4 i4 SCIENCE PROGRESS 



farmer will have to cope successfully with their growth. The 

 habit of growth as well as the habitat and climatic conditions 

 all play their part in regulating the distribution of weeds. 



At the outset some definition of the word "weed" is 

 desirable, as the plants that are classified as such differ 

 according as the land on which they grow is arable or pasture 

 land. A good pasture bears a turf or herbage, consisting essen- 

 tially of plants of nutritive value as animal food, which is either 

 consumed on the field or cut and preserved as hay or silage. 

 The larger proportion of such plants are grasses, though many 

 others belonging to the Leguminosae and other families find 

 a definite place in the herbage. Under such circumstances, the 

 term "weed" is necessarily somewhat elastic and adaptable, as 

 some plants may be weeds in some places and under certain 

 conditions whilst they may be welcomed and encouraged in 

 other districts where different conditions prevail. 

 The weeds of pasture land are of various types : 

 i. Plants that are of a poisonous nature harmful to animals 

 feeding on the herbage. 



2. Species that are of very low nutritive value, which lower 

 the quality of the herbage as food if they occur in any 

 abundance. 



3. Species that are valuable enough in themselves yet tend 

 to occur in such superabundance that the balance of the com- 

 position of the herbage is destroyed, other plants of equal or 

 greater value being crowded out more or less completely. 



If this elastic definition be accepted, it is evident that pasture 

 weeds may be of any nature. At times certain species of grass 

 even will fall into the category, if for any reason they are 

 deficient in value as food and occur too abundantly. 



On arable land the conditions are different, as a rule ; one 

 species only is grown as the crop and the admixture of any other 

 is undesirable and harmful. All such alien plants are classed 

 as weeds, " plants growing in the wrong place " — perhaps they 

 may be defined as " plants other than those sown, which grow 

 up naturally with the crop and if unchecked prove exceedingly 

 detrimental to the crop." The weeds are not always what are 

 usually termed "wild" plants, e.g. if seed be shed at autumn time 

 from a crop of barley and spring up among a succeeding crop 

 of vetches, the intrusive barley must be designated as a weed. 



