126 PHYSIOLOGICAL BOTANY. 



From the above experiment it will be seen that the root 

 obeys the force of gravitation, while the stem opposes this 

 force. 



Gravitation is therefore a force which determines the 

 direction of growing parts. 



4. ELECTRICITY. 



Plants are good conductors of electricity and equalise the 

 difference of the electric tension of earth and air ; large trees 

 are therefore often struck by lightning. 



Besides the above mentioned forces, wind, various living beings, and 

 others also exercise much influence on plants. Thus, wind may modify 

 the direction of the stem, may disperse fruits and seeds to distant places, 

 may carry the pollen grains of one flower to another, and may perform 

 other actions ; and other living beings may disperse fruits and seeds to 

 distant places, may carry pollen-grains, may become the hosts of parasitic 

 plants, and may do other actions. 



Section 3. GROWTH. 



Growth is brought about by internal processes which 

 enlarge and alter permanently any portion of plants. 



A sufficient quantity of raw material and water is first 

 indispensable to growth; and proper temperature is next 

 important. 



In the process of growth, new cells are first formed by 

 repeated cell-division ; these new cells are next enlarged in 

 size ; and they are variously modified in the last. The first 

 stage may be termed Growing ; the second stage, Elongat- 

 ing ; and the last, Fully-developed. 



Section 4. IRRITABILITY. 

 If the leaves of the Nemurigusa (Mimosa p adieu) be 

 touched by a foreign body, their leaflets will be immediately 



