STRUCTURE OF ROOTS AND LEAVES. 167 



may be wise to revert to the idea of that famous pioneer in 

 agriculture, Jethro Tull, who believed that manure might be 

 profitably superseded by suitable mechanical means for pul- 

 verizing and aerating the soil. 



The striking peculiarity in the structure of the root is the 

 absorbent power of the young rootlets, which are either 

 covered with a thick spongy layer of cellular tissue, or 

 furnished, as is commonly the case, with exceedingly minute 

 but innumerable hairs, which penetrate the crevices of the 

 earth in every direction in search of food. The extreme tips 

 of the rootlets, about one-sixth of an inch in length, are not 

 clothed with hairs, nor capable of absorption, but serve as 

 entering wedges for the advancing root, which lengthens only 

 near the extremity. The bark of the larger roots becomes 

 thick and impervious, like that of the trunk and its older 

 branches, and the inner portion of the wood both above and 

 below ground gradually solidifies, and becomes unfitted for 

 the free transmission of fluids. It is then called heart- wood 

 in distinction from the sap-wood through which fluids are 

 transmitted freely. The farther any layer of wood or bark 

 is removed from the living cambium, the less vitality does it 

 retain, and consequently the less useful is it in the economy 

 of the plant. 



The leaf has been said with some propriety to be an 

 expansion of the bark, and consists of a frame- work of fibro- 

 vascular tissue forming the stalk and veins, with a double 

 layer of loose cellular tissue covered with a distinct epider- 

 mis or skin. The vessels in the leaf-stalk and the veins, 

 which are its branches, are also in two layers, the upper con- 

 necting the leaf with the vessels surrounding the pith, which 

 are called spiral because of their peculiar markings, and the 

 lower which are united to the cambium layer through the 

 tissue of the inner bark. The leaves of a singular plant 

 called Hermas, in South Africa, have the two layers of the 

 leaf so loosely attached that they readily separate, except 

 where united at the edges. 



The distinctive feature of the leaf is the presence of stom- 

 ata or breathing pores, which are usually most numerous on 

 the under side. The leaf of the lilac has none on the upper 

 .surface, but the surprising number of one hundred and sixty 



