140 



PLANT ANATOMY 



Kr.,J 



«m. vin^T^?>Ti"1}^4 ^^^*^^o^ o^ Glycyrrhiza glabra showing, g. vessel with pitted and border pore 

 iiiai kings in the walls, hp, wood parenchyma, i. wood fibres, libriform. (Tschirch). 



pores may occur. 



tcristic shape iu the Ferns. Their length 

 varies. In pines it is 4 mm. The ends 

 of the Tracheids are pointed, and sheath 

 into one another. The walls are gener- 

 ally thickened and the lumen of the cell 

 is generally extensive. The thickening 

 of the walls is due to hgnitication which 

 takes place irregularly, sometimes form- 

 ing spiral markings and sometimes thick- 

 ening the whole wall save in certain 

 spots where border 

 (Fig. 52.) 



Simple slit-like pores, such as are 

 found in Libriform tissue, are generally 

 wanting in tracheids. This fact is of 

 practical diagnostic importance. 



In some plants the water-conducting 

 tissues consist entirely of tracheids. This 

 is the case in the pines, in the root of 

 Ipecac, and the nerve endings of leaves. 

 The sides of tracheids which border on 

 ducts or libriform tissue are apt to con- 

 form in their appearance to those con- 

 tiguous tissues, therefore sharp distinc- 

 tions between them cannot always be 

 made. 



Dncts. Under this head are included 

 cells which resemble tracheids in many 

 respects, but differ in the fact that their 

 end walls are absorbed, thus forming 

 long open tubes which may run the en- 

 tire length of the plant without any 

 cross walls. Although Ducts were orig* 

 inally closed cells, by the absorption of 

 their cross walls they cease to be cells 

 in one sense of the word. 



The modification which Ducts have 

 undergone, through thickening of the cell 

 wall by Hgnitication, are so marked that 

 si)ecial names have been given to cer- 

 tain characteristic forms. The principal 

 types are Annular, Spiral, Reticulated 

 and Pitted Ducts, with intermediate 

 forms between. (Fig. 53.) 



An Annular duct (r.) is one in which 



the thickening of the cell wall takes 



place in the form of a ring, making a 



series of bands around the inside of the 



tubular vessel. This form occurs in the 



stem of Conium, and Radix Podophyl- 

 lum. 



Spiral ducts have the lignification dis- 



