ECONOMIC WOODS OF THE UNITED STATES 9 



branes. Between the ends of the sieve-tube segments (and some- 

 times between adjacent side walls as well) are thin plates dotted 

 with pits, resembling a sieve. The pit membranes are finally 

 absorbed, allowing free communication from one cell to another. 

 Unlike vessels, the segments of the sieve tubes remain alive for a 

 year or more, though they lose their nuclei. This unusual phe- 

 nomenon may be due to some influence of the companion-cells 

 or to associated parenchyma cells. The function of the sieve 

 tubes is the vertical (especially downward) distribution of elab- 

 orated food materials. After the first year the cells usually be- 

 come crushed by the pressure of the surrounding tissues, their 

 places being taken by new cells generated by the cambium. 



In addition to the structure just mentioned, many other 

 elements and structures may enter the composition of the bark. 

 Among these may be mentioned resin ducts, latex tubes, stone cells, 

 crystals, mucilage sacs, and tannin sacs. Bast rays are also present, 

 being continuous with the rays of the xylem. They increase in 

 width uniformly and gradually as they recede from the cambium. 



In practically all cases of growth in thickness the epidermis 

 is destroyed at an early period and is replaced by cork. Cork is 

 suberised tissue formed by a special meristem called cork cambium 

 or phellogen, which originates in the epidermis or in the cells just 

 beneath the epidermis. All parenchymatous cells, however, 

 wherever located, appear to possess the ability to form cork. 

 Wound surfaces are closed and healed by it, and diseased and 

 dead parts are isolated from those in living condition. 



The formation of cork cambium in the bark usually occurs 

 during the first year's growth of the stem. As a result of its 

 activity a layer of cork cells is generated on the outside, and fre- 

 quently a layer of thin-walled parenchyma cells — the phelloderm — 

 on the inside. Collectively these new tissues, including the cork 

 cambium, are called the periderm. The effect of the development 

 of cork is to cut off from the interior mass of tissue portions of the 

 cortex, which dry up and are eventually thrown off as outer bark. 

 This action may occur only once, as in Fagus and Carpinus, but 

 usually is repeated, and successively deeper layers of the cortex 

 and eventually of the pericycle and phloem are cut off. 



In some species the successive formations of cork extend 

 more or less uniformly around the stem, cutting off in each case 

 an annular layer of cortex — sometimes called ring bark. In other 

 species the successive internal layers are very irregular, and cut 



