GROWTH OF WEEDS 101 



under the shade of a tree gives raw food material to the plants, 

 and they must have sunlight in order to make it into food. Over- 

 crowding is often seen in the garden where young beet or lettuce 

 plants are growing. The gardener assists nature by thinning out 

 the young plants so that they may not be handicapped in their 

 battle for life by an insufficient supply of air, light, and food. 



It is evidently of considerable advantage to a plant to be able 

 to place its progeny * at a considerable distance from itself, in order 

 that the young plants may be provided with sufficient space to 

 get nourishment and foothold. Some plants accomplish this, 

 particularly weeds, more completely than others, and thus they 

 are the more successful ones in the battle of life. Besides depriv- 

 ing other plants of soil salts and water, weeds do much harm. 

 Some are poisonous to cattle and sheep, as the loco-weed, hemlock, 

 and laurel. Other weeds, as the wild onion or garlic, may be eaten 

 by cows, and the milk produced will be ruined in flavor. Some 

 weeds are hosts to injurious parasitic insects or fungi ; witness the 

 Hessian fly, which lives in some wild grasses, and the wheat rust, 

 which lives in the barberry. The pollen of the ragweed and of 

 other weeds undoubtedly cause some people to have "hay fever." 



Weeds are introduced often into lawns and fields because their 

 seeds are mixed with the good seeds which are sown there. We 

 should use every method possible to prevent weeds from producing 

 seeds. Poisons are used in some cases ; sheep, which seem to prefer 

 some weeds to grass, are also a great aid in keeping down these 

 pests ; and birds that eat weed seeds are the most valuable of all. 



PROBLEM I. WHAT ARE WEEDS AND WHAT DO THEY DO? 



Weeds are plants that grow in places where they are not wanted. 

 They are generally stronger and faster growing than other plants 

 and therefore they rob crops of food, moisture, and sunlight. 

 Any vacant lot near the school will make a good laboratory for the 

 study of weeds. In such an area we shall find numerous plants, 

 many alike, and all growing closely together in soil that not infre- 

 quently appears so dry and stony that it hardly seems possible 



1 Progeny (proj'e-nl) : offspring. 

 B. BIO — 8 



