below the truth, 



