42 



TJIE GEOLOGICAL HISTORY OF PLANTS. 



supplies of graphite, of petroleum, and of illuminating 

 gas, useful to man at the present day. We may write 

 of them and draw their forms with the carbon which 

 they themselves supplied. 



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. ^ NOTE TO CHAPTER XL 



Examination of Prototaxites {Nematopliyton), by Prof. Pen- 

 hallow, OF McGiLL University. 



Prof. Penhallow, having kindly consented to re-examine my 

 specimens, has furnished me with elaborate notes of his facts and 

 conclusions, of which the following is a summary, but which it is 

 hoped will be published in full : 



" 1. Concentric Layers. — The inner face of each of these is com- 

 posed of relatively large tubes, having diameters from 13'6 to 34"6 

 micro-millimetres. The outer face has tubes ranging from 13'8 to 

 27"6 mm. The average diameter in the lower surface approaches to 34, 

 that in the outer to 13-8. There is, however, no abrupt termination 

 to the surface of the layers, though in some specimens they separate 

 easily, with shining surfaces. 



"3. Minute Structure. — In longitudinal sections the principal 

 part of the structure consists of longitudinal tubes of indeterminate 

 length, and round in cross-section. They are approximately parallel, 

 but in some cases may be seen to bend sinuously, and are not in 

 direct contact. Finer myceloid tubes, 5'33 mm. in diameter, trav- 

 erse the structure in all directions, and are believed to branch off 

 from the larger tubes. In a small specimen supposed to be a branch 

 or small stem, and in which the vertical tubes are somewhat distant 

 from one another, this horizontal system is very largely developed; 

 but is less manifest in the older stems. The tubes themselves show 

 no structure. The ray-like openings in the substance of the tissue 

 are evidently original parts of the structure, but not of the nature of 

 medullary rays. They are radiating spaces running outward in an 

 interrupted manner or so tortuously that they appear to be inter- 

 rupted in their course from the centre towards the surface. They 

 show tubes turnmg into them, branching into them, and approxi- 

 mately horizontal, but tortuous. On the external surface of some 

 specimens these radial spaces are represented by minute pits irregu- 



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