118 THE GERM THEORY OF DISEASE. 



to be expected. In many instances, however, it is found that 

 individual plants are poisonous to others growing in their 

 neighborhood. It is rare to see a very large black walnut 

 tree that has not a clearing around it, wherever it may stand 

 in the forest. This is not on account of its shade, but some- 

 thing eliminated by the tree that is hurtful to other trees. I 

 remember well an effort to raise corn on the south side of a 

 row of walnut trees. The experiment was continued for 

 many years. The corn was injured seriously for many feet 

 distant, where it was never shaded by the trees. Very many 

 such instances are known. Enough has been demonstrated 

 to show conclusively, that most of the substances formed in 

 the fermentations are also excreted in very small amounts, by 

 various plants. Even alcohol and ammonia are formed in 

 this way, in small quantities. Although these substances, as 

 a rule, contain no nitrogen, their chemical construction is such 

 as to show their relationship to urea, and leave no doubt that 

 they are properly analogues of that product in the animal 

 kingdom. 



ALKALOIDS. 



By far the most important compounds to be considered in 

 this connection are the alkaloids. These bear the same rela- 

 tion to plants that urea bears to the animal forms. There is 

 seen in their chemical conformation and characteristic prop- 

 erties a marked relationship; and yet among them is found 

 a wide divergence of poisonous effects. All possess, in some 

 degree, the power of intoxication, while a few are the most 

 virulent poisons known. Nearly all are composed, as urea, 

 of carbon, hydrogen, nitrogen, and oxygen. In a few of 

 them oxygen is lacking; and in all the nitrogen is in smaller 

 proportion than in urea. As we descend the scale, the nitro- 

 gen disappears, the oxygen is increased, and we have the 

 organic acids and alcohols. All of these are, properly, waste 



