44 STRUCTURE AND PHYSIOLOGY OF SEED PLANTS 



B. roiuit the rings of wood on the cnt-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 {Principles, Fig. 76). 

 Plant food, in the case of trees, is mainly produced in the 

 leaves, and the course through the trunk of sugar or other 

 food in solution is mainly straight down along the sieve 

 tubes of the young wood. This would account for more rapid 

 growth on the more leafy side. Sometimes the inequality 

 may be 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? 



C. In all the woods examined look for : 



1. Contrasts in color between the heartwood and the sapwood. 



2. The narrow lines running, in very young stems, pretty 

 straight from pith to bark ; in older wood extending only a 

 little of the way from center to bark, — the medullary rays. 



3. The wedge-shaped masses of wood between these. 



4. The pores which are so grouped as to mark the divisions 

 between successive rings. These pores indicate the cross 

 sections of vessels or ducts. Note the distribution of the 

 vessels in the rings to which they belong, and decide at 

 what season of the year the largest ducts are mainly pro- 

 duced. Make a careful drawing of the end section of one 

 billet of wood, natural size. 



D. Cut off a grapevine several years old and notice the great 

 size of the vessels. 



E. Examine the smoothly planed surface of a billet of red oak 

 that has been split through the middle of the tree, and note 

 the large, shining plates formed by the medullary rays. 



