166 BOARD OF AGRICULTURE. 



world with caoutchouc, gutta-percha, and other useful gum- 

 resins. 



Surrounding the inner bark is a layer of cellular tissue in 

 which the rays from the pith terminate, and which is named 

 the green layer, because it .often exhibits this color in young 

 shoots, and then performs the same functions with the green 

 tissues of the leaf. Outside of all is the corky layer, con- 

 sisting of dry, dead cellular tissue, and developed annually 

 from the green layer. This is not usually of much thickness 

 or consequence, but sometimes, as in the cork-oak of Spain, 

 it becomes an important article of commerce. 



The growth of our trees goes on in the cambium layer, 

 from which is produced annually a layer of wood and a layer 

 of bark, each formed of longitudinal fibro-vascular tissue and 

 horizontal cellular tissue. As the trunk expands the outSr 

 bark cracks and falls off, as in the shag-bark hickory, or dis- 

 tends and envelops it with a somewhat smooth covering, as 

 in the beech and birch. In these latter cases, the annual cor- 

 tical la3'ers are quite thin, and the outer layer very gradually 

 wastes away under the influence of winds and storms. In 

 the cork-oak the outer layer is specially thickened, and, if re- 

 moved every eighth year, may be obtained in stout, elastic 

 sheets, which Vvould crack and fall to the ground in the 

 process of time, if not harvested. 



The structure of the root is not unlike that of the stem, 

 except that the pith is usually wanting, as well as the green 

 layer of the bark, which could not be formed, nor be of any 

 use in the dark earth where the root makes its home. This 

 part of the plant develops an annual layer of wood and bark 

 with rays of cellular tissue like the stem. The number and 

 extent of root-branches in the soil depend much upon its fer- 

 tility and adaptation to the plant. Thus, Hellriegel found 

 that a plaut of barley, in a rich, porous soil, had one hun- 

 dred and twenty-eight feet of roots, while a similar specimen 

 in coarser and heavier land had only eighty feet. He also 

 measured the roots of winter- wheat, rye and clover, and 

 learned that they penetrated the earth from three to four 

 fe.et. As the vigor of vegetable growth depends chiefly on 

 the action of the roots, the importance of thorough tillage is 

 apparent. With the present enormous cost of fertilizers it 



