TFIE NUTRITION OF PLANTS 153 



pListid uses this energy in tlie m'ui-I< (j! jilmios^-nthesis. It 

 is evident, tlierefore, tliat pliotosjni thesis goes (ni only in 

 the sunlight, and is suspended entirely at night. It is 

 found that any intense light eau lie used as a suljstitute 

 for sunlight, and plants have Ijeen deserved io eany on 

 the work of photosynthesis in the presence of electric 

 light. 



(Ij) Ih'gulf of j)ho/os///it/icsis. — 'I'he result ol: (his work 

 can bo stated only in a very general way. Carh(jn dioxide 

 is composed of two elements, carbon ajid oxvgen, in the 

 proportion one part of carbon to two parts tif nxygen. 

 A\ ater is a.lso composed of two elements, hydrogen and oxv- 

 gen. In i)hotosynthcsis the elements composing these sulj- 

 stances are separated from one auothei', and rceombined in 

 a new way. In the jirocess a certain amount (^f oxygen is 

 liberated, just as much as was in the I'arbou dioxide, and a 

 new substance is formed, known as a carboliydrato. The 

 oxygen set free escapes from the plant, and may be re- 

 garded as waste product in the process of pliotosvnthesis. 

 It will be remembered that the external changes in this 

 firooess are the absorjitiou of carbon dioxide and the giving 

 otf of oxygen (see §35). 



(4) Carbohydrates and jiroteids. — The carbohydrate 

 formed is anorganic substance; that is, a substance made 

 in miture only by life processes. It is the same kind of 

 substance as sugar or starch, and all are known ascarljoliy- 

 drates ; that is, substances composed of carljon, and of hy- 

 drogen and oxygen in the same proportion as in water. 

 The work of photosynthesis, therefore, is to form cai-l)ohy- 

 drates. The cai'boliydrates, such as sugar and starch, rep- 

 resent but one ty]ie of food nraterial. Proteids represent 

 another jiromiueiit type, substances Avliicli contain carbon, 

 hydrogen, and oxygen, as do carbohydrates, but which also 

 contain other elements, notably nitrogen, suljihur, and 

 j)hosiihorus. The white of an egg nuiy lie taken as an ex- 

 ample of proteids. They seem to be made from the carbo- 



