YEGETATITE OKGANS OF PLANTS. 307 



guishable in shape, they are but little unlike in other 

 external characters. 



In forest trees, we find the most obrioas and striking 

 differences between -the stem and leaves. 



Color of Leaves. — A peculiarity most character- 

 istic of the leaves of the higher orders of plants, so long 

 as they are in vigorous discharge of their proper vegeta- 

 tive activities, is the possession of a green color, due to 

 the presence of Ghlorophyl. (See p. 124.) This color 

 is also proper in most cases to the young bark of the 

 stem, a fact further indicating the connection between 

 these parts, or rather demonstrating their identity of 

 origin and function, for it is true, not only in the case 

 of the cactuses, but also in that of all other young 

 plants, that the green (young) stems perform, to some 

 extent, the same offices as the leaves, the latter being, in 

 fact, growths from and extensions of the bark. 



The loss of green color that occurs in autumn, in the 

 foliage of our deciduous trees, or on the maturing of the 

 plant, as with the cereal grains, is related to the cessa- 

 tion of growth and death of the leaf, and results from 

 chemical changes in the chlorophyl-pigment. 



Plants naturally destitute of chlorophyl, like Indian 

 pipe {Monotropa), Dodder {Guscuta), Mushrooms, 

 Toadstools, and fungi generally, are parasites on living 

 or dead organisms, from which they derive their nour- 

 ishment. Such plants cannot construct organic sub- 

 stances out of inorganic matters, as do the plants having 

 chlorophyl. 



When leaves, ordinarily green, are totally excluded 

 from light, or develop at a low temperature, they have a 

 pale yellow color; on exposure to light and warmth they 

 become green. In both cases the GhloropTiyl-granules 

 are formed, but the chlorophyl-pigment appears only in 

 the latter. In absence of iron, leaves are white, contain 

 no chlorophyl granules, and growth is arrested. 



