CHAPTER XXI 

 WEEDS AND POISONOUS PLANTS 



346. Weeds. A weed has been defined as a " wild plant 

 that has the habit of intruding where not wanted." It does 

 not follow that weeds are necessarily useless or harmful ; 

 indeed, many of them may at times be useful ; and some 

 annoying weeds belong to species which are or have been 

 cultivated but which make a nuisance of themselves when 

 they grow in the wrong place. Some plants that are not 

 particularly troublesome in their original home grow so 

 rankly as to be very annoying weeds when introduced into a 

 new country. Practically all our weeds are herbaceous ; they 

 may therefore be either annuals, growing rapidly from seed 

 and dying at the end of the season, or biennials or perennials 

 whose above-ground parts die each fall. To be successful 

 as a weed, a plant must have some very efficient method 

 of increasing its numbers ; and for this purpose it depends 

 usually either upon the production of a great number of seeds, 

 which are often specially well provided with devices for dis- 

 tribution, or upon multiplication by means of long, branch- 

 ing underground stems. 



347. Injury Caused by Weeds. As a general thing, the 

 greatest objection to weeds is that they reduce the crop of the 

 cultivated plants among which they grow. They do this 

 by crowding the useful plants and so reducing their supply 

 of light, moisture, and the food substances that the plants 

 must take from the soil. It has been suggested, too, that 

 weeds give off to the soil substances that may be poisonous 

 to other plants. The presence of such a weed as the Canada 



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