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substratum are numerous and the root tip is most richly endowed with 

 various forms of irritability and movement in response to such needs. 

 Thus in finding its way, through the soil a root may encounter a stone 

 which blocks its way or splintered fragments which might pierce and 

 destroy the tip. The root tip is irritable to contact or injury and when 

 it meets such objects as those named above, it bends away from them, 

 and then once more resumes its former direction past the obstacle. 



Certain portions of soil may contain more water or food material than 

 others and it will be found that the roots curve in such manner as to 

 direct their tips toward these portions. Then again a laterally growing 

 root may find itself exposed to sunlight filtering down between the 

 particles of the soil with injurious consequences. To avoid danger 

 from this source the root has the power of bending away from the light. 



The root tip responds by movement to many other forces, and it is so 

 delicately adjusted and has acquired so many forms of irritability that 

 Chas. Darwin compared it to the brain of an animal in his classic work 

 " Movement in Plants." 



Let us now turn our attention to some of the movements which are 

 exhibited by the shoot and which may be seen without any troublesome 

 detail of experimentation. 



If the compound leaves of the Logwood (Haematoxylon), Shameweed 

 (Mimosa), or Quango (Pithecolobium Saman) are examined shortly after 

 sunrise, it will be found that the leaflets are spread open in a horizontal 

 position, and that they are exposed to the full force of the sun's rays. 

 (See Fig. 1.) 



Fig. 1 Extended position of leaf of Pithecolobium Saman, Gu mgo, occnrhi* 

 in the morning or in diffuse light throughout the day. 



