CELL-CONTENTS AND FORMS OF CELLS. 179 



CHEMISTRY OF PLANT COLOR PRINCIPLES. The substances 

 giving colors in plants may be divided into two classes: I. Sub- 

 stances having a specific color, as chlorophyll in leaves, chromo- 

 phyll in flowers and fruits, and anthocyanin, the cell-sap color 

 in flowers. 2. Substances which, while they themselves are color- 

 less and known as leuco-compounds, yet form color derivatives. 

 Some of these are more or less readily oxidized, forming charac- 

 teristic color-substances, as brazilin. Again, some occur as gluco- 

 sides which through the action of ferments yield color-substances, 

 as quercitrin, which yields the yellow pigment quercetin. All of 

 the researches which have been made seem to indicate that the 

 color in plants is due to a basic substance or nucleus, and that 

 the variation in color is due to the nature and arrangement of 

 certain groups and side chains. For example, benzo-quinhydrone 

 is green in color, while thymo-quinhydrone is of a purplish color 

 (Pharm. Review, 1908, p. 330). Again, the hydroxybenzoquinones 

 vary in color with the number of hydroxy groups, hydroxybenzo- 

 quinone being of a light yellow, di-hydroxybenzoquinone is of a 

 dark yellow, tri-hydroxybenzoquinone is of a black color, and 

 tetra-hydroxybenzoquinone is of a bluish-black color. 



In the artificial dyes there are a number of so-called chromo- 

 phores or chromophorous groups (radicals) characteristic of the 

 various classes. In plant color-substances, on the other hand, we 

 find the carbonyl group (C = O), the imido group (NH), and 

 C = C group. The introduction of the chromophorous radical 

 gives a basic compound which is more or less colored and known 

 as a chromogen. The intensity of the color varies on the intro- 

 duction of certain salt-producing groups or auxochromes. In 

 the case of plant pigments, the most important of these is the 

 hydroxyl group, and, as we have just seen, the intensity of color 

 varies according to the number of these groups. 



According to their constitution, plant color principles may be 

 arranged into six different classes: i. Phenol derivatives, includ- 

 ing orcin, the coloring principle in many lichens, that forms color- 

 less prisms which become red ; and thymo-quinone found in 

 Monarda (Kremers, in Pharm. Rev., 1908, p. 329). 2. Naphtha- 

 lene derivatives, including juglon, which forms garnet-colored 

 needles. 3. Anthracene derivatives, including alizarin, the color- 



