78 



STRUCTURAL BOTANY. 



[65, 66. 



tion. Hence, while the seed should be covered, for the 

 sake of the moisture and shade, the covering should 

 be thin and light, for the sake of a free access to air. 



196. The cause of the downward tendency of the root is a theme of much 

 discussion. Some have referred it to the principle of gravitation ; others to 

 its supposed aversion to light. But it is a simple and satisfactory explanation 

 that its growth or cell-development takes place most readily on the moist side 

 of its growing-point, and consequently in a downward direction, so long as the 

 soil in contact with its lower surface is more moist than that above. Hence, 

 also, the well-known tendency of roots toward springs and water-courses. 



CHAPTER XIV. 



THE ROOT, OR DESCENDING AXIS. 



197. The Root is the basis of the plant, and the 

 principal organ of nutrition. It originates with the 



33 



233, White Clover an axial root (with minute tubers). 234, Buttercups fibrous roots, inaxial. 235, Eri- 



genia root tuberous. 



radicle of the seed, the tendency of its growth is down- 

 ward, and it is generally immersed in the soil. Its 



