160 BOTANY FOR BEGINNERS CHAP. 



cut a slice of soil away near the centre, it will be found full of roots with 

 the tap-root growing downwards. 



(iv) This shows that the ordinary roots are negatively heliotropic and 

 the tap-root is positively geotropic. 



Hydrotropism. It has already been seen (p. 59) that 

 roots growing in dry soil are attracted by moisture. The move- 

 ment of any part of a plant towards moisture is termed hydro- 

 tropism, and roots possess this property to a far greater extent 

 than the other organs of plants. 



Movements Caused by Contact. Just as animal bodies 

 respond to contact so do the organs of a few plants. This is 

 well shown in the case of climbing organs (p. 26). When a 

 tendril comes in contact with a solid body the side of the tendril 

 touching the object has its growth arrested, while the side 

 away from the object grows more quickly. This produces an 

 elongation of the side away from the object and causes the 

 tendril to curve and twine round the object. 



THE IRRITABILITY OF MATURE ORGANS. 



Special Cases. Most full grown organs are incapable of 

 moving, but the organs of a few plants are endowed with the 

 power of vigorous movement. These movements are due to 

 changes in the amount of water which the cells of the organs 

 contain. When cells are full of water, it presses on the elastic 

 walls and they become greatly distended, their cavities becom- 

 ing enlarged. If the cells lose water, the walls shrink and the 

 cavities are diminished. It is to the changes in the size of the 

 cells of an organ that the movements under consideration are 

 due. 



The amount of water in the cells depends upon the temperature 

 and amount of light which they receive. The change in the 

 position of leaves and the opening and closing of flowers are 

 due to changes in the turgidity of the cells on different sides of 

 the leaf. The turgidity, depending as it does upon the amount 

 of water absorbed, is evidently produced by the amount of 

 light and by the temperature. Many flowers and leaves show a 

 periodic movement. 



The Opening and Closing of Flowers. Many flowers 

 and some inflorescences (p. 1 56) change their position from day 



