THE STEM. 27 



tant part in their development. In the old part it is dry, 

 shrivelled, and seems incapable of taking any part in the 

 process of vegetation, and this appears evident from the 

 fact that trees often continue to nourish after the center, 

 containing the pith, has begun to decay. 



/Structure of the Stein. The stem is composed of 

 woody fibre and cellular tissue, a substance similar to the 

 pith. The woody fibre is arranged in perpendicular lay- 

 ers, and the cellular tissue in horizontal layers, running 

 from the pith to the bark and connecting them. The 

 mingling of these two systems gives to the surface of the 

 cross section of a stem the 

 beautiful veined or netted ap- 

 pearance observable in fig. 3, 

 which represents the cross- 

 section of an oak branch. The 

 perpendicular layers of woody 

 fibre are most clearly observ- 

 able when we cut a stem ver- 

 tically; they are then easily 

 separated from one another. Fig. 3. SECTION OF A BKANCH 

 The layers, or plates of tissue OF OAK- 



radiating from the center to the circumference of the stem 

 and inner bark are called the medullary rays. Two of these 

 are shown in fig. 2, marked by the dotted lines H. 



Growth of the Stem. The stem of a tree is originally 

 the extension of the cellular tissue of the seed. As soon 

 as leaves are formed they organize new matter, which de- 

 scends and forms woody fibres : the layers sent down 

 from the first leaves are covered by those sent down 

 from the next, and so on, one layer after another is pro- 

 duced until the end of the season, when the leaves fall and 

 growth ceases. A yearling tree has, therefore, a greater 

 number of layers of woody fibre at the collar than at the 

 top, and is, consequently, thicker; the second year the 

 buds on the first year's growth produce shoots, and these 



