THE SIMPLEST LIVING THINGS 205 



tion (to cite a commercial instance) in the liqueur known 

 as " creme de menthe," being used to give its fine green 

 colour to that preparation. Sunlight shining on to the 

 green parts of plants is " screened " or " strained " by the 

 leaf-green, so that only some of the coloured rays pass 

 through it, and it is only by this peculiarly " strained " 

 green sunlight that the protoplasm of the cells of the 

 leaf is stimulated to its remarkable chemical activity. 

 The carbonic acid in the air or in the water in which 

 the green plant is living is taken up by the protoplasm. 

 Carbonic acid consists of oxygen and of carbon. The 

 protoplasm, when the green sunlight acts on it, actually 

 takes out of carbonic acid and throws off as a gas (seen 

 as bubbles in the case of a water plant) some of its con- 

 stituent oxygen, thus keeping up the supply of free 

 oxygen in air and water. Then at the same time it 

 combines the carbon and the rest of the oxygen with 

 water (hydrogen and oxygen) inside itself, forming solid 

 starch, which, with the microscope, we can see actually 

 manufactured as little oblong grains in the green cells. 

 Not only this, but the element nitrogen is, so to speak, 

 " forced " in other cells of the plant to combine with the 

 three elements of the newly-formed starch (carbon, 

 hydrogen, and oxygen), and thus the first steps leading 

 to the building up of those wonderful bodies, the proteids, 

 are passed. Nothing of the sort can be done by the 

 protoplasm of an animal cell. 



Consequently we distinguish among the simplest 

 living things those which are provided with leaf-green, 

 and feed, as do the larger green plants, on dissolved 

 " mineral " solids and gases. There are many thousands 

 of kinds of them single simple cells. Some are known 

 to microscopists as Diatoms and Desmids often of 

 curious spindle or crescent-shape, others star-like. The 

 diatoms form on their surface a delicate, wonderfully- 



