100 TEXTBOOK OF BOTANY 



why discovery of the chemistry of Hving organisms has progressed so 

 slowly is the presence of such large complex molecules. Another reason 

 is that within the colloidal system of protoplasm with its surface energy 

 and electrical energy there are many chemical transformations that are 

 dijBficult or impossible to duplicate in the laboratory. Furthermore, growth 

 hormones and other substances may affect life processes in a most strik- 

 ing way when present in amounts too small to be detected by the or- 

 dinary methods of chemical analysis. Owing to the complexity of the 

 problems of biochemistry, progress is slow; but students of this phase of 

 biology may well be proud of the advances made during the last half- 

 century. 



Elements found in plants. About half of the known chemical elements 

 have been found in plants through chemical analyses of a great variety 

 of specimens. But this fact is of little significance. The roots of a plant 

 are in contact with the water in the soil, and any soluble substance in 

 the soil water is likely to pass into the water within the plant and thus 

 be reported in a chemical analysis of the plant. On the other hand cer- 

 tain of these elements are known to be in the compounds of which the 

 plant is made. First in abundance are carbon, hydrogen, and oxygen 

 which occur in most plant compounds. Nitrogen and sulfur occur in all 

 known plant proteins. Phosphorus occurs in some proteins and in various 

 lipoids (phosphatides). IMagnesium is one of the elements in the 

 chlorophyll molecule. Calcium forms salts with the acid substances, such 

 as pectic acid in the cell walls of plants. Any other metal may similarly 

 form salts with acid substances in plants, but its presence may not be 

 essential to the plant. Finally there are a few elements — namely, po- 

 tassium, iron, manganese, boron, and sometimes copper and zinc — which 

 are essential to the development of plants; but their definite relations to 

 plant processes are still inadequately known. Some of them are toxic to 

 the plant except when present in very minute amounts. 



A list of the elements essential to the development of plants tells us 

 little about the chemical composition of plants. Thev do not occur in 

 plant tissues as free elements, but owe their importance to the part they 

 play in the formation of compounds and the processes they aflFect. With 

 the exception of oxygen they are not absorbed and used by green plants 

 as elements. Thev enter the plant either in the form of compounds or 

 as dissociated ions of compounds. A part of the oxygen used by plants 

 also enters the plant in the form of the compounds HiO and CO2. 



