THE WORK OF STEMS 235 



air- bubbles, which would hinder the rise of the water, 

 are trapped in these pits and so rendered harmless. 

 The tracheids and wood parenchyma are also pitted so 

 as to allow water and dissolved substances to pass 

 readily from one to another. The great elongation of 

 the tracheids especially fits them for conducting mate- 

 rials lengthwise through the stem, while their dovetailed 

 arrangement increases their surface-contact and so pro- 

 motes the diffusion of liquid while at the same time it 

 gives them great mechanical strength. Let us now in- 

 vestigate in the same manner the wood of the Pine 

 (Fig. 138). Here we find only tracheids. The only 

 thing which resembles a duct is the resin duct {rd) , 

 which does not convey water, but contains resin, scat- 

 tered in irregular drops. The tracheids are large and 

 are provided with very large bordered pits. Make a 

 careful comparison of the wood of the Oak and the 

 Pine. 



A very good way to study wood -cells is by isolating 

 them. Place some rather thick longitudinal sections in 

 a dish, add a few crystals of potassium chlorate, and 

 pour in enough nitric acid to cover them. Set the mix- 

 ture outside the window until fumes cease to come off, 

 then wash the sections in water and tease out with 

 needles, if necessary. The wood-cells will then have 

 separated from each other and may be studied in their 

 isolated condition ; in this way we may learn the length 

 of the tracheids ; to ascertain the length of the ducts 



