CHAPTER VI. 



Structure of the Stem. 



Stem of Dicotyledonous Plants. 



I. 



75. General Structure. — Cut smooth, rather thick, sections from a twig 

 of apple one year old. Place in focus under the magnifying glass and make 

 a sketch to show the relative position and amount of bark, wood, and pith. 



From a twig of cherry a year or two old peel off the brown outer coat- 

 ing. This is the corky layer of the bark, more distinct in the cherry tree 

 than in the apple. 



Notice on the outer surface of the twig the rough oval or lens-shaped 

 spots. These are the lenticels, spots in which the inner and more porous 

 layers of the bark protrude through the corky layer and allow air to 

 penetrate to the interior of the branches. ^ Notice the green layer or 

 middle bark in the peeled portion of the cherry twig, and expose this 

 layer in the apple twig by carefully scraping off the corky layer. 



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 

 (§ 58), and count the rings of wood above and below each ring of scars. 



How do the numbers correspond ? What does this indicate ? 



Count the rings of wood on the cut-off ends of large billets of some of 

 the following woods : locust, chestnut, sycamore, 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 reason In the case of an upright stem (trunk) 

 is perhaps 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.' 



' See Gregory's Plant Anntomii, pp. 138-141. 2 See § 145. 



' This is admirably shown in black walnut, barberry, and osage orange. 



