58 



Fruit-growing in Arid Regions 



The large tubes shown at 2 form a part of the new or 

 sapwood. It is through these vessels that the water taken 

 in by the roots passes upward to the leaves, and they form 

 a continuous passage, interrupted only at rather long in- 



FIG. 6. A Cross-section of a Stem. (After Stevens.) 



tervals by thin cross-partitions. As the plant grows older, 

 these tubes become thickened or woody, and new ones are 

 produced, until a more or less solid layer of wood is formed. 

 After a few years what is now sapwood becomes heart- 

 wood, and its activities then cease. 



The peculiar markings on these vessels are thin places in 

 the walls, and they allow water to escape. They may also 



