THE WOBK OF STEMS 



237 



we must employ another method. For this purpose we 

 force one end of the stem through a hole in the 

 center of a rubber stopper which fits into the air- 

 pump in the manner shown in Fig. 139. 

 If we now place the free end of the stem 

 in water and withdraw the piston, water 

 will be drawn up into the ducts. We must 

 place in the fluid some coloring matter 

 which cannot pass through the cell -wall; 

 for this purpose we may use India ink or 

 cinnabar rubbed up fine in water and 

 filtered through filter paper so as to take 

 out all the coarser particles. We now ex- 

 haust for about half an hour, making as 

 many strokes with the piston as necessary. 

 If none of the coloring matter comes 

 through, shorten the stem by cutting a 

 piece off the end and proceed as before ; 

 repeat this until the stem is short enough 

 to permit the coloring matter to pass 

 through.^ If we now remove the stem and 

 cut it open, we can easily ascertain the 

 length of the ducts, tracheids, etc., and 

 whether there is open communication between them. 

 We should suppose that water would travel faster 

 in the open ducts than in the closed tracheids; the 



iThe liquid may also be forced into the stem by means of \h.e apparatus 

 shown in Fig. 141. 



-k 



%. 



139. Method of 

 injecting a 

 twig by means 

 of the a i r • 

 pump. 



