50 THE ELEMENTARY STRUCTURE OF PLANTS. 



only by the form of the markings ; and these vary so greatly in 

 the same tissue, and even in the very same duct (Fig. 44), that it 

 would be an endless and useless task to describe all their varieties. 

 A continuous dotted duct with oblong spots is nearly the same as 

 the large ducts with rather larger markings, disposed so as to form 

 a series of regular bands, which abound in Ferns (Fig. 42). When 

 the markings are a little longer, and the walls are rendered pris- 

 matic by mutual pressure (as in parenchyma) we have the Scala- 

 riform Ducts of Ferns (Fig. 41), so named because the lines (or 

 slits as they become in old tissue) form transverse bars resembling 

 the rounds of a ladder. In many cases, it is uncertain whether the 

 lines or narrow bands are spots thinner than the rest of the wall, 

 as they certainly are in dotted ducts, and probably in the scalari- 

 form vessel ; or whether they are places where the secondary 

 deposit is thickened. Probably there are Reticulated Ducts (those 

 where the lines branch and run together here and there, forming a 

 network) of both sorts ; certainly of the latter ; for we occasion- 

 ally meet with such markings (as in the middle of Fig. 44) on a 

 part of the walls of true 



60. Annular and Spiral Ducts (Trachea). The nature of their 

 markings is explained in Paragr. 46. They are elongated cells 

 (or ducts formed by the confluence of several cells), with their 

 delicate membranous walls strengthened by the deposition of fibres 

 within. Sometimes the fibre is deposited in unbroken rings (as in 

 the middle of Fig. 43, and in Fig. 48, d), which forms the Annular 

 Duct. More commonly it is deposited as a continuous spiral coil, 

 producing the Spiral Duct or Spiral Vessel (Fig. 45-47) ; which 

 is taken as the typical or pattern form of vascular tissue, because 

 of its universal occurrence in Flowering Plants, and because of the 

 general tendency of such definite secondary deposits to assume a 

 spiral form. That these markings are thickened, and not thinner 

 lines, is well shown in those remarkable cells from Cacti, already 

 described (Fig. 29, 30), in which the fibre thickens into a band, with 

 its edge, as it were, applied to the wall : also in those cells which 

 have a loose spiral fibre generated within (48). Moreover, in 

 what is called the true Spiral Vessel (Fig. 45-47), the fibre is so 

 strong and tough, in comparison with the delicate cell-wall on 

 which it is deposited, that it may be torn out and uncoiled when 

 the vessel is pulled asunder, the membrane being destroyed in the 

 operation. This is seen by breaking almost any young shoot or 



