CHAPTER XII 



THE PHYSIOLOGY OF GROWTH AND MOVEMENT 



Growth. The permanent change of form which takes place 

 in living plants is called growth. The change of size in a dead 

 seed which takes place when it is placed in water is not a 

 permanent change, for if the water is removed it returns to its 

 original size. On the other hand, if a living seed is supplied 

 with water, the young embryo it contains commences to develop. 

 Root, stem and leaves are produced, and a permanent change 

 in shape and size takes place, or in other words, it grows. // is 

 only living things which can grow. 



Conditions which are necessary for Growth. 

 i. Heat. The plant and surrounding air must be at a certain 

 temperature. In winter the temperature of the soil is too low 

 for absorption to take place, and growth is arrested. The 

 lowest temperature at which plants can grow is said to be the 

 minimum temperature of growth. There is a temperature 

 above which no growth can take place. This cessation of 

 growth may be caused either by the activity of the protoplasm 

 being arrested, or by the cells losing water so that they are no 

 longer turgid. The highest temperature at which plants can 

 grow is called the maximum temperature of growth. The ex- 

 tremes of temperature above and below which no growth can 

 take place vary for different plants. Between the minimum 

 and maximum temperatures there is one at which plants grow 

 best ; this is said to be the optimum temperature for growth. 



2. Water. No plant can grow without water, since this 

 substance enters into the composition of all protoplasm. Water 

 is also necessary as a medium for carrying nutritive materials to 

 those parts of the plant where growth is taking place, and it i 



