244 PLANT AND ORGANIC CHEMISTRY 



The function of chlorophyll may be briefly stated : * It ab- 

 sorbs certain rays of light, and thus enables the protoplasm of 

 the cell to avail itself of the radiant energy of the sun's rays for 

 the construction of organic substances from carbon-dioxide 

 and water. 



Plants which are grown in the dark, or at low temperature, 

 are usually of a yellow color. Such plants are said to be etio- 

 lated. There is reason to think that this yellow substance, 

 etiolin, is an intermediate substance in the formation of chlo- 

 rophyll, for if it is exposed to light it is converted into a green 

 color; these complex coloring- matters are probably derivatives 

 of protoplasm. 



The autumnal change of leaves is owing to a third coloring- 

 matter, called xanthophyll; in many cases, the leaves also con- 

 tain a red coloring- matter, erythrophyll. 



The importance of chlorophyll in the plant will be admitted 

 when it is said that the absorption of carbon-dioxide, the evo- 

 lution of oxygen, and the formation of many organic substances 

 are effected solely by chlorophyll corpuscles. 



The organic acids occur in plants free and also in combina- 

 tion with bases. It is to the presence of these bodies that the 

 acid reaction of plant tissues is due. Some organic acids are 

 assimilated by plants ; the turgidity of cells is to be ascribed to 

 their presence, and the acid sap in root hairs renders possible 

 the solution and absorption of mineral substances insoluble in 

 water. 



The primary function of the resins 2 of Coniferae and analo- 

 gous juices of other plants is to render service in cases of injury, 

 and, by covering the wound with a protecting coating, to favor 

 its healing. 



During my studies on the Yucca, 3 resins and saponin were 

 separated from each part of the plant. Experiments were made 

 to determine the emulsive power of saponin on resins. It was 

 found that aqueous solutions of saponin were able to emulsify 



1 Cambridge edition, p. 157. 



2 H. De Vries, Chem. Centr. Bl., III, xiii, 565 (Jour. Chem. Soc., 1883, p. 



)- 



3 Trans. Am. Phil. Soc., Dec. 1885. See ante, p. 126. 



