66 ORIGIN OF COAL. SPIRAL VESSELS. 



must have been formed from resinous woods, even though the 

 remains of such were very scanty and imperfect. Now on apply- 

 ing the microscope to transparent sections of such fragments of 

 coal, as most distinctly exhibit the fibrous structure, it is seen that 

 they present the character which has been .described, as peculiar to 

 the resinous woods, the glandular form of woody fibre, as it is 

 technically termed ; and hence it is established beyond doubt, 

 that the immense masses of coal which now contribute so much, 

 in every way, to the comfort and the social improvement of the 

 human race, are but the remains of vast forests, probably the 

 growth of many successive centuries, which chiefly, if not en- 

 tirely, consisted of trees of the Pine and Fir kind. It is even 

 possible, by the peculiarities of the arrangement of the dots, to 

 say which of the subdivisions of that tribe at present existing, 

 those primeval trees most nearly resembled. The reason why 

 the remains of Ferns have been so well preserved, whilst those of 

 other plants and trees should have lost all definite structure, has 

 been already explained (. 26). 



80. The third kind of primary tissue, is that denominated 

 the Vascular. Its typical form is the Spiral 

 Vessel, which is only found in Flowering-Plants ; 

 but modifications of this exist in the lower tribes. 

 The essential character of the Vascular tissue is 

 the possession of a spiral fibre, coiling more or 

 less regularly within its thin membranous tubes, 

 from one extremity to the other. The true 

 spiral vessel much . resembles the woody fibre in 

 form, being a long narrow tube drawn to a 

 point at both ends. But the membranous wall 

 is much thinner, and is easily torn asunder. 

 The spiral filament is usually single ; it is 

 a b sometimes, however, double, or even triple : 



FIG. 27- PORTIONS / 



OF SPIRAL VKSSELS: and in the very large spiral vessels 01 the Chinese 

 ^rS S Pitcher-Plant (Nepenthes, . 242) it is quad- 

 partly drawn out ; ru pl e . These tubes in their perfect state contain 



6, from Nepenthes, . * . 



with the quadruple air only, which finds its way from one to an- 

 fibre - other, like fluid through the woody tubes. They 



