TBI<J WORK OF STEMS 



281 



in the stem, fix a short piece of stem air-tight in a 



cork and fit this in a lamp-chimney (as shown in 



Fig. 139). Pour in water, and 



exhaust. Or force air through 



the stem by an arrangement 



similar to that shown in Fig. 



141, in which case we may ob- 

 serve with a hand - lens just 



where the air issues. 



It sometimes happens that in 



coating trees with tar to protect 



them from insects and fungi, 



they suffer severely from too 



liberal an application, which 



deprives the stem of air. 



To determine the effect of ex- 

 cluding air from the stem, we may take 

 Willow cuttings and suspend them in 

 moist air (see Fig. 157), by attaching 

 them to a cork fitted into a lamp- 

 chimney which stands in water. As the 

 experiment is to last for several weeks, 

 the constant level apparatus described 

 on page 27 (Fig. 27) may be used. 

 Let some of the cuttings be completely 

 smeared over with vaseline, so as to en- 



157. Willow twig tirely exclude air. Place all the cuttings 



suspended ma.' o 



saturated atmos- ,^^^^^ favorable couditions of growth. 



156. Method of investigating the 

 entrance of air into the stem 

 by way of the stomata and 

 lenticels. 



