148 PLANT ORGANS OR PARTS OF PLANTS. 



nothing but tracheids. The woody cylinder in the root 

 of Ipecac consists entirely of tracheids. 



(3) The libriform or wood fibres make the largest mass 

 of the wood. They are generally packed about the 

 vessels, supporting them. They are elongated elements 

 with rounded or sharpened ends, thickened lignified 

 walls, and provided with left oblique, slit pores, in all 

 respects similar to the bast fibres, from which they can be 

 distinguished with difficulty. 



(4) The parenchyma may be of two kinds : (a) Woody ; 

 (b) simple. 



(a) Woody parenchyma is, as a rule, found in small 

 quantities in the wood, immediately surrounding the 

 vessels or arranged in radial rows between the vessels. 



Parenchyma may be difficult to distinguish from 

 libriform in long section. Shortness is diagnostic. The 

 simple pores are also diagnostic. Medullary ray cells 

 closely resemble woody parenchyma in cross-section in 

 many woods. 



(b) Simple parenchyma may be irregularly scattered in 

 small amounts among the various elements of the wood. 

 Its thin-walled isodiametric cells with simple pores are 

 sufficiently distinctive. Rarely does the simple paren- 

 chyma become sclerotic. 



Transition forms among the elements are to be ex- 

 pected, and to these much of the difficulty in determina- 

 tion of powders is due. 



Excretory reservoirs (oils, mucilage, tannin) are rare 

 in the official woods (Sassafras). 



Crystals are sometimes found in the woody paren- 

 chyma of the medullary rays and sometimes in the simple 

 parenchyma. 



Secretory reservoirs, both schizogenous and lysigenous, 

 are met with. In Pinus, Eucalyptus, etc., the oil, 

 resins, and balsams are contained in schizogenous reser- 

 voirs, as also in the root of Arnica, Angelica, etc. 



