160 LIVING PLANTS 



structed at greatexpenseof energy, and which 

 is in a form of the highest possible usefulness 

 to the plant. To this class belong the com- 

 pounds in the protoplasm, the green color 

 bodies, and whatever surplus food may not 

 have been previously conveyed away. The 

 substances which the plant must needs dis- 

 card are in the form of nearly insoluble crys- 

 tals, and, by remaining in position in the leaf, 

 drop with it to the ground and pass into 

 that great complex laboratory of the soil 

 where by slow^ methods of disintegration, use- 

 ful elements are set free and once again may 

 be taken up by the tree and travel their 

 devious course through root hairs along the 

 sinuous roots and up through million-celled 

 columns of the trunk out through the twigs 

 to the leaves. 



The plastic substances within the leaf which 

 would be a loss to the plant if thrown away, 

 undergo quite a different series of changes. 

 These substances are in the extremest parts 

 of the leaf, and to pass into the plant body, 

 must penetrate many hundreds of membranes 

 by diffusion into the long conducting cells 

 around the ribs or nerves, and then down 

 into the twigs and stems. The successful re- 

 treat of this great mass of valuable matter is 

 not a simple problem. These substances con- 

 tain nitrogen as a part of their compounds 



