REPRODUCTION AND DEVELOPMENT OF PLANTS 287 



Under the microscope the protoplasm in cells is seen to be 

 constantly flowing around inside the cell wall. The growing 

 tips of roots and stems are almost constantly in motion. 

 These movements are usually very slight, though one 

 example may be quoted to show that they are sometimes 

 easily seen. On the banks of the Ganges River there grows 

 a plant related to the bean, the tips of whose leaves make 

 constant movements in the air, so that they describe a com- 

 plete circle in less than three minutes. 



330. The Sun, the Source of Energy for Life Processes. 

 How the living protoplasm is able to carry out the life 

 processes of nutrition, reproduction, irritability, and spon- 

 taneous motion is a mystery. We know in regard to 

 it, however, that the protoplasm ceases its activity 

 and dies unless a certain amount of internal energy is 

 available. 



This energy, in the form of heat, is derived from oxidation 

 of the protoplasm itself and of food material not needed for 

 growth and reproduction. No amount of energy supplied 

 externally can take 'the place of the necessary internal 

 energy. If growth and movement are rapid, more energy 

 is required and therefore a greater amount of material is 

 consumed than if movement is sluggish or growth slow. 



The sun is the main source of energy for living bodies. 

 The internal energy, heat, set free during respiration is 

 obtained from the oxidation of tissues made from food. 

 This food is largely starch, made only under the in- 

 fluence of light, and proteid made from starch and from 

 salts taken from the soil. When the tissues of the body are 

 broken up or decomposed in the process of oxidation, this 

 energy is set free and is used by the plant. Thus it ap- 

 pears that the most important thing about food making is 

 that it is a process of storing up the sun's energy by the 

 plant, just as the coal making of the past stored energy for 

 present use. The plant requires energy by night as well as 



