PROBLEM 2. Otir Relationship to Other Organisms 



385 



Fig. 334 These tall stalks of 

 vmllein, a covivion weeJ, 

 compete with tree seedlings 

 at the edge of a forest. Why 

 do we think of them as 

 weeds? (Brooklyn botanic 

 garden) 



chemical preservatives such as benzoate 

 of soda are used commercially. To pro- 

 tect us against the use of harmful chemi- 

 cals the Food, Drug, and Cosmetic Act 

 was passed. Heat is used in pasteurization 

 and in canning. Extreme heat kills bac- 

 teria. If appHed while food is in an air- 

 tight can, there will be no further bac- 

 terial action. 



While we have learned much about 

 food preservation, you know that there 

 is a constant struggle to keep our foods 

 from spoiHng, We struggle, also, for the 

 preservation of other articles. Against 

 the decay of wood we use paint and such 

 disinfectants as creosote. None of the 

 methods thus far discovered is perfectly 

 satisfactory and all take time and cost 

 monev. To review this paragraph do 

 Exercise 5. 



Weeds are harmful to us. Our desire to 

 raise only the plants we choose makes us 

 regard some plants as weeds. You prob- 

 ably know the ragweeds, tarweeds, bind- 



weeds, tumbleweeds, thistles, and many 

 other weeds. Why do we call them 

 weeds? 



A weed is best defined as a plant that 

 is not wanted or a plant that is out of 

 place. A corn plant is a weed if it is 

 growing in a field of wheat. In general 

 a weed is a plant that is prolific (repro- 

 duces in large numbers) and that is well 

 fitted to grow in many environments. 

 For these reasons weeds are so common 

 in gardens and fields. Weeds may be 

 edible and usable. Milkweed and purs- 

 lane, two very common weeds in many 

 parts of the country, are sometimes 

 eaten. Since plants that are not wanted 

 take useful minerals from the soil, they 

 can cause serious financial loss to the 

 farmer. The farmer spends large sums 

 of money in labor and time to destroy 

 the weeds that interfere with his desired 

 crops. Do Exercise 6 to find out why 

 the unwanted plants we call weeds are so 

 common. 



