FOLIAGE LEAVES: THE LIGHT-RELATION. 7 



do not develop this color. Even when leaves have devel- 

 oped the green color they lose it if deprived of light, as is 

 shown by the process of blanching celery, and by the ell'ect 

 on the color of grass if a board has lain upon it for 

 some time. It seems plain, therefore, that the green colur 

 found in working foliage leaves depends upon light for its 

 existence. 



We conclude that at least one of the essential life-rela- 

 tions of a foliage leaf is what may be called tlic lighl-rela- 

 lion. This seems tn explain satisfactorily why such leaves 

 are not develojied in a subterranean position, as are many 

 steins and most roots, and why plants Avhich produce them 

 do not grow in the dark, as in caverns. The same green, 

 and hence tlic same light-relation, is oljserved in other 

 parts of the plant as well, and in }ilants without leaves, the 

 only difference being that leaves display it most conspicu- 

 ously. Another indication that the green color is con- 

 nected with light may Ijc obtained from the fact that it is 

 found only in the surfac'e region of plants. If one cuts 

 across a living twig or into a cactus body, the green color 

 will be seen only in the outer part of the sertion. The con- 

 clusion is that the leaf is a s}iecial organ for the light-re- 

 lation. Plants sometimes grow in such situations that it 

 would be unsafe for them to display' leaves, or at least large 

 leaves. In such a case the work of the leaves can be thrown 

 upon the stem. A notable illustration of this is the cactus 

 plant, which produces no foliage leaves, but whose stem dis- 

 plays the leaf color. 



11. An expanded organ. — Another general fact in refer- 

 ence to the foliage leaf is that in most cases it is an expanded 

 organ. This means that it luis a great amount of surface 

 exposed in comparison with its mass. As this form is of 

 such common occurrence it is safe to conclude that it is in 

 some way related to the work of the leaf, and that whatever 

 work the leaf does demands an exposure of surface rather 

 than thickness of body. It is but another step to say that 

 2 



