STEM. 13 



4 or 5 years growth) we observe the following circum- 

 stances : 



Fig. 9. 



70. In the centre of the stem is seen the pith; 

 around that a thin layer, the medullary sheath; next 

 come the more or less woody layers, traversed by lines 

 radiating from the centre of the stem to its circumfe- 

 rence called medullary rays ; and, lastly, the bark ; the 

 whole structure is harder at the centre than at the 

 circumference. 



71. Now all stems exhibiting such structure are 

 termed Exogenous or dicotyledonous, and examples are 

 to be found in such plants as the Oak or the Elm. 



72. The medulla, or pith, is a continuous column of 

 cellular tissue, extending from where the stem and root 

 join each other to the extremities of the twigs and 

 branches. When young it is of a greenish white co- 

 lour, and its cellules are filled with watery matter. At 

 about the third or fourth year, the pith arrives at the 

 condition in which it will remain during the whole 

 period of its existence ; and as a general rule we may 



c 



