526 ECOLOGY 



theories light has been regarded as the direct source of energy, but recently it has 

 been suggested that the absorbed Hght rays are transformed into electricity, which 

 then becomes the direct agent of synthesis. This theory has been given notable 

 support by the experimental demonstration of the reduction to formaldehyde of 

 carbonic acid (formed by the union of carbon dioxid and water) through the opera- 

 tion of a silent electric discharge. Still more recently, formaldehyde has been 

 synthesized in the laboratory by the use of ultra-violet light. The formation 

 of formaldehyde, accompanied by the emission of oxygen, has been observed in a 

 chlorophyll layer deposited from solution on a gelatin plate, which was exposed to 

 light, though it is to be noted that the validity of this observation is called in ques- 

 tion. Some investigators also have claimed that the exposure to sunlight of pulver- 

 ized dead leaves and glycerin leaf extract results in the absorption of carbon dioxid, 

 the emission of oxygen, and the formation of formaldehyde. The data here given 

 make increasingly probable the theory that the manufacture of food from carbon 

 dioxid and water is not of necessity a, vital process, though it must be confessed 

 that the food-making processes of chemical laboratories are as yet crude and im- 

 perfect as compared with those within plant cells. 



Whatever the office of chlorophyll in carbohydrate synthesis, its importance 

 is beyond doubt; yet it is not indispensable, for carotin and xanthophyll play a 

 similar though less important part, and in the brown algae phaeophyll plays the 

 usual r61e of chlorophyll. The blue-green algae apparently do not have ordinary 

 chloroplasts, yet they manufacture carbohydrates, probably through the agency of 

 chlorophyll disseminated with other pigments throughout the cell; until recently 

 the same has been supposed to be true of the purple bacteria, which now are re- 

 garded as dependent organisms. Certain nitrifying bacteria {e.g. Nitrosomonas) 

 and Bacillus oligocarbophilus manufacture carbohydrates without the aid of any 

 pigment. Some chlorophyll, on the other hand, seems to have little or no part in 

 synthesis, as in some orchids and in the ovary of Ornithogalum arabicum. In the 

 green, partially parasitic Scrophulariaceae there have been thought to be stages 

 between functional chlorophyll and chlorophyll with no synthetic rdle.' 



The influence of external factors upon carbohydrate synthesis. — 



Light. — Whatever may be its specific role, light is fundamentally 

 important in the early stages of food-making.- There is a minimum 

 intensity of light below which synthesis is impossible, and a certain 



^ Carbohydrate synthesis now is known to take place in some animals that contain 

 chlorophyll, notably in certain species of flatworms (Convoluta), in which algae living with 

 them symbiotically are believed to be responsible for the food-making. 



2 It has been discovered that plants can utilize small amounts of formaldehyde 

 supplied to their absorptive organs in the manufacture of more complicated carbohy- 

 drates; this process of condensation may take place even in the dark, appearing to 

 indicate that light is necessary only for the reduction of carbonic acid to formaldehyde. 

 Light is unnecessary even for this first step in food-making in some bacteria, as Bacillus 

 oligocarbophilus and Thiobacillus, where energy is derived by the oxidation of sulfur or 

 hydrogen sulfid, and in nitritsring bacteria, where energy is derived by the oxidation of 

 ammonia. In one of these forms, Bacillus pantatrophus, formaldehyde is produced. 



