104 EXPERIMENTAL PLANT PHYSIOLOGY 



At intervals of twenty-four hours the seedling is 

 carefully sketched. 



As the Indian ink marks tend to get blurred it is 

 well to re-mark them frequently. 



Observations. — In Fig. 54 are given drawings of a 

 bean-seedling made at intervals of twenty-four hours. 



On the first day fifteen equal lengths of one milli- 

 metre were measured off from the tip. 



After twenty-four hours it was seen that no growth 

 in length had taken place in the division nearest the 

 tip nor in the five divisions furthest from it, whereas 

 the greatest growth had taken place in the third and 

 fourth divisions from the tip. 



The next reading showed that the second and third 

 divisions only were still growing in length, the fourth 

 division and those above it had ceased elongating. 



In the final reading taken growth was confined to 

 the second division. 



It was also observed that the root continued to grow 

 in thickness after it had ceased to grow in length. 



Inferences. — A root does not grow in length at the 

 tip but in the region immediately behind the tip. 



The elongating region is very short, extending only, 

 in the case of the bean, for a distance of about ten 

 milHmetres. 



The region of greatest elongation is about three 

 millimetres from the tip. 



Each part of the root in turn very soon reaches its 

 maximum length. 



Any part of the root continues to grow in thickness 

 after ceasing to grow in length. 



Experiment 68 



Aim. — To find the connection between the growing 

 part of the root and the region that is sensitive to 

 gravity. 



Method. — A broad-bean seedling having a radicle 

 whose length is about one and a half to two inches 



