THE ROLE OF GREEN PLANTS 



247 



The latter conducts water, while the heart-wood functions merely as 

 a supporting tissue. As the tree increases in diameter, the area of 



,_bark 



.-Cambium 

 layer- 

 annual 



pith 

 rays 



Section through a dicotyledonous stem. Explain its method of growth. 



heart-wood increases while the sap-wood, although greater in cir- 

 cumference, gets proportionately smaller in extent. 



The bark, or area outside the cambium, is made up of several 

 different tissues, which have a somewhat different 

 arrangement in conifers than in deciduous trees. The 

 area known as phloem is formed immediately outside 

 the cambium. This area contains many living sieve 

 tubes through which elaborated food is carried down 

 from the upper part of the plant. The sieve tubes 

 in the conifers are more or less regular in arrangement 

 while in deciduous trees they are scattered. In both 

 stems they are all surrounded by parenchyma. 

 Scattered through the bark of deciduous trees are 

 masses of tough, stringy schlercnchyma cells of two 

 types, phloem fibers — fibrous, elongated cells that 

 give strength and elasticity to the trunk — and thick- 

 walled, hard stone cells. Outside the latter area is 

 formed the corky layer, produced by a layer of growing 

 cells known as the cork cambium. Cork cells, which 

 have their walls impregnated with an insulating sub- 

 stance called suherin, are of great value to the tree 

 because they prevent a rapid loss of water from the 

 tissues. It is this layer in the Spanish cork oak which 

 is of commercial value. In some trees, such as the 

 redwoods, the bark forms a coating highly resistant 

 to fire. 



H. w. H. — 17 



Above, sieve 

 \ (' s s e 1 (of 

 phloem) with 

 c()mf>anion cell; 

 below, sieve 

 plate, with 

 section of com- 

 panion cell. 

 (After Stras- 

 burger.) 



