INTRODITCTORT. 47 



ber and extent of root branches in the soil (Icju'nd nmcli upon 

 its fertility and adaptation to the plant." 



" As the vigor of vegetal)le growth depends chiefly on the 

 action of the roots, the inii)ortance of thorough tillage is 

 apparent." 



" The striking peculiarity in the structure of the root is the 

 absorbent power of the young rootlets, which are either cov- 

 ered with a thick, spongy layer of cellular tissue, or furnished, 

 as is commonly the case, with exceeding minute l)ut innumer- 

 able 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 absori^tion, but serve as entering wedges 

 for the advancing root, which lengthens onlv near the extrem- 

 ity." 



" The bark of the larger roots becomes thick and impervi- 

 ous, like that of the trunk and its older branches, and the 

 inner portion of the Avood, both above and below ground, 

 gradually solidifies, and becomes unfitted for the free trans- 

 mission of fluids. It is then called heartwood, in distinction 

 from the sapwood, 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 conse- 

 quently the less useful is it in the economy of the plant." 



" The leaf has been said, with some propriety, to be an 

 extension of the bark, and consists of a framework of fibro- 

 vascular tissue forming the stalk and veins, with a double 

 layer of loose cellular tissue covered with a distinct epidermis 

 or skin. The vessels in the leaf stalk and the veins, which are 

 its branches, are also in two layers, the upper connecting the 

 leaf with the vessels surrounding the pith, which are called 

 spiral on account of their peculiar markings, and the lower 

 which are united to the cambium layer through the tissue of 

 the inner bark." 



" The distinctive features of the leaf is the presence of sto- 

 mata or breathing pores, which are usually more numerous on 

 the under side. These stomata are furnished with openings, 

 so constructed as to close in very dry air, and open in that 

 which is moist, but they always remain shut, except under the 

 stimulus of light. As the chief function of the rootlets is to 

 absorb the liquid food of the plant from the earth, so it is the 

 special w^ork of the stomata to transpire the surplus water of 

 the crude sap, which has been employed as a carrier of food 

 from one extremity of tlie countless series of cells which liuilcl 

 up the plant, to the other, in some cases a distance of five hun- 

 dred feet, through im})erf()rate meml)ranes, and against the 

 force of gravitation." 



