244 PLANT AND ORGANIC CHEMISTRY 
The function of chlorophyll may be briefly stated : 1 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 3 of Coniferse 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? 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. 
365 )- 
3 Trans. Am. Phil. Soc., Dec. 1885. See ante, p. 126. 
