THE WOBK OF STEMS 



279 



of them (Fig. 135, I) shows that it consists of a mass 



of loose cells which by their growth have ruptured 



the epidermis. This process always begins under one 



of the stomata, and a greatly 



enlarged opening is formed by 



tearing. The figure also shows 



that a layer of cork has been 



formed under the epidermis ; it 



is this which gives the brown 



color to the stem. The cork is 



a necessary protection to the 



stem, for, as it grows older, the 



epidermis falls off. When the 



stem gets still older the hark 



is formed; this consists of a 



mixture of cork, strengthening cells and the dead cells 



of the bast. The growth of the stem causes the bark 



to split and fissure, thus allowing air to enter. 



The lenticels are very clearly shown by sealing 

 both ends of a short piece of stem with sealing-wax, 

 placing it under water in the air-pump and exhaust- 

 ing. In some cases merely placing the stem in hot 

 water suffices. 



In order to see how readily air may travel down 

 from the leaf into the stem, we may fix a leaf air- 

 tight in a rubber stopper (as described on page 205), 

 and fix this in a tul)e to which we fit a piston (con- 

 structed as described on page 188) ; pour in a little 



154. Method of investigating lenti- 

 cels (air is pumped into the sub- 

 merged stem by means of the 

 bicycle pump). 



