GROWTH OF ROOT AND STEM rata 
If the tip is removed, will the root turn? Now place 
some more seedlings with the roots in a horizontal posi- 
tion, or, 1f you choose, this experiment can be carried 
on along with the others. With sharp scissors, or a 
very sharp knife, cut off the extreme tip of the root. 
In twenty-four hours afterwards observe the roots. 
They have elongated, but they have not turned down- 
ward. They have continued to grow in 3 
the horizontal position in which they were 
placed, although the motor zone was not 
cut away. Why is this? It must be that 
the tip of the root is the part which is sen- 

sitive to the influence or stimulus of gravity. 
For this reason the tip of the root 1s called 
the perceptive zone. 
The upward growth of the stem. Ii the , 
stem is well developed in any of the seed- E ane 
lings placed in a horizontal position, we see ; 

ee, 
that the stem turns up while the root turns Sunflower seed. 
lings turning 
down. The maize seedling shows this well — upwara. 
in Fig. 52. It is more convenient in studying stems to 
take seedlings grown in pots. Peas, marrow, maize, 
bean, sunflower, etc., are excellent for this study. Place 
the pot on its side. In twenty-four hours observe the 
plants. They have turned straight upward again, as 
shown in Figs. 57 and 58. In the case of the stems 
the part which turns is at a much greater distance 
