SECT. I. ORGANOGRAPHY AND GLOSSOLOGY. 



21 



tain intermediate forms, which establish the fact of a 

 most intimate connection, and even appear to indicate 

 a common origin. The two kinds of vessels alluded 

 to, are the spiral vessels and the ducts. 



(23.) Spiral Vessels. These are generally termed 

 " tracheae/' from the resemblance which they bear to 

 the windpipe, and more especially to the air-cells of 

 insects, which are called by the same name. They 

 consist of a membranous tube, on whose inner surface 

 a cylindrical fibre is spirally coiled {fig. 12. a) ; and 

 the whole so completely 

 united, that if the vessel 

 be ruptured, and the thread 

 uncoiled, no trace of the 

 membrane is to be seen, 

 excepting towards the co- 

 nical extremity of the ves- 

 sel, where the coils of the 

 fibre are wider apart. In 

 some tracheae, indeed, the 

 successive coils are not in 

 contact with each other, 

 and then the investing 

 membrane is sufficiently 

 apparent. Sometimes the 

 fibre branches into two 

 threads (6), and each continues its course in separate 

 but contiguous coils; and instances may be found, 

 where the contiguous coils of separate threads range (c) 

 between this number and twenty-two ! The diameters 

 of these vessels vary from the ^^ up to the ^ 

 of an inch. They may be detected with the greatest 

 facility upon tearing asunder the leaves of many plants, 

 and especially are very visible in some species of Ama- 

 ryllis, when they form a set of parallel fibres, nearly as 

 conspicuous as the threads in a spider's web, and are 

 strong enough to support the weight of a considerable 

 portion of the leaf. By carefully unravelling them, 

 they may sometimes be extended to eighteen inches in 

 c 3 



