84 ELEMENTARY AGRICULTURAL CHEMISTRY 



8.0, 21° at 8.45, and 25-5° at 9.15, 28° about 9.30, and there- 

 after gradually rose to 37° at 11 a.m. 



Obviously an unshaded plant would not only pass more 

 gradually through the transition from the too low night to 

 the too high day temperature, but would be maintained at a 

 temperature near its optimum for a much longer period than 

 the screened plant. Consequently the conditions for its growth 

 would be much more favourable. In fact, there can be little 

 doubt that in countries like the Transvaal many plants 

 actually suffer from excessive sunshine, with its attendant 

 heat, while in less sunny lands — e.g.^ England — they rarely 

 attain even their optimum temperature, and probably never 

 exceed their maximum temperature of growth. 



The Constituents of Plants —The elements present in 

 plants have been enumerated in the first chapter, but of far 

 greater importance are the chemical compounds actually 

 existent in the plant. These compounds may conveniently 

 be grouped as follows : 



.^ 



Non-nitrogenous 

 substances. 



I. Carbohydrates. 

 II. Fats and waxes. 



III. E^scntial oils and resins. 



IV. Oiganic acids and their salts. 

 V. Inorganic salts. 



( VI. Albuminoids or proteids. 

 Nitrogenous sub- J VIT. Amides and amino-compounds. 

 stances. jVlII. Alkaloids. 



t IX. Chlorophyll and othercolouring-matters. 



A brief account will now be given of these substances. 



I. The Carbohydrates. — This is a large group of substances, 

 each of which contains carbon, hydrogen and oxygen, the two 

 latter present in the ratio by weight of 1 : 8, the same ratio as 

 they have in water. Most of them contain some multiple of 

 five or six atoms of carbon in a molecule. They can be sub- 

 divided into two classes — 



(1) the starches, amyloses, or poly saccharoses ; 



(2) the sugars. 



