100 FOUNDATIONS OF 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-off ends 
:C of large billets of some 
of the following woods: 
locust, chestnut, syca- 
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 
Fic. 71.— Cross-Section of a Three-Year-Old all the way round the stick 
Linden Twig. (Much magnified.) of wood? If not, the rea- 
P, epidermis and corky layer of the bark ; Phl, bast; gon in the case of an up- 
C, cambium layer ; JR, annual rings of wood. 
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. 
2This is admirably shown in red cedar, black walnut, barberry, black 
locust and osage orange. 
