142 INFLUENCE OF CROPS UPON EACH OTHER. 



various substances from the soil, but impart to it a portion of the 

 juices, which they have formed within themselves. A well- 

 marked instance of this is the Oak ; which so completely im- 

 pregnates the soil around its roots with tannin (the substance 

 which gives to oak-bark its peculiar power of converting animal 

 skin into leather), that few trees will grow in the spot from 

 which it has been rooted up ; since this agent, even when a very 

 minute quantity of it is dissolved in water, produces an effect 

 like tanning upon the delicate tissue of the spongioles, and 

 destroys their peculiar properties. It is probable that every 

 species of forest-tree produces a similar effect ; since it is well 

 known that, when a wood .composed of one kind has been cleared 

 by the hatchet or by fire, the new growth which soon springs 

 up, is not of the same, but of a different species. Again, some 

 of the plants which are known as the rankest weeds, secrete 

 from their roots substances equally injurious to plants around 

 them ; thus the Poppy tribe impregnates the soil around with a 

 substance analogous to Opium, which is easily shown by expe- 

 riments, to have as injurious an effect upon Plants, as an over- 

 dose of this powerful medicine has upon Animals ; and the 

 Spurge tribe exudes an acrid resinous matter. 



210. The Excretions of all Plants seem injurious to them- 

 selves, as well as to others of the same species grown in the 

 same spot ; and in many instances, as in those just quoted, they 

 are injurious to plants of other tribes also. But there are many 

 instances, in which they are absolutely beneficial to plants of dis- 

 tinct tribes. Thus most of the Leguminous tribe exude from 

 their roots a matter analogous to gum ; as may be easily shown, 

 by growing a pea or bean in water, which soon becomes turbid ; 

 and this product is beneficial to plants of almost every other 

 tribe. Hence, therefore, the benefit which the farmer derives, 

 from taking off a crop of beans between two crops of corn, is not 

 restricted to the value of the former ; since the succeeding crop of 

 corn is absolutely improved by this proceeding. And, on the 

 other hand, the exhaustion of the soil by rank-growing weeds 

 is not their only evil ; since they impart to it some of their own 

 injurious properties. 



