THE STEM. 



159 



These are : 



1. Protection against insects and fungi. 



2. Prevention of the escape of water. 



3. Ascent of sap. 



4. Supply of food material. 



5. Storage of reserve food. 



6. Provision for increase of thickness. 

 Protection against the attack of insects, etc., and 



prevention of the escape of water, are insured by the 

 continuous cork sheath which surrounds the entire stem 



>V ^^;(y^ Direction of Water 

 Pi(-f^' *Z^ rood Materials 



;■- Annual Ring 

 /?!?- Z"Mancl 3'?' Medullary Rays 



/ ,- Cambium / 



CORKFORMING 

 LAYER 



Fig. 20. — Diagram. Transverse Section of a Stem. Pith, medullary rays 

 and cortex are composed of ordinary living cells. 



of most trees. As shown in the diagram the arrange- 

 ment of the living cells may be roughly compared to 

 the circumference, spokes, and centre of a wheel. These 

 living cells are used for storage of fats, oils, or some- 

 times starch in winter, and are, probably, essential for 

 carrying on the work in all parts of the trunk. Be- 

 tween the " spokes," or medullary rays, are packed in the 



