outside of the circle, first, tlie rough bark practically without life 



hut nevertheless an important protection; next, a greenish ring of 



hark recently formed ; next to this a thin green ring of wood. 



Several other gi-eenish rings are usually present (sap-wood), then the 



color of the wood changes to a liglit or dark brown which we 



recognize under the name of heart-wood. 



What is the difference between sap-wood and 



heart-wood ? What do the annnal rings 



mean ? An apple tree in the process of 



growth produces, under normal conditions, 



each year two j'ings — one of new wood and 



the other of new bark. The ring of wood 



each year encloses — is laid over — that of the 



preceding season. The ring of harl^ is laid 



under last year's. This latter action results 



in pushing out and thickening the bark cover- 



ing. As the stem expands, rifts and cracks 



are developed in the outer bark. Thus we 



have the shag-bark of the hickory, the ridges 



of the oak and pine and maple, and the scales 



of the apple and pear. Under these rough 



scales insects sometimes find comfortable 



winter quarters. Tliat this rough bark protects the living parts 



within is show^n by the lessened amount of sunscalding as the tree 



advances in years. It does not follow that all this rough scaly bark 



is necessary nor is it desirable that it should always remain. 



The livmg jpart of the stem. Not cdX parts of the stem are alive. 

 — Lesson 5, page 6, described the manner which the raw materials 

 from the soil and those from the air unite and are gradually trans- 

 formed into food and fibre for the sustenance and mechanical sup- 

 port of the tree. This elaborated food forms the new circle, called 

 cambium at first, and by the inward j^assage and deposition of food 

 thickens the cell walls in the older wood. In this way thin-walled 

 sap-wood cells are changed to the dense and dead tissue making up 

 the heart- w^ood. Which wood is most duraljle, sap-wood or heart- 

 wood ? If a piece of bark is scraped off the stem of an apple tree 



359 



6, — The deeply ridged 

 hark, of the locust. 



