304 



PRACTICAL BOTANY 



280. The branch and stem. By means of transverse sec- 

 tions of branches of different ages the general structures of 

 the stem may be observed. Innermost is a small pith region, 

 which in older stems is compressed until it is not usually 

 noticeable. Around this is the woody tissue, — the xylem of 



the fibrovascular bundles. 

 The xylem from all bundles 

 is joined in such a way as to 

 form a solid woody (xylem) 

 cylinder. It is possible to 

 determine the age of the 

 twig by counting the layers 

 or rings of wood. If two 

 distinct growth periods oc- 

 cur, as rarely takes place 

 in one season, two rmgs 

 of wood are formed; hence 

 this is not always an abso- 

 lutely accurate method of 

 determining the age of a 

 stem. At the outer edge of 

 the woody tissue is a thin 

 layer of cells, the cambium, 

 which separates the xylem 

 and phloem cells. The cam- 

 bium is actively growing 

 tissue which produces new 

 xylem withm and new 

 phloem without. The tis- 

 sues outside the phloem, 

 which we need to notice in this connection, are the green bark 

 and dead bark. Dead bark is constantly being formed from 

 green bark within, as green bark is from tissues within itself. 

 This results in making the dead bark constantly thicker, until 

 finally in older branches and stems light penetrates through 

 it poorly if at all, and chlorophyll ceases to be developed. 



Fig. 250. The "grizzly giant big tree" 

 (Sequoia Washingtoniana) 



The 20 X 100 foot scale will help to show the 

 size of the tree, especially if one pictures 

 the height of some known object at the side 

 of this scale. This tree is 275 feet high, 93 

 feet in circumference at the base, and 64 feet 

 3 inches in circumference at 11 feet from 

 the ground. Original negative by Mode 

 Wineman 



