248 introduction to plant geography [chap. 



Significance and Distribution of Weeds and Plant Diseases 



Although there are many tens of thousands of species of higher 

 plants in the world, only a few dozens of these are really troublesome 

 weeds which are able to reproduce and thrive in the presence of 

 cultivation and other interfering human activities. More numerous 

 by far are the weedy plants and ' escapes ', both herbaceous and 

 woody, which for the present purpose may be considered with the 

 more noxious weeds, and which link the latter with the categories 

 of cultivated plants. Yet the annual loss due to weeds is enormous, 

 often amounting to millions of dollars in a relatively small area. 



Weeds are injurious to agriculture, e.g. by robbing crops of 

 needed water and nutrients, by crowding them out through root- 

 competition or overgrowing, by choking and pulling them down in 

 the case of (sometimes parasitic) climbers, by having seeds or fruits 

 so similar to those of the crop that they are difficult to separate and 

 so adulterate it and reduce its value, by harbouring undesirable 

 insects or plant diseases, by being poisonous or injurious to stock, 

 by tainting milk, and so on. The nuisances caused by Bindweeds 

 and Couch-grass are all too familiar to almost every gardener as 

 well as farmer in the temperate belt ; the Prickly-pears (species of 

 Opuntia) which were introduced into South Africa and Australia as 

 a stock feed now usurp the ground ; Barberries and Currants 

 harbour (as alternative ' hosts ') the devastating Wheat Stem-rust 

 and White Pine Blister-rust, respectively {see pp. 251-2). 



Regardless of the common ' effects of cultivation ' listed near the 

 beginning of this chapter, annual weeds often produce numerous 

 seeds whose germination may be distributed over many years — for 

 example, after being buried for decades.^ Moreover, the seeds or 

 fruits of many weeds, such as Thistles and Dandelions, are provided 

 with efficient means of dispersal. Otherwise their wide distribution 

 seems to be largely due to Man's transportation activities — such as, 

 for example, the shipment of commercial seeds and grain. These 

 are often of much the same size and shape as the disseminules of 



^ The longevity of seeds and fruits comprises an interesting study for which 

 more and more authentic data are needed. Discounting claims of longer periods 

 of burying in marshes etc. which are not fully authenticated, and stories of 

 ' mummy Wheats ' which are clearly bogus, the record seems to be held by a 

 ' seed ' reputedly of Nelumho nucifera (syn. Nelumbiuni speciosum, the Sacred or 

 Indian Lotus) which was germinated in the British Museum (Natural History), 

 South Kensington, London, during the bombing of 1940, apparently about 250 

 years after it had been collected. 



