100 



FOUNDATIONS OV BOTANY 



a solid steel or iron rod of the same size would be too 

 stiff for this use. 



111. The Dicotyledonous Stem, thickened by Secondary Growth. — 

 Cut off, as smoothly as possible, a small branch of hickory and one of 

 white oak above and below each of the rings of scars already mentioned 



(Sect. 77), and count the 



p rings of wood above and 



below each ring of scars. 



How do the numbers 



correspond? What does 



Phi this indicate ? 



Count the rings of 

 wood on the cut-ofE ends 

 ^' of large billets of some 

 of the following woods : 

 locust, chestnut, sycar 

 more, oak, hickory. 



Do the successive rings 

 of the same tree agree in 

 thickness ? 



Why? or why not? 

 Does the thickness of 

 the rings appear uniform 

 all the way round the stick 

 of wood? If not, the rea^ 

 son in the case of an up- 

 right stem (trunk) is per- 

 haps that there was a greater spread of leaves on the side where the 

 rings are thickest ' or because there was unequal pressure, caused by 

 bending before the wind. 



Do the rings of any one kind of tree agree in thickness with 

 those of all the other kinds? What does this show ? 

 In all the woods examined look for : 

 (a) Contrasts in color between the heartwood and the sapwood.* 



1 See Sect. 118. 



2 This is admirably shown in red cedar, black walnut, barberry, black 

 locust and osage orange. 



4 



* 



% 



\JR 



Fig. 71. — Cross-Section o£ a Tliree-Year-Old 

 Linden Twig. (Much magnified.) 

 P, epidermis and corky layer of tliebark ; PM, bast ; 

 C, cambium layer ; JR, annual rings of wood. 



