214 TEXTBOOK OF BOTANY 



bark," and the layers of cortex and bast that lie between the 

 cork cambium and the cambium proper are called " green 

 bark." The dry bark protects the living tissues within from 

 injuries by insects and larger animals ; from infection by 

 parasitic fungi ; from the too rapid loss of water by evapora- 

 tion, and to some extent from the effects of extreme cold. A 

 wound that penetrates the bark and injures the tissues be- 

 neath is healed (provided it is not too extensive) by the growth 

 and division of the living cells of the various tissues (cortex, 

 bast, medullary rays, cambium, and wood) that are in con- 

 tact with the wound. Ultimately bark is formed over the 

 surface of the new wound tissue. " Girdling " consists in re- 

 moving a strip of the bark and inner tissues down to the wood. 

 If the girdling extends all the way around the trunk, the move- 

 ment of food through the bast from the upper part of the tree 

 to the roots is cut off ; in time the roots, being starved, will 

 die, and the tree will be killed. 



The bark of many trees contains large amounts of tannin, 

 which is used in the curing or tanning of leather. The bark, 

 first stripped from the tree, is so treated as to obtain from it a 

 solution which contains the tannin as well as some other sub- 

 stances that affect the quality of the leather. Tanbark is 

 obtained in largest quantities from the hemlock and the 

 oaks ; but the barks of the larch, willows, and other trees 

 are also largely used, as well as various woods and other 

 parts of plants, each producing a leather of a particular qual- 

 ity or color. Another important bark product is quinin, 

 obtained from several species of trees belonging to the genus 

 Cinchona. They are natives of South America, for which 

 reason the bark is called " Peruvian bark " ; but there are 

 Cinchona plantations in other warm countries as well. The 

 drug cascara is obtained from the bark of a tree that grows 

 near the Pacific coast from California to British Columbia. 



233. Lenticels. The surface of the trunks of many young 

 trees, such as willows and poplars, is marked by small 



