68 EXPERIMENTAL PLANT PHYSIOLOGY 



Thus it is seen that plants only can live indepen- 

 dently on this planet. Without them the simple con- 

 stituents of air and water could not be used for food 

 and animal life could not be sustained. 



The important conclusion is now reached that all 

 animal life, our own included, depends for its very 

 existence on the activity of the green plamt. 



It is now possible to consider more fully the cause of 

 this dependence of the animal world upon plant-life. 



Carbon-dioxide and water cannot of themselves form 

 starch. Whenever it is required to build up any sub- 

 stance from two simpler substances some form of energy 

 must be supplied. Familiar instances of this occur in 

 the experiments made in the chemical laboratory, where 

 energy, in the form of heat, is constantly being applied 

 to bring about chemical combination. 



So it is in the case of starch formation. 



To write the equation 



Carbon-dioxide + water = Starch -f oxygen 



is not to state the case correctly. 

 The right equation is : 



Carbon-dioxide+ water-1- energy = Starch+ oxygen. 



What then is this energy ? and why cannot animals 

 make starch in a similar way ? 



The answer to these questions is that the ultimate 

 source of all this world's energy is the sun, and only 

 chlorophyll is able to absorb this energy for the building 

 up of food. 



Health Value of keeping Plants Indoors. — There is 

 one interesting and very important point which may be 

 referred to here. 



It will be remembered from the lime-water test (Ex- 

 periment 21) that in breathing, or respiration, a large 

 proportion of carbon-dioxide is given out. 



Further, we know from experience that a room gets 

 " stuffy " when several people are in it. 



