170 



APPLIED BIOLOGY 



in certain cone-bearing trees that no annual rings are marked 

 off, because at no time is there extra-rapid growth such as 

 produces the large wood-tubes in trees of the temperate zone 

 in the spring. 



163. Gross Structure of Dicotyledon Stems. (L) Materials : 

 Stems of oak, pine, beech, maple, and as many others as obtainable, 



selecting stems of various 

 diameters, from half inch 

 to two inches or more. 

 These should be cut off 

 with a fine-tooth saw and 

 then smoothed by rubbing 

 on sandpaper which has 

 been glued or tacked to a 

 board. Also, some sec- 

 tions or cut-ends of twigs, 

 one, two, and three years 

 old. In many high schools 

 the boys who know how 

 to use carpenters' tools 

 prepare a neat collection 

 of woods. Dead and 

 well-dried (' ' seasoned ' ' ) 

 branches are best. 



The hand-lens shows 

 that the annual rings are 

 marked off by large pores 

 (the wood-tubes). The 

 large wood-tubes of the in- 

 ner ring were formed while the stem was soft and still elongating (dur- 

 ing its primary growth) . Usually only this ring will be found in stems 

 taken during the first season's growth. Compare stems of varying 

 diameters, and count the rings. Each season's growth begins 

 with the large tubes, formed when growing rapidly in spring, and 

 extends outward to the next ring of large wood-tubes. In many 

 woody stems some pith remains in the center. 



Most woody stems have delicate radiating lines, starting from the 

 pith at the center ; and in older stems some of these radiating lines 

 start in annual rings and do not extend to the center. These lines 

 are the pith-rays or medullary rays, which are so named because they 

 are composed of the same kind of cells as those found in the central 



FIG. 46. From four-year-old pine stem cut in 

 winter. Annual rings (1, 2, 3, 4) ', pith-rays 

 (m) ; cambium (c) ; bark with bast (6) ; lines 

 between wood of successive years (i) ; center 

 of stem (p). (From Strasburger.) 



