l6 BOTANY OF CROP PLANTS 



is either naked or surrounded by modified leaves (bud 

 scales). There are other characters which will be mentioned 

 further on. 



Classification of Roots Based upon Their Medium of 

 Growth.^ — The medium of growth of most roots is soil. Such 

 roots may be called soil roots. It is customary for us to 

 think of the root system of a plant as growing in the soil, 

 just as we associate the shoot system with the air above 

 ground. However, not all roots live in the soil, and not all 

 shoots hve in the air. There are water roots, and air roots, 

 as well as the ordinary sort, soil roots. Water roots occur 

 in such floating plants as the duckweeds, water hyacinth 

 {Eichhornia speciosa). Water roots produce but a few 

 branches. They possess no root hairs; absorption takes place 

 through any cells on the surface. Air roots occur in many 

 plants, such as corn (Fig. 56, B), Virginia creeper, tropical 

 orchids, the banyan and other species of Ficus. Air roots are 

 well shown in corn. In addition to the ordinary underground 

 (soil) roots, corn develops aerial (air) roots, the so-called prop 

 or brace roots (Fig. 56, B). These arise at successive levels 

 above the surface, extending obliquely downward. As aerial 

 roots, they are unbranched, but they branch profusely when 

 they strike the soil. They have the role of absorption, then, 

 as well as anchorage. In the banyan, for example, the air 

 roots are often very large, and arise from branches far above 

 the ground. They grow downward, and when they strike 

 the ground, become firmly attached, and act as a support 

 or prop to the heavy branches. 



Hence, we learn that not all roots have soil as their medium 

 of growth, but that air and water may be the media for some. 



Structure of Roots. — ^Let us cut a median (middle) length- 

 wise section of a young root. It will appear as in Fig. 5. 

 We shall see then that the root has a cap of loose cells at the 



