40 GROWTH OF THE PLANT 



amides, etc. Another class of plants is called perennials, 

 which live on from year to year, bear seeds from time to 

 time, and also maintain life by means of succulent storage 

 organs. Trees are plants of this kind. Their reserve car- 

 bohydrates and other material are stored just under the 

 bark in the so-called cambium layer. 



III. FUNCTIONS OF THE ESSENTIAL ELEMENTS 



36. Value of a Study of These Functions. — In the previous 

 discussion of plant growth something has been stated about 

 how the principal plant constituents are manufactured, 

 but only in part has been noted the specific function played 

 by each of the essential elements in this process. 



A study of the physiological function of each of the essential 

 elements is of practical importance in the proper feeding 

 of a crop with fertilizers. It is possible by judicious feeding 

 to produce special, desirable results, and the more facts 

 known about the effect of fertilizers, the more economically 

 can they be applied. As yet our knowledge of the subject is 

 not very extensive, although some progress has been made. 



37. Carbon. — This element is the principal constituent 

 (45 per cent.) of the dry matter of plants. All of the com- 

 pounds manufactured by plants contain carbon. Every 

 naturally occurring organic compound owes its carbon content 

 directly or indirectly to the activity of plants. Whether 

 these compounds are tissues of the animal body, or coal 

 and oil in the earth, all of them are originally from plants. 

 Animals use as food either other animals or plants, and thus 

 directly or indirectly are dependent on plants as the source 

 of carbon. Coal and oil in the earth are decomposed remains 

 of plants. The very numerous "coal tar derivatives" were 

 originally plant compounds, decomposed in part by heat 

 and pressure within the earth, in part by the chemist. 



38. Hydrogen. — This is another of the elements which is an 

 essential constituent of plant compounds. Every organic com- 

 pound in plants, except neutral oxalates, contains hydrogen. 

 And yet in spite of its wide distribution, plants average only 



