72 ZESSOJ}fS WITH PLANTS 



consider the ridges which he now and then finds 

 upon trees, and determine whether they have any 

 significance. 



77. The pupil should now turn to Fig. 22, and 

 consider the ring of tissue which rolls out over the 

 wound. This tissue in time covers the wound. It 

 forms most rapidly and uniformly when the wound 

 is smooth and regular. Observe the healing on 

 broken and splintered limbs; also the difference in 

 rapidity of healing between wounds on strong and 

 weak limbs. There is difference in the rapidity 

 of the healing process in different kinds of trees. 

 Compare the apple tree and the pine. This tissue 

 may in turn become bark-bound, and the healing 

 may stop. On large wounds it progresses more 

 rapidly the first few years than it does later. Could 

 not the point of the knife -blade be used to re- 

 lieve the pressure upon this callus (as the roll of 

 healing tissue is called)? 



Suggestions. — Are the trunks of trees ever perfectly cylindrical? 

 If not, what may cause the irregularities? Do trunks often grow 

 more on one side than the other ? Slit a rapidly -growing limb, in 

 spring, with a knife blade, and watch the result during the season. 

 Consult the woodpile again, and observe the variations in thickness 

 of the annual rings, and especially of the same ring at different 

 places in the circumference. Cross -sections of horizontal branches 

 are interesting in this connection. Note the enlargement at the base 

 of a branch, and determine if this enlargement or bulge is larger on 

 long, horizontal limbs than on upright ones. Why does this bulge 

 develop? Does it serve as a brace to the limb, and is it de- 

 veloped as the result of constant strain ? 



