XIII. HOW PLANTS BENEFIT AND HARM MANKIND 



Problem XXII. The relations of fungi to man. (Laboratory 

 Manual, Prob. XT//.)' 

 (a) Yeast, 

 (&) Other fungi. 



The Economic Value of Plants. Besides the other relations 

 existing between plants and animals, there is a relation between 

 man and plants measurable in dollars and cents. Plants are of 

 direct value or harm to man. We call this an economic relation. 

 We have seen how they supply him with his cereals and flour, 

 his fruits and garden vegetables, his nuts and spices, his beverages 

 and the sugar to sweeten them, his medicines and his dyestuffs. 

 They supply the material out of which many of his clothes are 

 made, the thread with which they are sewed together, the paper which 

 covers the package in which they are delivered, and the string with 

 which the package is tied. The various uses of the forest have been 

 mentioned before; the need of trees to protect the earth, their 

 usefulness in the holding of the water supply, their direct economic- 

 importance for lumber and firewood. Many of us forget, too, that 

 much of the energy released on this earth to man as heat, light, or 

 motive power comes from the dead and compressed bodies of plants 

 which thousands of years ago lived on the earth and now form coal. 

 Plants are thus seen to be of immense direct economic importance 

 to mankind. 



The Harm Plants Do. Unfortunately, plants do not all benefit 

 mankind. We have seen the harm done by weeds, which scatter 

 their numerous seeds far and wide or by other devices gain a foot- 

 hold and preempt the territory which useful plants might occupy 

 were they able to cope with their better-equipped adversaries. 

 Plants with poisonous seeds and fruits are undoubtedly responsi- 

 ble for the death of many animals. 



But by far the most harmful plants to mankind are the fungi. 



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