154 THE CARBOHYDRATES 



presence of formaldehyde by oxidizing it to formic acid, which 

 was recognized by the usual tests. 



A critical review of the investigations regarding the 

 presence of formaldehyde in plants has been published by 

 Polacci.* 



PHOTOSYNTHESIS. 



The formation of carbohydrates takes place in living chloro- 

 phyll-containing cells on exposure to sunlight : the initial 

 substances are the carbon dioxide of the atmosphere and the 

 water contained within the cells ; the end products are oxygen 

 and carbohydrate, the volume of oxygen evolved being prac- 

 tically equal to the volume of carbon dioxide used up ; the 

 processes take place with extraordinary rapidity, oxygen being 

 evolved almost immediately after exposure to light, and the 

 carbohydrate appearing very quickly afterwards. 



This rapidity of action means that, whatever the inter- 

 mediate products may be, they must be very transient, so that 

 small amounts rather than large quantities are to be looked for 

 in healthy leaves which are actively engaged in photosynthesis. 

 It appears from the investigations of Friedel f that the quantity 

 of oxygen of the air is immaterial to the process. The amount 

 of this oxygen may be decreased to 2 per cent or increased 

 to 50 per cent without interfering with the photosynthetic 

 processes. 



In the laboratory it is convenient to take the presence of 

 starch as an indicator of the fact of photosynthesis in micro- 

 scopic experiments ; but the first recognizable products of 

 photolysis are soluble carbohydrates, which may, later on, be 

 converted into starch, as in the potato, bean, etc., or may re- 

 main as sugar, as in Allium, Scilla, and many other plants. In 

 these latter, starch may be formed provided the sugar in the 

 leaves be allowed to become concentrated. 



In addition to sugars, other compounds such as mannite, 

 oils, tannin, and organic acids have also been described as 

 direct products of photosynthesis ; whether they have been 

 so described on sufficient grounds or not, their occurrence in 



* Polacci : " Atti. R. Ace. Lincei," 1907, [v.], 16, i. 199. 

 t Friedel: " U.S. Dept. Agric," 1901, Bull. 28. 



