FOLIAGE LEAVES : THE LIGHT-RELATION 7 



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

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

 shown by tlie process of blanching celery, and Ijy tlie effect 

 on the color of grass if a Ijoard has lain npon it for 

 sonic time. It seems plain, therefore, tliat the green color 

 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 the JiijJtl-rela- 

 tion. Tills seems to explain satisfactorily why such leaves 

 are not developed in a subterranean position, as are manv 

 stems and most roots, and wliy plants M'liich produce them 

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

 and hence the same light-relation, is ol;>servcd in other 

 jiarts of the plant as well, and in plants without leaves, the 

 only ditference being that leaves display it most conspicu- 

 ouslv. Another indication tliat the green color is con- 

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

 found only in the surface region of plants. If one cuts 

 across a living twig or into a cai.'tus body, the green color 

 will be seen only in the miter part (if the sectimi. The con- 

 clusion is that the leaf is a special organ for tlie 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 tlie foliage leaf is that in most cases it is an expanded 

 organ. This means tliat it has a great amount of surface 

 exjjosed 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 



