AN INTRODUCTION TO PLANT BIOLOGY 115 



higher animals all excretory and excess substances containing 

 nitrogen, sulphur, phosphorus, calcium, etc., are in solution 

 in the water eliminated by the kidneys. Land plants usually 

 give off water only by evaporation, and certain excretory 

 and excess matters are left behind in the plant (in the leaves) 

 just as lime is left in a tea-kettle. Plants living in water 

 may have some of these substances absorbed by the surround- 

 ing water. Also, plants may eliminate some substances by 

 the roots, for the roots of some plants (e.g., oat seedlings) will 

 etch or corrode polished marble or limestone by the action of 

 acid substances which osmose from the roots. Possibly these 

 corroding substances enable a plant to dissolve useful minerals. 

 Also, it has been shown quite recently that grasses give off 

 from their roots to the soil some substances which poison 

 and check the growth of young orchard trees.* 



It seems certain that some of the substances formed by 

 the oxidation of cell-substances are stored in the plant, 

 but often changed so as to render them harmless, or even 

 useful, to the plant which produces them. Our knowledge of 

 a large number of peculiar substances found in plants is still 

 very incomplete; but it seems probable that many of them 

 are modified and stored waste products which could not be 

 eliminated along with water as animals eliminate similar 

 substances by the kidneys. Examples of such substances 

 which are not known to be of further use to the plants 

 which contain them, are the active constituents of tea (in 

 leaves), coffee (in seed), cocoa (in seeds), the poisonous sub- 

 stance in the pits (seeds) of almonds, peaches, and plums; 

 quinine from the bark of a tree; nicotine in tobacco plants; 

 strychnine and many other poisons in various plants. These 

 and many other peculiar products of plants contain the ele- 

 ment nitrogen, and some of them are chemically similar to the 



* For an account of this see bulletins published by U. S. Dept. of Agri- 

 culture. 



