FRUITS AND COLONISATION BY SEED. 55 



there are not infrequently four generations in a year, 

 may produce 10,000 to 30,000 fertile seeds. 



Man himself is responsible for great vegetable migra- 

 tions : not only his own cultivated favourites, but many 

 weeds are carried inadvertently by him. From the 

 economic side, this carrying of weeds is of great 

 importance. In America much careful observation has 

 been directed to the seeds of common weeds ; and 

 the results of these observations are to be found in 

 the United States year-books of agriculture. It can 

 scarcely be denied, that the untidiness and wastefulness 

 of man is solely responsible for a large number of these 

 agricultural pests. Railway trucks, the packing of 

 cases in which all sorts of goods have been conveyed, 

 and waste products of any kind are all capable of 

 greatly damaging the crops. 



Often the seeds of weeds closely resemble cultivated 

 forms. The seed of Agrostemma Githago is much like a 

 grain of wheat, and several wild Avenas have fruits very 

 like those of the Common Oat. Quite apart from this 

 economic side of the question, which, it may be remarked 

 in passing, is wholly neglected in Britain, is another 

 very important deduction. It has been shown in this 

 chapter that, on the emigration of the young, plants 

 seem to spare no sacrifice of material. To further this, 

 colour, scent, all sorts of beautiful flying and clinging 

 organs are produced in abundance. It is very difficult 

 to discover any plant which, after close study, is found 

 to be without some special arrangement for carrying 

 the young plants away. They are never supposed to 

 remain on the ground where they would infallibly come 

 into collision, and compete with, their long-established 

 predecessors. 



