MORPHOLOGY OF ANGIOSPERMS. 



95 



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

 materials, and of fixing the plant to the soil, determine in a meas- 

 ure the direction of its growth. The tendency of the root to grow 

 downward is a characteristic which distinguishes it from other 

 parts of the plant and it is said to be positively geotropic (Fig. 

 6i,A). 



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 



Fig. 6i. 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 PfefEer. 



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. While 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. 

 It is well known that whatever the position of the seed at the 

 time of germination the young radicle begins to grow perpen- 



