80 THE VEGETATIVE ORGANS 



and give off oxygen, whereas non-green, living parts or 

 green leaves kept in darkness are unable to bring about 

 these changes. This absorption of carbon dioxide by green 

 plants under the influence of sunlight is called photo- 

 synthesis (Gr. phos, photos = light, synthesis = a putting 

 together), or carbon-assimilation. 



That plants do give off oxygen under such conditions 

 as above described may be shown by an experiment 

 arranged as in Fig. 50. Take a jar or beaker filled with 

 tap-water and saturate the water with carbon dioxide. 

 Place in the water a branch or two of Canadian Waterweed 

 (Elodea canadensis), with the cut ends uppermost, and over 

 these a funnel with a shortened stem. Completely fill 

 a test tube with water and invert it over the stem of the 

 funnel ; then expose to sunlight. Set up a similar experi- 

 ment, but instead of charging the water with carbon 

 dioxide, drive off all the dissolved air by first boiling the 

 water, and then allowing it to cool, or simpler still add 

 lime-water to it so as to remove all the free carbon dioxide. 

 Place the bottles side by side and compare. 



In the second case it will be found that few or no bubbles 

 are given off, but in the former, bubbles are given off freely 

 and displace the water in the inverted test tube. When 

 sufficient gas has accumulated, test it with alkaline pyro- 

 gallic acid and note how suddenly the brown colour is 

 produced, or insert a glowing match which at once bursts 

 into flame ; these are proofs that the gas given off is oxygen. 

 We thus see that such a plant, when growing in water 

 containing carbon dioxide, and exposed to sunlight, gives 

 off oxygen. 



Formation of starch in green leaves. Starch prints. We 

 have seen in a former chapter that starch is commonly 

 present in the tissues of roots and stems. We have now 

 to consider how this starch makes its appearance and under 

 what conditions it is formed. For this purpose, plants of 



