70 BOTANY: PRINCIPLES AND PROBLEMS 



supplies present in the complex organic materials of humus, can 

 apparently be drawn upon by ordinary green plants. Carbon, 

 oxygen and hydrogen, together with the seven essential elements 

 derived from the soil, constitute the necessary chemical basis for 

 plant life. 



Mechanism. — The mechanism or apparatus by which water and 

 carbon dioxide are combined is the remarkable green pigment 

 chlorophyll. This is present only in the chloroplasts, portions of 

 the cytoplasm slightly denser than the rest. Chloroplasts may 

 be few and large in certain lower plants but in the higher ones are 

 almost always small, numerous, and more or less spherical in 

 shape. They are most abundant in the palisade layer of the leaf. 

 As to chlorophyll itself we know comparatively little except that 

 it is a complex protein and contains magnesium. Iron is essen- 

 tial for its production but apparently does not enter into the 

 construction of the substance itself. The presence of light is 

 also necessary for the full development of chlorophyll, as is shown 

 by the pale color of leaves which have grown in darkness. We are 

 even more ignorant as to the manner in which chlorophjdl oper- 

 ates in bringing about the union of carbon dioxide and water, 

 nor have we yet succeeded in imitating this process in the labo- 

 ratory. We know, however, that chlorophyll does not con- 

 tribute material to the product formed and that it is not used up 

 itself in the process, and we may therefore infer that it acts 

 somewhat as does a catalyzer. 



Associated with chlorophyll is usually another pigment or 

 group of pigments, j^ellow in color instead of green, to which the 

 general terms xanihophyll or carotin are given. These are not 

 concerned with photosynthesis and their function is poorly 

 understood. To them are due most of the yellow colors which 

 occur in plants. Chlorophyll is a very unstable compound and 

 tends to break down quickly when extracted from the leaf or 

 when the leaf loses its vitality, but the yellow pigments are much 

 more resistant and often survive long after chlorophyll has 

 disintegrated. 



Energy. — Energy is necessarily expended in the process of 

 breaking up the molecules of water and carbon dioxide and recom- 

 bining their atoms into a new compound. We know that this 

 energy is derived not from heat, as in so many cases, but entirely 

 from light, which thus plays an essential part in the physiology 

 of plants. According to the most widely accepted theory, light 



