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A TEXT-BOOK OF BOTANY. 



the theory has been established (as a result of Knight's experi- 

 ments) that the root grows downward by reason of the influence 

 of gravity. In addition it may be said that the principal functions 

 of the root, namely, those of absorbing inorganic food materials 

 and of fixing the plant to the soil, determine in a measure the 

 direction of its growth. The tendency of the root to grow down- 

 ward is a characteristic which distinguishes it from other parts 

 of the plant, and it is said to be POSITIVELY GEOTROPIC (Fig. 163, 

 A). 



FIG. 163. A, seedling of Brassica nigra in which root and stem have curved into a 

 vertical position after being laid horizontally. B, seedling of Sinapis alba, the hypocotyl 

 showing a positive, the root in water a negative heliotropic curvature. The arrows show 

 the direction of the incident rays of light. After Pfeffer. 



The influence which gravity has on plants may be best under- 

 stood by bearing in mind that gravity is a constant force which 

 acts perpendicularly to the surface of the earth, and that all parts 

 of the plant are subject to its influence. The organs of plants 

 respond in different ways to the action of gravity, but a clear 

 distinction should be made between mere mass attraction, or that 

 manifestation of the force of gravity whereby the heavily laden 

 branch of a fruit tree bends downward, and the stimulus which 

 causes the primary root of a plant to grow downward and the 

 shoot to grow upward. \Yhile all parts of the plant are subject 

 to the influence of gravity, not all the organs of plants respond 

 in an equal degree. This is well illustrated by roots themselves. 



