10 SPIROGYRA. [CH. I 



animal is deprived of food, the degree to which it suffers 

 from the deprivation can be roughly gauged by estimating 

 the amount of fat in its body. When the degree of 

 starvation is severe the amount of fat is small. In a 

 green plant starvation may in the same way be estimated 

 by the amount which it contains of another carbon-com- 

 pound, namely starch. By applying this test it is found 

 that in water containing no C0 2 the Spirogyra soon loses 

 its starch, which reappears when C0 2 is added to the 

 water. 



The same tests are of value in determining the conditions 

 under which assimilation of carbon from C0 2 can be carried 

 on. Thus no green plant can live permanently in darkness. 

 Even dull light is injurious, as may be seen in the 

 dwarfed miserable appearance of shrubs, etc. growing in 

 deep shade as compared with specimens in brighter light. 

 Here again the starch test is of value. If a green plant 

 is placed in the dark it soon loses the starch it possessed, 

 even though the water in which it lives contains C0 2 : 

 and the starch will not re-appear until the plant is once 

 more exposed to light. 



On the other hand a green plant can feed on sugar in 

 darkness, so that light seems to be a condition especially 

 connected with the extraction of carbon from CO 2 . The 

 fact is that the chloroplasts which give the green 

 colour to plants are machines, the motive power of which 

 is the energy of light, and whose special quality is the 

 power of robbing C0 2 of its carbon. 



It is easily proved that this power resides in the 

 chloroplasts. In the leaves of variegated plants are 



