178 A TEXT-BOOK OF BOTANY. 



ently from tannin toward other reagents, such as organic acids, 

 alkalies, lime water, and solution of alum. 



An examination of the color-substances of a large number of 

 plants shows that the flower color-substances are distributed in 

 all parts of the plant. For example, the flower color-substance of 

 the rose occurs in the leaves and prickles as well as in the petals. 



The color-substance in the root of the radish closely corre- 

 sponds to that in the flowers, while the one in the grains of black 

 Mexican corn corresponds to that in corn silk. 



The cell-sap color-substances are usually found in greatest 

 amount at the tips of the branches, this being well marked in the 

 foliage of the rose, and may be said to be rather characteristic 

 of spring foliage. Not infrequently in the purple beech the young 

 leaves will be of a distinct purplish-red color and almost entirely 

 free from chlorophyll, suggesting a correspondence in position 

 and color to a flower. 



Color in Autumn Leaves. The coloring matters in both 

 spring and autumn leaves closely resemble the cell-sap color- 

 substances of flowers, although it is the spring leaves which give 

 the most satisfactory results when examined. The fact that in 

 the autumn leaves there is little or none of the plastid pigment 

 present would point to the conclusion that the color-substances 

 occurring in these leaves are in the nature of by-products and of 

 no further use to the plant. Of course, in the case of autumn 

 leaves, we know that these products cannot be further utilized 

 by the plant, and for this reason we are justified in regarding 

 them as waste products. 



So-called White Colors. The so-called white colors in plants 

 do not properly belong to either class, but may be said to be 

 appearances due rather to the absence of color, and depending 

 upon the reflection of light from transparent cells separated by 

 relatively large intercellular spaces containing air. In other words, 

 the effect produced by these cells may be likened to that produced 

 by the globules in an emulsion. The white appearance is most 

 pronounced in the pith cells of certain stems, where on the death 

 of the cells the size of the intercellular spaces is increased and 

 the colorless bodies in the cells as well as the walls reflect the 

 light like snow crystals. 



