72 ORGANIC COMPOUNDS IN PLANTS. 



peculiar, green, coloring matter. This green coloring 

 matter is formed under the immediate action of light, 

 and its depth of color seems to depend upon the inten 

 sity of the light. Hence the innumerable shades of 

 green, from the delicate yellowish green of early spring 

 to the deep greens of midsummer ; and hence the striking 

 changes in the color of leaves, after some days of cloudy, 

 warm weather, when succeeded by clear sunshine. 



258. The yellow leaves in autumn contain proportion 

 ally more wax than the green leaves of summer, and the 

 yellow rinds of ripe fruits more than the green rinds of 

 unripe fruits. The rich, gorgeous colors of the autumnal 

 foliage have been attributed to the action upon chlorophyl 

 of various vegetable acids and alkalies, under the influ 

 ence of the sun s light. They are not produced by frost. 



259. From the roots, wood, bark and leaves of various 

 plants are extracted very many coloring 1 substances used 

 in the arts. Certain plants, as, for example, the indigo 

 plant and woad, are cultivated extensively, in some coun 

 tries, for this very purpose. 



260. Tannin. This is the substance with which tan 

 ners convert the hides of animals into leather. It is 

 found in the bark of several kinds of oak, and also of 

 hemlock, spruce and some other trees of the Pine Family, 

 and in the leaves of tea and of some plants of the Heath 

 Family. It is of a sourish, astringent taste, and has this 

 remarkable property of converting the animal gelatine 

 of the skin into leather. Tannin is found only in the 

 older wood and bark, and is supposed to be formed by 

 the commencement of decay in cellulose. 



261. How the vital principle in plants, with the agency 

 of the osmotic power and chemical attraction, forms the 



