The Carboniferous Arborescent Lepidodendra. 39 



Lepidodendron Selaginoides. 



L. brevijolium. 

 L. Wunschianum. 



L. Harcourtii. 



L. Juliginosum. 



L. mundmn. 



L. parvulum. 



But it may be well to explain how I have ob- 

 tained some of the statistics recorded in the following 

 pages. My measurements are chiefly made in milli- 

 metres. So far as regards the mean diameters of 

 the medulla?, the Primary and Secondary Xylem cylinders, 

 and the thickness of the walls of these cylinders, 

 reliance may be placed upon their strict accuracy. 

 More difficulty was experienced in counting, with approxi- 

 mate accuracy, the number of the tracheids visible in 

 transverse sections of those cylinders. The difficulties arose 

 from the fact that, in young growths especially, the peri- 

 pheral tracheids are so small and their cell walls so thin 

 that they were sometimes almost undistinguishable from 

 the cells of the surrounding cortex. But I am certain that 

 I have made a sufficiently close approximation to their true 

 numbers to shew the enormous differences between what we 

 find in the younger twigs and the larger and older growths. 

 In cases where those tracheids were numbered by hundreds, 

 and even by thousands, a different method had to be 

 employed. I measured with the utmost care the circum- 

 ference of the section of the cylinder, and the mean'thickness 

 of the cylinder wall. I then counted the number of the 

 tracheids enclosed in each of six squares of the. twentieth 

 of an inch in diameter, and I placed these data in the hands 

 of my friend Professor Hudson, of King's College, who 

 kindly superintended the calculations based upon them, and 

 placed in my hands the results recorded in the following 

 pages. 



