82 



Vegetable Physiology. 



appear like little dots, and are supposed to be formed!)^}? 

 growth of matter in the inside of the tubes. Their form a A 

 arrangement can readily be observed by placing a thin pi 

 shaving under a magnifier, and even with the naked eye the 6 

 can be distinguished. Their nature and use do not seem as vet 

 to be clearly understood, but they are of considerable interest as 

 having assisted in establishing the true nature of coal. Th 

 formation of this substance, after it was acknowledged to b 

 of vegetable origin, was at first supposed to have been p rc ! 

 duced by the decay of plants of the Fern tribe ,♦ but the ques- 

 tion then was, how to account for the bituminous matter so 

 often combined with it. Nothing of this kind is contained i n 

 any of the Ferns, and therefore it was evident that some resin. 

 ous wood must have afforded it. On examining those por- 

 tions of coal which present the strongest traces of woody 

 structure, they are found to display the dots and all the 

 other peculiarities of the glandular form of woody fibre 

 and hence it may be considered as satisfactorily ascertained 

 Fig. 5. that coal is only the remains of im- 



m®m mense forests of those trees in which 

 ®Pj| alone this species of tissue is found, 

 c The arrangement of the dots being 

 1 | different in different divisions of that 

 tribe of trees, it may even be possible 

 to say to which of these their coal 

 producing ancestors belonged. 

 Another variety of elementary tissue is that called the Spi- 

 ral Vessel. This is a membranous tube, taperingto a point 

 at each end, and having within it a cylindrical fibre, spirally 

 rolled, and capable of being untwisted. This fibre has been 

 already described on page 43. To this vessel has also been 

 given the name of Trachea, or windpipe, from a fancied resem- 

 blance in its structure and function to that organ. It is consid- 

 ered by some to be formed only of fibre spirally twisted, without 

 any membrane, but the better opinion seems to be that which 

 we have Mowed in the above definition. The fibre itself has 

 been also variously represented as cylindrical, flat, or tubular. 

 It is generally formed of a single thread, as in Figure 6, c, but 

 sometimes it is double, or triple, as in d, and in some plants it 



