Growth 311 



nificance of this phenomenon may be seen. Polarity is 

 known merely through the behavior of organs. 



179. Elongation of roots. The nature and distribu- 

 tion of the tissues at the apex of the root axis have been 

 noted. The elongation of the root may be readily followed 

 by a simple experiment. The tip of a young seedling 

 should be divided into zones by a half dozen or more 

 parallel marks about 2 to 3 mm. (about yV inch) apart, the 

 first mark, however, being practically at the tip. The 

 marks can be made with India ink and a pen, or a very 

 fine brush. 



Observations after from 6 to 24 hours, under favorable 

 conditions, will show that the region of elongation is con- 

 fined to a space covering usually only a few millimeters 

 back of the tip. The zone farthest away may have 

 already ceased to elongate, or practically so; while those 

 nearest the tip elongate at first slowly, then faster, to a 

 maximum, after which they decline. In fact, if new zones 

 were constantly marked off each would be seen to go 

 through a certain grand period of growth. The impor- 

 tance of the shortness of this region of growth with respect 

 to the ability of the root to penetrate the soil has been 

 pointed out in the discussion of the relation of the root to 

 water and to the soil (section 30). 



New lateral roots form in the quiescent region behind 

 the root-tip ; such lateral roots originate in the pericycle, 

 just within the endodermis, and as they push out they 

 literally break through the cortical tissues, which latter 

 are in part broken down and dissolved. Lateral roots are, 

 therefore, said to arise endogenously. 



The growth and branching of the roots of agricultural 



