STRUCTURES 



25 



more widely distributed throughout the country are good illustra- 

 tions of this type of roots. 



Aquatic Roots. — Some floating plants produce roots which 

 never penetrate the soil. They are known 

 as aquatic roots and absorb water and the 

 yarious food substances in solution which 

 are necessary for plant growth. The very 

 small but widely distributed duck weeds 

 and the much larger water hyacinth of the 

 South are excellent examples of floating 

 plants. However, many aquatic plants, 

 such as the pond lily, cat-tails, etc., have 

 roots which penetrate the mud, while the 

 plant either floats or stands upright. 



Structures. — Roots are made up of 

 three parts, a central cylinder composed pri- 

 marily of woody tissue and surrounded by 

 a sheath or cellular cortex, and covered 

 with a very thin epidermis or skin-like 

 coating. (Fig. 15.) They grow in length 

 much more rapidly than in thickness and 

 the very delicate tip is protected by a mass 

 of cells known as the root cap. All parts 

 of the root do not elongate with equal ra- 

 pidity. There is a zone just back of the 

 tip in which the growth is most rapid.' 

 The tips of roots are soft and delicate and 

 easily broken, yet they must exert an enor- 

 mous force which we cannot appre- 

 ciate. The surface of the young roots, just back of the tip, is 

 covered with numerous very delicate root-hairs or trichomes. 

 (Figs. 4, 16 and 19.) These root-hairs are very numerous and 

 should be studied on very young plants that have been grown in 



Fig. 15. — Diagramm- 

 atic longitudinal sec- 

 tion of a root tip; a, 

 central cylinder; c, 

 cortex; e, epidermis; 

 r. c, root cap. 



