igo BOTANY AND PHARMACOGNOSY. 



assimilable materials, comprise the sieve and conducting paren- 

 chyma, although there are other elements^ which sometimes assist 

 these two groups of cells in the work of conduction (Fig. 104, 

 T,S). 



The tracheae or ducts are formed by the disintegration and 

 removal of the transverse walls between certain superimposed 

 cells, forming an elongated cell or tube, which occasionally retains 

 some of the transverse walls (Fig. 102, A, B). The longitudinal 

 walls are relatively thin and consist of lignocellulose, giving more 

 or less pronounced reactions with phloroglucin or aniline sulphate. 



The thickenings of the longitudinal walls of ducts are quite 

 characteristic, several forms being distinguished: Those having 



Fig. 105. I, cross section of a bast fiber of Begonia as viewed by means of the micro- 

 polariscope. 2, polariscopic view of a sphere-crystal of inulin in" Helianthus tuberosus. 

 The crystals are produced by allowing the roots to remain in alcohol for some months. — 

 After Dippel. 



the thickenings in the form of horizontal or oblique rings are 

 known as annular ducts; those having the thickenings in the 

 form of spirals, which usually run from right to left, are known 

 as SPIRAL DUCTS ; those having the thickenings in the form of a 

 reticulation are known as retiCulated ducts, and those with 

 horizontal, disconnected thickenings which occur in parallel lines, 

 resembling a flight of steps, are known as scalariform ducts. 

 In those ducts in which but few of the transverse walls are 

 obliterated, the walls are marked by both simple and bordered 

 pores, which latter are described under tracheids. Ducts contain 

 water, water-vapor and air; in some cases they contain sugar, 

 tannin, mucilage or resin. 



