CHAPTER XII 



THE CARBOHYDRATES 



The two noblest things ... are sweetness and light. 



— Swift. 



The carbohydrates make up the bulk of the dry matter of the 

 plant. They are the first products of photosynthesis, and it is 

 to be expected that they will play a fundamental part in the nutri- 

 tion and life of the plant. The word implies that carbohydrates 

 are composed of carbon and water, i. e., they contain carbon, 

 hydrogen, and oxygen, the latter two elements in the same propor- 

 tions as in water. Not all, however, have twice as many hydrogen 

 as oxygen atoms: rhamnose has the formula C 6 Hi 2 5 . Acetic 

 acid, on the other hand, with the formula C 2 H 4 2 is not a car- 

 bohydrate, even though the hydrogen and oxygen atoms are in 

 the same proportion as in water. 



The carbohydrates are mostly open chain compounds, with the 

 carbon atoms linked together in the manner of compounds of 

 the aliphatic series. From the point of view of the chemist they 

 may also be considered as aldehyde or ketone derivatives of al- 

 cohols. (Compare the formula of glucose which is an aldehyde 

 with fructose which is a ketone.) 



Use of Carbohydrates.— The carbohydrates are widely dis- 

 tributed in plants and, as mentioned above, make up the bulk of 

 their dry matter. They have in general five functions: 



1. They are used in building up the cell walls. Cellulose is a 

 carbohydrate which composes the bulk of certain tissues such as 

 wood and straw. The carbon burned in wood is the carbon of the 

 carbohydrate and comes mostly from cell walls. 



2. They supply energy. Sugars and starches are the chief energy 

 producers of the plant and make up the greater part of the food 

 placed in special storage organs, such as roots tubers, and many 

 seeds. They are also the basis for the construction of fats and pro- 

 teins, which are the other food sources of energy and building 



materials. 



3. They hold water, and in many desert plants, in the form of the 

 pentosans, play a prominent role in conserving a scanty water 



supply. 



130 



