CHAPTER XVI. 



'CARBON-FIXATION,' 'ASSIMILATION/ OR 

 ' PHOTOSYNTHESIS.' 



I. The source from which plants obtain the large quantity of 

 carbon of which more than half their dry weight consists, has 

 been the subject of extensive investigation for a long time. 



Parasitic plants, such as dodder, broomrape and many fungi, 

 attach themselves to other living organisms and absorb the 

 carbon they need in the form of sugar, proteids and other 

 elaborated carbon compounds from their victims. Saprophytes, 

 such as the bird's-nest orchis {Neotiia), mushrooms, and the 

 majority of common fungi, which like the above-mentioned 

 parasites are devoid of chloroplasts, obtain their carbon in a 

 similar elaborated form from the carbon compounds present in 

 the remains of dead plants and animals upon which they grow. 



It is probable also that all green plants absorb and utilise 

 organic carbon compounds from the humus or decaying vegetable 

 and animal remains within the soil, although it has been proved 

 that this source is insufficient to supply all the carbon needed 

 for the perfect healthy nutrition of plants of this kind. 



By the method of water-culture or sand-culture it may be 

 readily shown that ordinary green plants flourish and increase 

 in carbon-content when their roots are supplied with a solution 

 of food-materials containing no carbon, so long as the solution 

 contains all other essential elements. 



Under these circumstances the only source of carbon is the 

 carbon dioxide of the atmosphere surrounding the leaves, and 

 although the proportional amount of this gas present in the 



