THE GRO"\\n^H OF THE PLANT 101 



Observations. — The tips of ho tli seedlings have turned 

 dowmvards. 



By measuring ott on the seedlings a' and h' the 

 length of the radieles of the seedlings a and h, it is seen 

 that the downward eurTe has begun in both cases just 

 behind the point which marks the }>ositiou Avhich the 

 root-tips occupied when placed on the cork. 



Inferences. — The root-tip and region immediately 

 behind it is sensitive to graTity. The remainder of the 

 root is unatieeted V'V cri'ii'^itational stimulus. 



EXPEKIMEXT 65 



Aim. — To fijid out what provision the plant makes 

 against injury of the primary root. 



Method. — A broad-hean seedling with a raditle of 

 about an inch long is seleeted. The growing part of 

 the radicle is cut off. The seedliui: is then suspended 

 over water in a gas-jar with part ot the radicle dipping 

 down into the water. The ijas-jar is eovered with 

 black paper. 



Observations. — The seedling is not killed by the re- 

 moTal of the growing part of the radicle. Very soon 

 secondary roots are i:iTen off from the radicle. Gener- 

 ally, but not invariably, one of the secondary roots 

 grows vertically downwards and in this way functions 

 as the primary root (Fig. 52). 



Inference. — One of the secondtiry roots which would 

 normallv grow horizontally usually takes a vertical 

 position if the primary root becomes injured. 



Experiment 66 



Aim. — To find out what provision is made, in some 

 plants at anv rate, against injury to the primary shoot. 

 Method. — Two broad-beans are planted in a pot. 



From one of the seedlings the shoot is cut off as soon 

 as it comes throui^h the soil. 



