38 PLANT GROWTH 



Anchorage may be incidental to the structure for con- 

 duction of the root but its importance cannot be questioned, 

 since it is necessary that a shoot be in a position to get an 

 abundance of air and Hght. Recent work in plant breeding 

 has stressed the character of a root system that provides 

 good anchorage, notably in the breeding of corn. 



Root hairs are tubular projections of the epidermal or 

 outer layer of cells of the young root (Figs. 9, 10, 11) . Root 

 hairs are frequently less than a sixteenth of an inch long, but 

 there may be in some cases a quarter of a million per square 

 inch of root surface, which will increase the absorbing surface 

 ten times or more. The figures show the normal position of 

 root hairs, as they begin just back of the elongating region 

 and extend some distance along the root. When the older 

 root develops the corky layer the root hairs die. From the 

 above it is clear that the absorbing region of a root system 

 moves farther and farther from the plant, or in other words 

 into new absorbing areas. 



With favorable conditions the root system grows sur- 

 prisingly large; in fact, studies have shown the root spread 

 to be much greater than the shoot spread, resulting, even in 

 larger trees, in a relation similar to the one described for the 

 lima bean. Studies of root growth all indicate that their 

 spread is more than twice as great as the ordinary planting 

 distances, resulting in severe competition in the soil in their 

 growth and for the absorption of water and salts. This is 

 true for most of our garden plants unless we supply excessive 

 amounts of available fertilizers, and apply frequently an 

 optimum amount of water, both of which reduce the growth 

 of root systems. 



Perennial grass plants grown with unlimited space have 

 been found to grow as much as a total length of one hundred 

 fifty miles of roots each year, but never exceeding a total root 

 length greater than about three hundred miles, since after 

 reaching that length the older portions die at about the same 



