GROWTH OF THE ROOT 23 



and of the influence of the main root. If the tip of the main 

 root is removed, one or more of the side roots curve down- 

 wards and continue in a vertical direction. The roots of 

 higher orders are ageotropic and grow outwards from their 

 respective parent axes. 



In the soil other stimuli are efl:ective in modifying the 

 direction of growth. A slight wounding of the tip leads 

 to a traiimatotropic curvature away from the exciting 

 influence. More gentle contact with a solid body leads to 

 a haptotropic curvature towards the stimulus. A gradation 

 in the degree of moisture induces a hydrotropic curvature 

 towards the greater saturation. An aeroiropic curvature 

 may bring the root towards a higher concentration of 

 oxygen. Under such influences, the root tip grows down- 

 wards through the soil, moving slightly from side to side, 

 and thus aided in its penetration. 



Penetration is further aided by the mode in which 

 growth takes place. The region of active elongation lies 

 3-5 mm. behind the tip. The pushing force is thus applied 

 close behind the penetrating point. This avoids the danger 

 of buckling which would be present if the growing zone 

 were long or situated further back, as is the case in many 

 shoots and in such roots as do not require to overcome the 

 resistance of the soil, e.g. the aerial roots of the epiphytic 

 orchids. The structure of the root tip also aids penetration. 

 It is covered by the root cap, an organ about 0*5-1 "o mm. 

 long, the outer cells of which are constantly sloughed as they 

 rub against soil particles and constantly renewed by a special 

 meristem at the root tip. The slimy nature of these cells 

 lubricates the tip. The force developed by growing roots 

 is considerable ; it is a familiar fact that they are capable 

 of splitting drains and stone walls if they gain entrance into 

 a crack. The exact extent of the force has been investi- 

 gated by Pfefter ; a pressure of 100 lbs. to the square inch 

 and more is commonly developed. 



Branching. — A few days after germination the growth 

 rate of the radicle slows down and it produces side roots. 

 These arise, as do all except the primary root, endogenously 



