62 



AGROXOMY 



Root hairs. Roots do not absorb through all parts of their 

 surface, as is commonly supposed. The waterproof epidermis 

 prevents the passage of moisture through all the older parts, 

 and only a small portion near the tip, where there are special 



structures, called root hairs, de- 

 signed for the purpose, is able to 

 perform this office for the plant. 

 At first glance a waterproof epi- 

 dermis might seem a disadvantage 

 to the plant, but its utility is seen 

 when we learn that it serves to 

 prevent moisture once absorbed 

 from passing out into the soil 

 again. The root hairs are tiny 

 tubes closed at the end, which 

 project from a zone of epidermal 

 cells just back of the root tip. As 

 the root adds to its length, new 

 root hams are developed on the side of the zone toward the 

 growing point, while on the other the old ones slowly shrivel 

 and disappear. The advancing root is thus always provided 

 with a zone of fresh root hairs with which to absorb. On 

 seedlings that have been 

 sprouted in moist air the 

 root hairs appear like a 

 fine white down. They are 

 very numerous and spread 

 out in all directions among 

 the soil particles, affording 

 a much larger surface for 

 absorption than would the epidermal cells alone. As the roots 

 push onward through the soil the development of fresh root 

 hairs constantly brings the plant into contact with new sources 

 of food materials. 



FIG. 35. A bit of epidermis with 

 young root hairs. (Enlarged) 



FIG. 36. A single root hair projecting 

 from an epidermal cell. (Much enlarged) 



