126 PLANTS AND MAN 



constant battle to save valuable plants enlists the services of an 

 army of trained specialists, an entire government department, 

 and requires annual expenditures of millions of dollars. 



Other harmful kinds of plants are those which produce 

 poisons (fig. 81). These are remarkably few, in comparison with 

 the total number of useful or harmless plants. Most of them are 

 found among the mushrooms, the most familiar examples being 

 the various species of Amanita. Some of these produce powerful 

 toxic substances which can cause human death. A few of the 

 Angiosperms likewise produce poisons, usually in the fruits or 

 seeds. Many of these poisons, such as that found in the opium 

 poppy, are used as drugs and have considerable medicinal 

 value. In other cases the poisonous substances are merely annoy- 

 ing, as in the irritating oils produced by the vegetative parts of 

 poison oak, poison ivy or poison sumac (all members of the genus 

 Toxicodendron). When a plant is not poisonous, yet persists in 

 growing where it is not wanted, it becomes one of those per- 

 nicious weeds which frequently makes cultivation of desired 

 plants a constant battle. Botanically, there is no distinction of a 

 weed; in many cases the plant considered a weed in one locality 

 becomes— usually because of its rarity — a prized ornamental in 

 another. 



In this and the preceding chapters we have discussed the 

 general considerations of what a plant is, how plants are struc- 

 turally fitted to get a living and reproduce their own kind, the 

 variety of plants which exist and the outline of their importance 

 to man. With this as an introduction we can proceed to a more 

 detailed consideration of certain aspects of these relations be- 

 tween plants and man. 



I 



