146 PLANT ORGANS OR PARTS OF PLANTS. 



central and usually lacerate hilums. The grains range 

 from 10 to 35 microns in size. In the artificial heat of 

 drying many of the starch grains are apt to be altered, 

 especially those near the epidermis of the tuber. The 

 resin masses are prominent, they are usually spherical, 

 light to dark-brown, and minutely irregular or with 

 larger drops of oil. The crystals vary; they are of the 

 spheroidal, spiny type and average from 20 to 40 microns. 

 The parenchyma is very abundant; it is lax, thin- 

 walled, and usually non-pored. The tracheids are few, 

 and have simple slit-like markings. The vessels are few 

 but large, the fragments sometimes showing the markedly 

 regular border pores. The cells of the epidermis are 

 generally elongated, hexagonal in shape, and are dark- 

 brown to black. 



WOODS. 



The woody tissues form, as a rule, the greater part of 

 the mechanical tissues of the plant, and in some plants by 

 far the greatest amount of material in the stem is made up 

 of woody fibres. 



There are but few official woods, as we know from the 

 physiology of the plant that the woody fibres serve mainly 

 to support and conduct air and water only; in the bark 

 the more active life processes of the plant are constantly 

 taking place, and hence more active chemical compounds 

 are found in the bark. This lack of active chemical 

 compounds accounts for the few official woods. 



Starch and sugar are sometimes found lying free in 

 the woods, but rarely nitrogenous substances nor alka- 

 loids. Heart woods frequently contain tannin and 

 resin, coloring-matter forming part of cells or the cell 

 walls. 



The proper contents of the woods are reserve-stuff 

 materials, aromatic bodies, ethereal oils (Juniper, Sassa- 

 fras), resin (Guaiac), tannin and coloring-matters (Haema- 



