LESSONS WITH PLANTS 



307 



ness of the stem takes place. Wood forms on the inside and 

 new layers of bark on the outside. x\s the wood witliin gets 

 larger, the old bark coat becomes too small and cracks into 

 fissures or peels off in layers. This is seen in all old trees. 

 In each species there is a characteristic mode of fissuring 

 of tire bark by wliich 

 alone we are often en- 

 abled to distinguish the 

 tree. The plum and the 

 birch peel off around 

 the stem. The soft 

 maple bark splits off in 

 long strips up and down 

 the stem. In other 

 trees the bark wears 

 away less conspicuously, 

 though just as truly. 



In sections of stems 

 or twigs of more than 

 one year's growth there 

 is seen a concentric ar- 

 rangement of the wood. Fig. 112. The White Elm. 

 This is called the annual rings, by which, as a rule, we can 

 tell the age of the stem, one ring being formed each year. In 

 the spring the cells of the new wood are thinner walled and 

 larger than the cells formed in the later growth of the sum- 

 mer.' Hence each spring's growth appears hghter and more 

 porous than the growth formed in the preceding fall. 



Bring in pieces of firewood or go to a sawed-off tree and 

 determine tlie age by counting the rings. 



Modified Stems for Propagation: After the typical 



