CHAPTEE III. 

 THE VEGETATIVE ORGANS OF PLANTS. 



§ 1- 

 THE BOOT. 



The roots of plants, with few exceptions, from the 

 first moment of their development, grow downward. In 

 general, they require a moist medium. They will form 

 in water or in moist cotton, and in many cases originate 

 from branches, or even leaves, when these parts of the 

 plant are buried in the earth or immersed in water. It 

 cannot be assumed that they seek to avoid the light, 

 because they may attain a full development without 

 being kept in darkness. The action of light upon them, 

 however, appears to be unfavorable to their functions. 



The Growth of Roots occurs mostly by lengthen- 

 ing, and very little or very slowly by increase of thick- 

 . ness. The lengthening is chiefly manifested toward the 

 outer extremities of the roots, as was neatly demonstrated 

 by Wigand, who divided the young root of a sprouted 

 pea into four equal parts by ink-marks. After three 

 days, the first two divisions next the seed had scarcely 

 lengthened at all, while the third was double, and the 

 fourth eight times its previous length. Ohlerts made 

 precisely similar observations on the roots of various 

 kinds of plants. The growth is confined to a space of 

 about one-sixth of an inch from the tip. {Linnea, 1837, 

 pp. 609-631.) This peculiarity adapts the roots to 

 extend 'through the soil in all directions, and to occupy 

 256 



