I.] MICROSCOPIC PLANTS. 13 



we can easily find out what gas it is. We test it, and 

 find it to be carbonic acid gas. How is this? There 

 was no carbonic acid gas in the fluid. We analyse 

 the fluid again, and find that the sugar has dis- 

 appeared. And so we proceed with our experiments ; 

 and, in the end, see clearly that the oxygen has been 

 taken out of the sugar by the Torula, and the car- 

 bonic acid gas set free ; in fact, it has been feeding 

 upon the sugar. Now Protococcus is able to get its 

 food from the carbonic acid gas, and sets free oxygen. 

 Torula cant obtain its oxygen from carbonic acid 

 gas, or it would have multiplied in the rain-water. 

 There is evidently some remarkable difference in 

 these plants. Protococcus possesses chlorophyll, by 

 means of which it is able under the influence of 

 sunlight to decompose carbonic acid gas. Torula 

 does not possess chlorophyll, and therefore has to get 

 its food from a substance already formed by plants. 

 This it finds in the sugar, which contains both carbon 

 and oxygen. Torula exists as well in the dark as 

 in the light ; Protococcus cannot exist in the dark. 

 If we apply a small quantity of iodine to both, we 

 find that Torula remains unchanged, while Proto- 

 coccus turns blue. This denotes that it contains 

 starch, a substance peculiar to green plants ; on the 

 other hand, Torula agrees with the Fungi in possess- 

 ing neither starch nor chlorophyll, and in being 

 independent of light. Protococcus we may take as 

 a simple type of the green plants, and Torula of the 

 Fungi. 



If we take a tiny drop of the yeast on the head 

 of a pin, touch a glass slide with it, and, carefully 



