68 



SPIRAL AND ANNULAR DUCTS. 



ness, even in the smallest organs of the smallest Insect (ANIM. 

 PHYTSIOL. . 320). 



82. There are other kinds of tissue which must be classed 

 under the same head, but which do not serve the same purposes, 

 or possess the same structure. Instead of tubes drawn to a point 

 at each end, we not unfrequently meet with long continuous 

 cylindrical canals, which serve for the conveyance of fluid, in- 

 stead of for the passage of air ; these are called Ducts. There 

 are several varieties of them ; of which those will be first de- 

 scribed, which evidently belong to the type of Vascular tissue. 

 In Ferns (which have no true spiral vessels), we find Ducts, 

 which very closely approach the spiral vessel in character ; 

 having an unbroken coil of spiral fibre throughout their whole 

 extent ; but, besides the important difference that these Ducts, 

 are long continuous tubes, they are further distinguished by the 

 brittleness of the spire, which snaps if we attempt to unrol it. 

 Such ducts are found in many other plants, and 

 may be easily distinguished in the leaf-stalk of the 

 Rhubarb. Another interesting modification of the 

 spiral, is what is termed the scalariform or ladder- 

 like duct ; this also consists of a spiral fibre inclosed 

 in a membrane, but has a space intervening be- 

 tween each turn of the spiral, so that the inner wall 

 of the membranous tube is not entirely covered by 

 it, but seems crossed by a series of bars. There 

 are other ducts, again, in which the spiral is 

 irregular, the coil sometimes terminating in a ring, 

 and then commencing again, with perhaps the inter- 

 vention of two or three rings (Fig. 29, b). Here it 

 would seem, as if the membrane had grown faster 

 than the spire could follow it ; so that the fibre, not 

 being elastic, had been occasionally broken. In 

 other ducts, again, we find no traces of a spiral fibre; 

 but the membranous walls are distended by rings 

 ft, spiral, with a t intervals sometimes tolerably regular. These are 

 vRia" a, annu- called annular or ringed vessels (Fig. 29, a). The 

 lar throughout. transition from the spiral to the annular vessels is 



FIG. 2D. 



