THE COMPOSITION OF PLANTS. 31 



in grapes, citric acid in lemons, and malic acid in 

 apples. These vegetable acids are numerous, but 

 all have the same elements, carbon, hydrogen, and 

 oxygen, that are found in sugar. 



59. Fats and oils, such as are found in olives, 

 cotton-seed, flax-seed, etc. These are composed of 

 the same elements as sugar, but have proportionally 

 less oxygen. Turpentine, resin, and different kinds 

 of wax, are included in this group. 



60. Albuminoid, or protein bodies, which dif- 

 fer from the group mentioned, in having nitrogen 

 as a constituent, and in some cases sulphur and 

 phosphorus. Albumen is found nearly pure in the 

 white of an egg, and a similar substance is contained 

 in the juices of plants. The term " albumen " is used 

 by botanists to mean any nutritive material found in 

 the seeds of plants without reference to its chemical 

 composition. Chemists apply the word only to sub- 

 stances containing nitrogen, and in this sense it is 

 here used. Gluten, a sticky substance found in 

 flour, belongs to this class, and vegetable casein, 

 similar to the white curd of milk, found in legu- 

 minous plants, such as the pea and bean. 



61. Notwithstanding the great variety of these 

 proximate principles, and their great difference in 

 physical and chemical properties, they are composed 

 of only a few elements. A knowledge of their chemical 

 constitution has very greatly simplified the changes 

 which take place in the growth and maturity of plants, 

 and shown that a very slight alteration in chemical 



